Merge #4865 from ZyX-I/file-buffered-read

Use buffered reading/writing for ShaDa files

10 files changed, 1333 insertions, 248 deletions

diff --git a/config/CMakeLists.txt b/config/CMakeLists.txt
index e794a8c5b9..cf84f8c6a4 100644
--- a/config/CMakeLists.txt
+++ b/config/CMakeLists.txt
@@ -27,12 +27,15 @@ if(NOT HAVE_SYS_WAIT_H AND UNIX)
 endif()
 check_include_files(sys/utsname.h HAVE_SYS_UTSNAME_H)
 check_include_files(utime.h HAVE_UTIME_H)
+check_include_files(sys/uio.h HAVE_SYS_UIO_H)
 
 # Functions
 check_function_exists(fseeko HAVE_FSEEKO)
 check_function_exists(getpwent HAVE_GETPWENT)
 check_function_exists(getpwnam HAVE_GETPWNAM)
 check_function_exists(getpwuid HAVE_GETPWUID)
+check_function_exists(uv_translate_sys_error HAVE_UV_TRANSLATE_SYS_ERROR)
+check_function_exists(readv HAVE_READV)
 
 if(Iconv_FOUND)
   set(HAVE_ICONV 1)
diff --git a/config/config.h.in b/config/config.h.in
index 867278de0d..4c35b3b1cb 100644
--- a/config/config.h.in
+++ b/config/config.h.in
@@ -30,6 +30,7 @@
 #cmakedefine HAVE_PUTENV_S
 #cmakedefine HAVE_PWD_H
 #cmakedefine HAVE_READLINK
+#cmakedefine HAVE_UV_TRANSLATE_SYS_ERROR
 // TODO: add proper cmake check
 // #define HAVE_SELINUX 1
 #cmakedefine HAVE_SETENV
@@ -48,6 +49,13 @@
 #cmakedefine HAVE_WORKING_LIBINTL
 #cmakedefine UNIX
 #cmakedefine USE_FNAME_CASE
+#cmakedefine HAVE_SYS_UIO_H
+#ifdef HAVE_SYS_UIO_H
+#cmakedefine HAVE_READV
+# ifndef HAVE_READV
+#  undef HAVE_SYS_UIO_H
+# endif
+#endif
 
 #cmakedefine FEAT_TUI
 
diff --git a/src/nvim/os/fileio.c b/src/nvim/os/fileio.c
new file mode 100644
index 0000000000..6cee102305
--- /dev/null
+++ b/src/nvim/os/fileio.c
@@ -0,0 +1,319 @@
+/// @file fileio.c
+///
+/// Buffered reading/writing to a file. Unlike fileio.c this is not dealing with
+/// Neovim stuctures for buffer, with autocommands, etc: just fopen/fread/fwrite
+/// replacement.
+
+#include <unistd.h>
+#include <assert.h>
+#include <stddef.h>
+#include <stdbool.h>
+#include <fcntl.h>
+
+#include "auto/config.h"
+
+#ifdef HAVE_SYS_UIO_H
+# include <sys/uio.h>
+#endif
+
+#include <uv.h>
+
+#include "nvim/os/fileio.h"
+#include "nvim/memory.h"
+#include "nvim/os/os.h"
+#include "nvim/globals.h"
+#include "nvim/rbuffer.h"
+#include "nvim/macros.h"
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/fileio.c.generated.h"
+#endif
+
+/// Open file
+///
+/// @param[out]  ret_fp  Address where information needed for reading from or
+///                      writing to a file is saved
+/// @param[in]  fname  File name to open.
+/// @param[in]  flags  Flags, @see FileOpenFlags. Currently reading from and
+///                    writing to the file at once is not supported, so either
+///                    FILE_WRITE_ONLY or FILE_READ_ONLY is required.
+/// @param[in]  mode  Permissions for the newly created file (ignored if flags
+///                   does not have FILE_CREATE\*).
+///
+/// @return Error code (@see os_strerror()) or 0.
+int file_open(FileDescriptor *const ret_fp, const char *const fname,
+              const int flags, const int mode)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  int os_open_flags = 0;
+  int fd;
+  TriState wr = kNone;
+#define FLAG(flags, flag, fcntl_flags, wrval, cond) \
+  do { \
+    if (flags & flag) { \
+      os_open_flags |= fcntl_flags; \
+      assert(cond); \
+      if (wrval != kNone) { \
+        wr = wrval; \
+      } \
+    } \
+  } while (0)
+  FLAG(flags, kFileWriteOnly, O_WRONLY, kTrue, true);
+  FLAG(flags, kFileCreateOnly, O_CREAT|O_EXCL|O_WRONLY, kTrue, true);
+  FLAG(flags, kFileCreate, O_CREAT|O_WRONLY, kTrue, !(flags & kFileCreateOnly));
+  FLAG(flags, kFileTruncate, O_TRUNC|O_WRONLY, kTrue,
+       !(flags & kFileCreateOnly));
+  FLAG(flags, kFileReadOnly, O_RDONLY, kFalse, wr != kTrue);
+#ifdef O_NOFOLLOW
+  FLAG(flags, kFileNoSymlink, O_NOFOLLOW, kNone, true);
+#endif
+#undef FLAG
+
+  fd = os_open(fname, os_open_flags, mode);
+
+  if (fd < 0) {
+    return fd;
+  }
+
+  ret_fp->wr = (wr == kTrue);
+  ret_fp->fd = fd;
+  ret_fp->eof = false;
+  ret_fp->rv = rbuffer_new(kRWBufferSize);
+  ret_fp->_error = 0;
+  if (ret_fp->wr) {
+    ret_fp->rv->data = ret_fp;
+    ret_fp->rv->full_cb = (rbuffer_callback)&file_rb_write_full_cb;
+  }
+  return 0;
+}
+
+/// Like file_open(), but allocate and return ret_fp
+///
+/// @param[out]  error  Error code, @see os_strerror(). Is set to zero on
+///                     success.
+/// @param[in]  fname  File name to open.
+/// @param[in]  flags  Flags, @see FileOpenFlags.
+/// @param[in]  mode  Permissions for the newly created file (ignored if flags
+///                   does not have FILE_CREATE\*).
+///
+/// @return [allocated] Opened file or NULL in case of error.
+FileDescriptor *file_open_new(int *const error, const char *const fname,
+                              const int flags, const int mode)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_MALLOC FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  FileDescriptor *const fp = xmalloc(sizeof(*fp));
+  if ((*error = file_open(fp, fname, flags, mode)) != 0) {
+    xfree(fp);
+    return NULL;
+  }
+  return fp;
+}
+
+/// Close file and free its buffer
+///
+/// @param[in,out]  fp  File to close.
+///
+/// @return 0 or error code.
+int file_close(FileDescriptor *const fp) FUNC_ATTR_NONNULL_ALL
+{
+  const int error = file_fsync(fp);
+  const int error2 = os_close(fp->fd);
+  rbuffer_free(fp->rv);
+  if (error2 != 0) {
+    return error2;
+  }
+  return error;
+}
+
+/// Close and free file obtained using file_open_new()
+///
+/// @param[in,out]  fp  File to close.
+///
+/// @return 0 or error code.
+int file_free(FileDescriptor *const fp) FUNC_ATTR_NONNULL_ALL
+{
+  const int ret = file_close(fp);
+  xfree(fp);
+  return ret;
+}
+
+/// Flush file modifications to disk
+///
+/// @param[in,out]  fp  File to work with.
+///
+/// @return 0 or error code.
+int file_fsync(FileDescriptor *const fp)
+  FUNC_ATTR_NONNULL_ALL
+{
+  if (!fp->wr) {
+    return 0;
+  }
+  file_rb_write_full_cb(fp->rv, fp);
+  if (fp->_error != 0) {
+    const int error = fp->_error;
+    fp->_error = 0;
+    return error;
+  }
+  return os_fsync(fp->fd);
+}
+
+/// Buffer used for writing
+///
+/// Like IObuff, but allows file_\* callers not to care about spoiling it.
+static char writebuf[kRWBufferSize];
+
+/// Function run when RBuffer is full when writing to a file
+///
+/// Actually does writing to the file, may also be invoked directly.
+///
+/// @param[in,out]  rv  RBuffer instance used.
+/// @param[in,out]  fp  File to work with.
+static void file_rb_write_full_cb(RBuffer *const rv, FileDescriptor *const fp)
+  FUNC_ATTR_NONNULL_ALL
+{
+  assert(fp->wr);
+  assert(rv->data == (void *)fp);
+  if (rbuffer_size(rv) == 0) {
+    return;
+  }
+  const size_t read_bytes = rbuffer_read(rv, writebuf, kRWBufferSize);
+  const ptrdiff_t wres = os_write(fp->fd, writebuf, read_bytes);
+  if (wres != (ptrdiff_t)read_bytes) {
+    if (wres >= 0) {
+      fp->_error = UV_EIO;
+    } else {
+      fp->_error = (int)wres;
+    }
+  }
+}
+
+/// Read from file
+///
+/// @param[in,out]  fp  File to work with.
+/// @param[out]  ret_buf  Buffer to read to. Must not be NULL.
+/// @param[in]  size  Number of bytes to read. Buffer must have at least ret_buf
+///                   bytes.
+///
+/// @return error_code (< 0) or number of bytes read.
+ptrdiff_t file_read(FileDescriptor *const fp, char *const ret_buf,
+                    const size_t size)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  assert(!fp->wr);
+  char *buf = ret_buf;
+  size_t read_remaining = size;
+  RBuffer *const rv = fp->rv;
+  while (read_remaining) {
+    const size_t rv_size = rbuffer_size(rv);
+    if (rv_size > 0) {
+      const size_t rsize = rbuffer_read(rv, buf, MIN(rv_size, read_remaining));
+      buf += rsize;
+      read_remaining -= rsize;
+    }
+    if (fp->eof) {
+      break;
+    }
+    if (read_remaining) {
+      assert(rbuffer_size(rv) == 0);
+      rbuffer_reset(rv);
+#ifdef HAVE_READV
+      // If there is readv() syscall, then take an opportunity to populate
+      // both target buffer and RBuffer at once, …
+      size_t write_count;
+      struct iovec iov[] = {
+        { .iov_base = buf, .iov_len = read_remaining },
+        { .iov_base = rbuffer_write_ptr(rv, &write_count),
+          .iov_len = kRWBufferSize },
+      };
+      assert(write_count == kRWBufferSize);
+      const ptrdiff_t r_ret = os_readv(fp->fd, &fp->eof, iov,
+                                       ARRAY_SIZE(iov));
+      if (r_ret > 0) {
+        if (r_ret > (ptrdiff_t)read_remaining) {
+          rbuffer_produced(rv, (size_t)(r_ret - (ptrdiff_t)read_remaining));
+          read_remaining = 0;
+        } else {
+          buf += (size_t)r_ret;
+          read_remaining -= (size_t)r_ret;
+        }
+      } else if (r_ret < 0) {
+        return r_ret;
+      }
+#else
+      if (read_remaining >= kRWBufferSize) {
+        // …otherwise leave RBuffer empty and populate only target buffer,
+        // because filtering information through rbuffer will be more syscalls.
+        const ptrdiff_t r_ret = os_read(fp->fd, &fp->eof, buf, read_remaining);
+        if (r_ret >= 0) {
+          read_remaining -= (size_t)r_ret;
+          return (ptrdiff_t)(size - read_remaining);
+        } else if (r_ret < 0) {
+          return r_ret;
+        }
+      } else {
+        size_t write_count;
+        const ptrdiff_t r_ret = os_read(fp->fd, &fp->eof,
+                                        rbuffer_write_ptr(rv, &write_count),
+                                        kRWBufferSize);
+        assert(write_count == kRWBufferSize);
+        if (r_ret > 0) {
+          rbuffer_produced(rv, (size_t)r_ret);
+        } else if (r_ret < 0) {
+          return r_ret;
+        }
+      }
+#endif
+    }
+  }
+  return (ptrdiff_t)(size - read_remaining);
+}
+
+/// Write to a file
+///
+/// @param[in]  fd  File descriptor to write to.
+/// @param[in]  buf  Data to write. May be NULL if size is zero.
+/// @param[in]  size  Amount of bytes to write.
+///
+/// @return Number of bytes written or libuv error code (< 0).
+ptrdiff_t file_write(FileDescriptor *const fp, const char *const buf,
+                     const size_t size)
+  FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ARG(1)
+{
+  assert(fp->wr);
+  const size_t written = rbuffer_write(fp->rv, buf, size);
+  if (fp->_error != 0) {
+    const int error = fp->_error;
+    fp->_error = 0;
+    return error;
+  } else if (written != size) {
+    return UV_EIO;
+  }
+  return (ptrdiff_t)written;
+}
+
+/// Buffer used for skipping. Its contents is undefined and should never be
+/// used.
+static char skipbuf[kRWBufferSize];
+
+/// Skip some bytes
+///
+/// This is like `fseek(fp, size, SEEK_CUR)`, but actual implementation simply
+/// reads to a buffer and discards the result.
+ptrdiff_t file_skip(FileDescriptor *const fp, const size_t size)
+  FUNC_ATTR_NONNULL_ALL
+{
+  assert(!fp->wr);
+  size_t read_bytes = 0;
+  do {
+    const ptrdiff_t new_read_bytes = file_read(
+        fp, skipbuf, MIN(size - read_bytes, sizeof(skipbuf)));
+    if (new_read_bytes < 0) {
+      return new_read_bytes;
+    } else if (new_read_bytes == 0) {
+      break;
+    }
+    read_bytes += (size_t)new_read_bytes;
+  } while (read_bytes < size && !file_eof(fp));
+
+  return (ptrdiff_t)read_bytes;
+}
diff --git a/src/nvim/os/fileio.h b/src/nvim/os/fileio.h
new file mode 100644
index 0000000000..2cffd5c851
--- /dev/null
+++ b/src/nvim/os/fileio.h
@@ -0,0 +1,72 @@
+#ifndef NVIM_OS_FILEIO_H
+#define NVIM_OS_FILEIO_H
+
+#include <stdbool.h>
+#include <stddef.h>
+
+#include "nvim/func_attr.h"
+#include "nvim/rbuffer.h"
+
+/// Structure used to read from/write to file
+typedef struct {
+  int fd;  ///< File descriptor.
+  int _error;  ///< Error code for use with RBuffer callbacks or zero.
+  RBuffer *rv;  ///< Read or write buffer.
+  bool wr;  ///< True if file is in write mode.
+  bool eof;  ///< True if end of file was encountered.
+} FileDescriptor;
+
+/// file_open() flags
+typedef enum {
+  kFileReadOnly = 1,  ///< Open file read-only. Default.
+  kFileCreate = 2,  ///< Create file if it does not exist yet.
+                    ///< Implies kFileWriteOnly.
+  kFileWriteOnly = 4,  ///< Open file for writing only.
+                       ///< Cannot be used with kFileReadOnly.
+  kFileNoSymlink = 8,  ///< Do not allow symbolic links.
+  kFileCreateOnly = 16,  ///< Only create the file, failing if it already
+                         ///< exists. Implies kFileWriteOnly. Cannot be used
+                         ///< with kFileCreate.
+  kFileTruncate = 32,  ///< Truncate the file if it exists.
+                       ///< Implies kFileWriteOnly. Cannot be used with
+                       ///< kFileCreateOnly.
+} FileOpenFlags;
+
+static inline bool file_eof(const FileDescriptor *const fp)
+  REAL_FATTR_PURE REAL_FATTR_WARN_UNUSED_RESULT REAL_FATTR_NONNULL_ALL;
+
+/// Check whether end of file was encountered
+///
+/// @param[in]  fp  File to check.
+///
+/// @return true if it was, false if it was not or read operation was never
+///         performed.
+static inline bool file_eof(const FileDescriptor *const fp)
+{
+  return fp->eof && rbuffer_size(fp->rv) == 0;
+}
+
+static inline int file_fd(const FileDescriptor *const fp)
+  REAL_FATTR_PURE REAL_FATTR_WARN_UNUSED_RESULT REAL_FATTR_NONNULL_ALL;
+
+/// Return the file descriptor associated with the FileDescriptor structure
+///
+/// @param[in]  fp  File to check.
+///
+/// @return File descriptor.
+static inline int file_fd(const FileDescriptor *const fp)
+{
+  return fp->fd;
+}
+
+enum {
+  /// Read or write buffer size
+  ///
+  /// Currently equal to (IOSIZE - 1), but they do not need to be connected.
+  kRWBufferSize = 1024
+};
+
+#ifdef INCLUDE_GENERATED_DECLARATIONS
+# include "os/fileio.h.generated.h"
+#endif
+#endif  // NVIM_OS_FILEIO_H
diff --git a/src/nvim/os/fs.c b/src/nvim/os/fs.c
index 08122828bd..49c42cb63d 100644
--- a/src/nvim/os/fs.c
+++ b/src/nvim/os/fs.c
@@ -1,14 +1,26 @@
 // fs.c -- filesystem access
 #include <stdbool.h>
-
+#include <stddef.h>
 #include <assert.h>
+#include <limits.h>
+#include <unistd.h>
 #include <fcntl.h>
+#include <errno.h>
+
+#include "auto/config.h"
+
+#ifdef HAVE_SYS_UIO_H
+# include <sys/uio.h>
+#endif
+
+#include <uv.h>
 
 #include "nvim/os/os.h"
 #include "nvim/os/os_defs.h"
 #include "nvim/ascii.h"
 #include "nvim/memory.h"
 #include "nvim/message.h"
+#include "nvim/assert.h"
 #include "nvim/misc1.h"
 #include "nvim/misc2.h"
 #include "nvim/path.h"
@@ -18,6 +30,20 @@
 # include "os/fs.c.generated.h"
 #endif
 
+#define RUN_UV_FS_FUNC(ret, func, ...) \
+    do { \
+      bool did_try_to_free = false; \
+uv_call_start: {} \
+      uv_fs_t req; \
+      ret = func(&fs_loop, &req, __VA_ARGS__); \
+      uv_fs_req_cleanup(&req); \
+      if (ret == UV_ENOMEM && !did_try_to_free) { \
+        try_to_free_memory(); \
+        did_try_to_free = true; \
+        goto uv_call_start; \
+      } \
+    } while (0)
+
 // Many fs functions from libuv return that value on success.
 static const int kLibuvSuccess = 0;
 static uv_loop_t fs_loop;
@@ -325,13 +351,190 @@ static bool is_executable_in_path(const char_u *name, char_u **abspath)
 int os_open(const char* path, int flags, int mode)
   FUNC_ATTR_NONNULL_ALL
 {
-  uv_fs_t open_req;
-  int r = uv_fs_open(&fs_loop, &open_req, path, flags, mode, NULL);
-  uv_fs_req_cleanup(&open_req);
-  // r is the same as open_req.result (except for OOM: then only r is set).
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_open, path, flags, mode, NULL);
+  return r;
+}
+
+/// Close a file
+///
+/// @return 0 or libuv error code on failure.
+int os_close(const int fd)
+{
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_close, fd, NULL);
   return r;
 }
 
+/// Read from a file
+///
+/// Handles EINTR and ENOMEM, but not other errors.
+///
+/// @param[in]  fd  File descriptor to read from.
+/// @param[out]  ret_eof  Is set to true if EOF was encountered, otherwise set
+///                       to false. Initial value is ignored.
+/// @param[out]  ret_buf  Buffer to write to. May be NULL if size is zero.
+/// @param[in]  size  Amount of bytes to read.
+///
+/// @return Number of bytes read or libuv error code (< 0).
+ptrdiff_t os_read(const int fd, bool *ret_eof, char *const ret_buf,
+                  const size_t size)
+  FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  *ret_eof = false;
+  if (ret_buf == NULL) {
+    assert(size == 0);
+    return 0;
+  }
+  size_t read_bytes = 0;
+  bool did_try_to_free = false;
+  while (read_bytes != size) {
+    const ptrdiff_t cur_read_bytes = read(fd, ret_buf + read_bytes,
+                                          size - read_bytes);
+    if (cur_read_bytes > 0) {
+      read_bytes += (size_t)cur_read_bytes;
+      assert(read_bytes <= size);
+    }
+    if (cur_read_bytes < 0) {
+#ifdef HAVE_UV_TRANSLATE_SYS_ERROR
+      const int error = uv_translate_sys_error(errno);
+#else
+      const int error = -errno;
+      STATIC_ASSERT(-EINTR == UV_EINTR, "Need to translate error codes");
+      STATIC_ASSERT(-EAGAIN == UV_EAGAIN, "Need to translate error codes");
+      STATIC_ASSERT(-ENOMEM == UV_ENOMEM, "Need to translate error codes");
+#endif
+      errno = 0;
+      if (error == UV_EINTR || error == UV_EAGAIN) {
+        continue;
+      } else if (error == UV_ENOMEM && !did_try_to_free) {
+        try_to_free_memory();
+        did_try_to_free = true;
+        continue;
+      } else {
+        return (ptrdiff_t)error;
+      }
+    }
+    if (cur_read_bytes == 0) {
+      *ret_eof = true;
+      break;
+    }
+  }
+  return (ptrdiff_t)read_bytes;
+}
+
+#ifdef HAVE_READV
+/// Read from a file to multiple buffers at once
+///
+/// Wrapper for readv().
+///
+/// @param[in]  fd  File descriptor to read from.
+/// @param[out]  ret_eof  Is set to true if EOF was encountered, otherwise set
+///                       to false. Initial value is ignored.
+/// @param[out]  iov  Description of buffers to write to. Note: this description
+///                   may change, it is incorrect to use data it points to after
+///                   os_readv().
+/// @param[in]  iov_size  Number of buffers in iov.
+ptrdiff_t os_readv(int fd, bool *ret_eof, struct iovec *iov, size_t iov_size)
+  FUNC_ATTR_NONNULL_ALL
+{
+  *ret_eof = false;
+  size_t read_bytes = 0;
+  bool did_try_to_free = false;
+  size_t toread = 0;
+  for (size_t i = 0; i < iov_size; i++) {
+    // Overflow, trying to read too much data
+    assert(toread <= SIZE_MAX - iov[i].iov_len);
+    toread += iov[i].iov_len;
+  }
+  while (read_bytes < toread && iov_size && !*ret_eof) {
+    ptrdiff_t cur_read_bytes = readv(fd, iov, (int)iov_size);
+    if (toread && cur_read_bytes == 0) {
+      *ret_eof = true;
+    }
+    if (cur_read_bytes > 0) {
+      read_bytes += (size_t)cur_read_bytes;
+      while (iov_size && cur_read_bytes) {
+        if (cur_read_bytes < (ptrdiff_t)iov->iov_len) {
+          iov->iov_len -= (size_t)cur_read_bytes;
+          iov->iov_base = (char *)iov->iov_base + cur_read_bytes;
+          cur_read_bytes = 0;
+        } else {
+          cur_read_bytes -= (ptrdiff_t)iov->iov_len;
+          iov_size--;
+          iov++;
+        }
+      }
+    } else if (cur_read_bytes < 0) {
+#ifdef HAVE_UV_TRANSLATE_SYS_ERROR
+      const int error = uv_translate_sys_error(errno);
+#else
+      const int error = -errno;
+      STATIC_ASSERT(-EINTR == UV_EINTR, "Need to translate error codes");
+      STATIC_ASSERT(-EAGAIN == UV_EAGAIN, "Need to translate error codes");
+      STATIC_ASSERT(-ENOMEM == UV_ENOMEM, "Need to translate error codes");
+#endif
+      errno = 0;
+      if (error == UV_EINTR || error == UV_EAGAIN) {
+        continue;
+      } else if (error == UV_ENOMEM && !did_try_to_free) {
+        try_to_free_memory();
+        did_try_to_free = true;
+        continue;
+      } else {
+        return (ptrdiff_t)error;
+      }
+    }
+  }
+  return (ptrdiff_t)read_bytes;
+}
+#endif  // HAVE_READV
+
+/// Write to a file
+///
+/// @param[in]  fd  File descriptor to write to.
+/// @param[in]  buf  Data to write. May be NULL if size is zero.
+/// @param[in]  size  Amount of bytes to write.
+///
+/// @return Number of bytes written or libuv error code (< 0).
+ptrdiff_t os_write(const int fd, const char *const buf, const size_t size)
+  FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  if (buf == NULL) {
+    assert(size == 0);
+    return 0;
+  }
+  size_t written_bytes = 0;
+  while (written_bytes != size) {
+    const ptrdiff_t cur_written_bytes = write(fd, buf + written_bytes,
+                                              size - written_bytes);
+    if (cur_written_bytes > 0) {
+      written_bytes += (size_t)cur_written_bytes;
+    }
+    if (cur_written_bytes < 0) {
+#ifdef HAVE_UV_TRANSLATE_SYS_ERROR
+      const int error = uv_translate_sys_error(errno);
+#else
+      const int error = -errno;
+      STATIC_ASSERT(-EINTR == UV_EINTR, "Need to translate error codes");
+      STATIC_ASSERT(-EAGAIN == UV_EAGAIN, "Need to translate error codes");
+      // According to the man page open() may fail with ENOMEM, but write()
+      // can’t.
+#endif
+      errno = 0;
+      if (error == UV_EINTR || error == UV_EAGAIN) {
+        continue;
+      } else {
+        return error;
+      }
+    }
+    if (cur_written_bytes == 0) {
+      return UV_UNKNOWN;
+    }
+  }
+  return (ptrdiff_t)written_bytes;
+}
+
 /// Flushes file modifications to disk.
 ///
 /// @param fd the file descriptor of the file to flush to disk.
@@ -339,9 +542,8 @@ int os_open(const char* path, int flags, int mode)
 /// @return `0` on success, a libuv error code on failure.
 int os_fsync(int fd)
 {
-  uv_fs_t fsync_req;
-  int r = uv_fs_fsync(&fs_loop, &fsync_req, fd, NULL);
-  uv_fs_req_cleanup(&fsync_req);
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_fsync, fd, NULL);
   return r;
 }
 
@@ -379,16 +581,9 @@ int32_t os_getperm(const char_u *name)
 int os_setperm(const char_u *name, int perm)
   FUNC_ATTR_NONNULL_ALL
 {
-  uv_fs_t request;
-  int result = uv_fs_chmod(&fs_loop, &request,
-                           (const char*)name, perm, NULL);
-  uv_fs_req_cleanup(&request);
-
-  if (result == kLibuvSuccess) {
-    return OK;
-  }
-
-  return FAIL;
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_chmod, (const char *)name, perm, NULL);
+  return (r == kLibuvSuccess ? OK : FAIL);
 }
 
 /// Changes the ownership of the file referred to by the open file descriptor.
@@ -397,13 +592,11 @@ int os_setperm(const char_u *name, int perm)
 ///
 /// @note If the `owner` or `group` is specified as `-1`, then that ID is not
 /// changed.
-int os_fchown(int file_descriptor, uv_uid_t owner, uv_gid_t group)
+int os_fchown(int fd, uv_uid_t owner, uv_gid_t group)
 {
-  uv_fs_t request;
-  int result = uv_fs_fchown(&fs_loop, &request, file_descriptor,
-                            owner, group, NULL);
-  uv_fs_req_cleanup(&request);
-  return result;
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_fchown, fd, owner, group, NULL);
+  return r;
 }
 
 /// Check if a file exists.
@@ -422,9 +615,8 @@ bool os_file_exists(const char_u *name)
 bool os_file_is_readable(const char *name)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  uv_fs_t req;
-  int r = uv_fs_access(&fs_loop, &req, name, R_OK, NULL);
-  uv_fs_req_cleanup(&req);
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_access, name, R_OK, NULL);
   return (r == 0);
 }
 
@@ -436,9 +628,8 @@ bool os_file_is_readable(const char *name)
 int os_file_is_writable(const char *name)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  uv_fs_t req;
-  int r = uv_fs_access(&fs_loop, &req, name, W_OK, NULL);
-  uv_fs_req_cleanup(&req);
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_access, name, W_OK, NULL);
   if (r == 0) {
     return os_isdir((char_u *)name) ? 2 : 1;
   }
@@ -451,16 +642,10 @@ int os_file_is_writable(const char *name)
 int os_rename(const char_u *path, const char_u *new_path)
   FUNC_ATTR_NONNULL_ALL
 {
-  uv_fs_t request;
-  int result = uv_fs_rename(&fs_loop, &request,
-                            (const char *)path, (const char *)new_path, NULL);
-  uv_fs_req_cleanup(&request);
-
-  if (result == kLibuvSuccess) {
-    return OK;
-  }
-
-  return FAIL;
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_rename, (const char *)path, (const char *)new_path,
+                 NULL);
+  return (r == kLibuvSuccess ? OK : FAIL);
 }
 
 /// Make a directory.
@@ -469,10 +654,9 @@ int os_rename(const char_u *path, const char_u *new_path)
 int os_mkdir(const char *path, int32_t mode)
   FUNC_ATTR_NONNULL_ALL
 {
-  uv_fs_t request;
-  int result = uv_fs_mkdir(&fs_loop, &request, path, mode, NULL);
-  uv_fs_req_cleanup(&request);
-  return result;
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_mkdir, path, mode, NULL);
+  return r;
 }
 
 /// Make a directory, with higher levels when needed
@@ -554,10 +738,9 @@ int os_mkdtemp(const char *template, char *path)
 int os_rmdir(const char *path)
   FUNC_ATTR_NONNULL_ALL
 {
-  uv_fs_t request;
-  int result = uv_fs_rmdir(&fs_loop, &request, path, NULL);
-  uv_fs_req_cleanup(&request);
-  return result;
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_rmdir, path, NULL);
+  return r;
 }
 
 /// Opens a directory.
@@ -599,10 +782,9 @@ void os_closedir(Directory *dir)
 int os_remove(const char *path)
   FUNC_ATTR_NONNULL_ALL
 {
-  uv_fs_t request;
-  int result = uv_fs_unlink(&fs_loop, &request, path, NULL);
-  uv_fs_req_cleanup(&request);
-  return result;
+  int r;
+  RUN_UV_FS_FUNC(r, uv_fs_unlink, path, NULL);
+  return r;
 }
 
 /// Get the file information for a given path
diff --git a/src/nvim/rbuffer.c b/src/nvim/rbuffer.c
index b3805a3a28..d4cc8c0d5d 100644
--- a/src/nvim/rbuffer.c
+++ b/src/nvim/rbuffer.c
@@ -153,7 +153,7 @@ void rbuffer_consumed(RBuffer *buf, size_t count)
 
 // Higher level functions for copying from/to RBuffer instances and data
 // pointers
-size_t rbuffer_write(RBuffer *buf, char *src, size_t src_size)
+size_t rbuffer_write(RBuffer *buf, const char *src, size_t src_size)
   FUNC_ATTR_NONNULL_ALL
 {
   size_t size = src_size;
diff --git a/src/nvim/shada.c b/src/nvim/shada.c
index 380d955f63..b5921eb810 100644
--- a/src/nvim/shada.c
+++ b/src/nvim/shada.c
@@ -5,7 +5,6 @@
 #include <stdint.h>
 #include <inttypes.h>
 #include <errno.h>
-#include <fcntl.h>
 #include <assert.h>
 
 #include <msgpack.h>
@@ -36,6 +35,7 @@
 #include "nvim/version.h"
 #include "nvim/path.h"
 #include "nvim/fileio.h"
+#include "nvim/os/fileio.h"
 #include "nvim/strings.h"
 #include "nvim/quickfix.h"
 #include "nvim/eval/encode.h"
@@ -409,7 +409,7 @@ typedef struct sd_read_def {
   ShaDaFileSkipper skip;  ///< Function used to skip some bytes.
   void *cookie;           ///< Data describing object read from.
   bool eof;               ///< True if reader reached end of file.
-  char *error;            ///< Error message in case of error.
+  const char *error;      ///< Error message in case of error.
   uintmax_t fpos;         ///< Current position (amount of bytes read since
                           ///< reader structure initialization). May overflow.
   vimconv_T sd_conv;      ///< Structure used for converting encodings of some
@@ -433,7 +433,7 @@ typedef struct sd_write_def {
   ShaDaFileWriter write;   ///< Writer function.
   ShaDaWriteCloser close;  ///< Close function.
   void *cookie;            ///< Data describing object written to.
-  char *error;             ///< Error message in case of error.
+  const char *error;       ///< Error message in case of error.
   vimconv_T sd_conv;       ///< Structure used for converting encodings of some
                            ///< items.
 } ShaDaWriteDef;
@@ -666,38 +666,14 @@ static ptrdiff_t read_file(ShaDaReadDef *const sd_reader, void *const dest,
                            const size_t size)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  size_t read_bytes = 0;
-  bool did_try_to_free = false;
-  const int fd = (int)(intptr_t) sd_reader->cookie;
-  while (read_bytes != size) {
-    const ptrdiff_t cur_read_bytes = read(fd, ((char *) dest) + read_bytes,
-                                          size - read_bytes);
-    if (cur_read_bytes > 0) {
-      read_bytes += (size_t) cur_read_bytes;
-      sd_reader->fpos += (uintmax_t) cur_read_bytes;
-      assert(read_bytes <= size);
-    }
-    if (cur_read_bytes < 0) {
-      if (errno == EINTR || errno == EAGAIN) {
-        errno = 0;
-        continue;
-      } else if (errno == ENOMEM && !did_try_to_free) {
-        try_to_free_memory();
-        did_try_to_free = true;
-        errno = 0;
-        continue;
-      } else {
-        sd_reader->error = strerror(errno);
-        errno = 0;
-        return -1;
-      }
-    }
-    if (cur_read_bytes == 0) {
-      sd_reader->eof = true;
-      break;
-    }
+  const ptrdiff_t ret = file_read(sd_reader->cookie, dest, size);
+  sd_reader->eof = file_eof(sd_reader->cookie);
+  if (ret < 0) {
+    sd_reader->error = os_strerror((int)ret);
+    return -1;
   }
-  return (ptrdiff_t) read_bytes;
+  sd_reader->fpos += (size_t)ret;
+  return ret;
 }
 
 /// Read one character
@@ -720,50 +696,26 @@ static ptrdiff_t write_file(ShaDaWriteDef *const sd_writer,
                             const size_t size)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  size_t written_bytes = 0;
-  const int fd = (int)(intptr_t) sd_writer->cookie;
-  while (written_bytes != size) {
-    const ptrdiff_t cur_written_bytes = write(fd, (char *) dest + written_bytes,
-                                              size - written_bytes);
-    if (cur_written_bytes > 0) {
-      written_bytes += (size_t) cur_written_bytes;
-    }
-    if (cur_written_bytes < 0) {
-      if (errno == EINTR || errno == EAGAIN) {
-        errno = 0;
-        continue;
-      } else {
-        sd_writer->error = strerror(errno);
-        errno = 0;
-        return -1;
-      }
-    }
-    if (cur_written_bytes == 0) {
-      sd_writer->error = "Zero bytes written.";
-      return -1;
-    }
+  const ptrdiff_t ret = file_write(sd_writer->cookie, dest, size);
+  if (ret < 0) {
+    sd_writer->error = os_strerror((int)ret);
+    return -1;
   }
-  return (ptrdiff_t) written_bytes;
+  return ret;
 }
 
 /// Wrapper for closing file descriptors opened for reading
 static void close_sd_reader(ShaDaReadDef *const sd_reader)
   FUNC_ATTR_NONNULL_ALL
 {
-  close_file((int)(intptr_t) sd_reader->cookie);
+  close_file(sd_reader->cookie);
 }
 
 /// Wrapper for closing file descriptors opened for writing
 static void close_sd_writer(ShaDaWriteDef *const sd_writer)
   FUNC_ATTR_NONNULL_ALL
 {
-  const int fd = (int)(intptr_t) sd_writer->cookie;
-  if (os_fsync(fd) < 0) {
-    emsgf(_(SERR "System error while synchronizing ShaDa file: %s"),
-          os_strerror(errno));
-    errno = 0;
-  }
-  close_file(fd);
+  close_file(sd_writer->cookie);
 }
 
 /// Wrapper for read that reads to IObuff and ignores bytes read
@@ -779,19 +731,20 @@ static int sd_reader_skip_read(ShaDaReadDef *const sd_reader,
                                const size_t offset)
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
-  size_t read_bytes = 0;
-  do {
-    ptrdiff_t new_read_bytes = sd_reader->read(
-        sd_reader, IObuff, (size_t) (offset - read_bytes > IOSIZE
-                                     ? IOSIZE
-                                     : offset - read_bytes));
-    if (new_read_bytes == -1) {
-      return FAIL;
+  const ptrdiff_t skip_bytes = file_skip(sd_reader->cookie, offset);
+  if (skip_bytes < 0) {
+    sd_reader->error = os_strerror((int)skip_bytes);
+    return FAIL;
+  } else if (skip_bytes != (ptrdiff_t)offset) {
+    assert(skip_bytes < (ptrdiff_t)offset);
+    sd_reader->eof = file_eof(sd_reader->cookie);
+    if (!sd_reader->eof) {
+      sd_reader->error = _("too few bytes read");
     }
-    read_bytes += (size_t) new_read_bytes;
-  } while (read_bytes < offset && !sd_reader->eof);
-
-  return (read_bytes == offset ? OK : FAIL);
+    return FAIL;
+  }
+  sd_reader->fpos += (size_t)skip_bytes;
+  return OK;
 }
 
 /// Wrapper for read that can be used when lseek cannot be used
@@ -824,37 +777,6 @@ static ShaDaReadResult sd_reader_skip(ShaDaReadDef *const sd_reader,
   return kSDReadStatusSuccess;
 }
 
-/// Wrapper for opening file descriptors
-///
-/// All arguments are passed to os_open().
-///
-/// @return file descriptor or libuv error on failure.
-static int open_file(const char *const fname, const int flags, const int mode)
-  FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ALL
-{
-  bool did_try_to_free = false;
-  int fd;
-open_file_start:
-  fd = os_open(fname, flags, mode);
-
-  if (fd < 0) {
-    if (fd == UV_ENOENT) {
-      return fd;
-    }
-    if (fd == UV_ENOMEM && !did_try_to_free) {
-      try_to_free_memory();
-      did_try_to_free = true;
-      goto open_file_start;
-    }
-    if (fd != UV_EEXIST) {
-      emsgf(_(SERR "System error while opening ShaDa file %s: %s"),
-            fname, os_strerror(fd));
-    }
-    return fd;
-  }
-  return fd;
-}
-
 /// Open ShaDa file for reading
 ///
 /// @param[in]   fname      File name to open.
@@ -865,11 +787,7 @@ static int open_shada_file_for_reading(const char *const fname,
                                        ShaDaReadDef *sd_reader)
   FUNC_ATTR_WARN_UNUSED_RESULT FUNC_ATTR_NONNULL_ALL
 {
-  const intptr_t fd = (intptr_t) open_file(fname, O_RDONLY, 0);
-
-  if (fd < 0) {
-    return (int) fd;
-  }
+  int error;
 
   *sd_reader = (ShaDaReadDef) {
     .read = &read_file,
@@ -878,8 +796,11 @@ static int open_shada_file_for_reading(const char *const fname,
     .error = NULL,
     .eof = false,
     .fpos = 0,
-    .cookie = (void *) fd,
+    .cookie = file_open_new(&error, fname, kFileReadOnly, 0),
   };
+  if (sd_reader->cookie == NULL) {
+    return error;
+  }
 
   convert_setup(&sd_reader->sd_conv, "utf-8", p_enc);
 
@@ -887,18 +808,12 @@ static int open_shada_file_for_reading(const char *const fname,
 }
 
 /// Wrapper for closing file descriptors
-static void close_file(int fd)
+static void close_file(void *cookie)
 {
-close_file_start:
-  if (close(fd) == -1) {
-    if (errno == EINTR) {
-      errno = 0;
-      goto close_file_start;
-    } else {
-      emsgf(_(SERR "System error while closing ShaDa file: %s"),
-            strerror(errno));
-      errno = 0;
-    }
+  const int error = file_free(cookie);
+  if (error != 0) {
+    emsgf(_(SERR "System error while closing ShaDa file: %s"),
+          os_strerror(error));
   }
 }
 
@@ -978,7 +893,7 @@ static int shada_read_file(const char *const file, const int flags)
   }
 
   if (of_ret != 0) {
-    if (of_ret == UV_ENOENT && (flags & kShaDaMissingError)) {
+    if (of_ret != UV_ENOENT || (flags & kShaDaMissingError)) {
       emsgf(_(SERR "System error while opening ShaDa file %s for reading: %s"),
             fname, os_strerror(of_ret));
     }
@@ -1421,7 +1336,7 @@ static void shada_read(ShaDaReadDef *const sd_reader, const int flags)
           }
         }
         if (!op_register_set(cur_entry.data.reg.name, (yankreg_T) {
-          .y_array = (char_u **) cur_entry.data.reg.contents,
+          .y_array = (char_u **)cur_entry.data.reg.contents,
           .y_size = cur_entry.data.reg.contents_size,
           .y_type = cur_entry.data.reg.type,
           .y_width = (colnr_T) cur_entry.data.reg.width,
@@ -2968,17 +2883,23 @@ int shada_write_file(const char *const file, bool nomerge)
 
   char *const fname = shada_filename(file);
   char *tempname = NULL;
-  ShaDaWriteDef sd_writer = (ShaDaWriteDef) {
+  ShaDaWriteDef sd_writer = {
     .write = &write_file,
     .close = &close_sd_writer,
     .error = NULL,
   };
-  ShaDaReadDef sd_reader;
-
-  intptr_t fd;
+  ShaDaReadDef sd_reader = { .close = NULL };
 
   if (!nomerge) {
-    if (open_shada_file_for_reading(fname, &sd_reader) != 0) {
+    int error;
+    if ((error = open_shada_file_for_reading(fname, &sd_reader)) != 0) {
+      if (error != UV_ENOENT) {
+        emsgf(_(SERR "System error while opening ShaDa file %s for reading "
+                "to merge before writing it: %s"),
+              fname, os_strerror(error));
+        // Try writing the file even if opening it emerged any issues besides
+        // file not existing: maybe writing will succeed nevertheless.
+      }
       nomerge = true;
       goto shada_write_file_nomerge;
     }
@@ -2996,15 +2917,11 @@ int shada_write_file(const char *const file, bool nomerge)
     // 2: Make sure that user can always read and write the result.
     // 3: If somebody happened to delete the file after it was opened for
     //    reading use u=rw permissions.
-shada_write_file_open:
-    fd = (intptr_t) open_file(tempname, O_CREAT|O_WRONLY|O_NOFOLLOW|O_EXCL,
-                              perm);
-    if (fd < 0) {
-      if (fd == UV_EEXIST
-#ifdef ELOOP
-          || fd == UV_ELOOP
-#endif
-          ) {
+shada_write_file_open: {}
+    sd_writer.cookie = file_open_new(
+        &error, tempname, kFileCreateOnly|kFileNoSymlink, perm);
+    if (sd_writer.cookie == NULL) {
+      if (error == UV_EEXIST || error == UV_ELOOP) {
         // File already exists, try another name
         char *const wp = tempname + strlen(tempname) - 1;
         if (*wp == 'z') {
@@ -3014,11 +2931,16 @@ shada_write_file_open:
                 fname);
           xfree(fname);
           xfree(tempname);
+          assert(sd_reader.close != NULL);
+          sd_reader.close(&sd_reader);
           return FAIL;
         } else {
           (*wp)++;
           goto shada_write_file_open;
         }
+      } else {
+        emsgf(_(SERR "System error while opening temporary ShaDa file %s "
+                "for writing: %s"), tempname, os_strerror(error));
       }
     }
   }
@@ -3042,23 +2964,29 @@ shada_write_file_nomerge: {}
       }
       *tail = tail_save;
     }
-    fd = (intptr_t) open_file(fname, O_CREAT|O_WRONLY|O_TRUNC,
-                              0600);
-  }
-
-  if (p_verbose > 0) {
-    verbose_enter();
-    smsg(_("Writing ShaDa file \"%s\""), fname);
-    verbose_leave();
+    int error;
+    sd_writer.cookie = file_open_new(&error, fname, kFileCreate|kFileTruncate,
+                                     0600);
+    if (sd_writer.cookie == NULL) {
+      emsgf(_(SERR "System error while opening ShaDa file %s for writing: %s"),
+            fname, os_strerror(error));
+    }
   }
 
-  if (fd < 0) {
+  if (sd_writer.cookie == NULL) {
     xfree(fname);
     xfree(tempname);
+    if (sd_reader.close != NULL) {
+      sd_reader.close(&sd_reader);
+    }
     return FAIL;
   }
 
-  sd_writer.cookie = (void *) fd;
+  if (p_verbose > 0) {
+    verbose_enter();
+    smsg(_("Writing ShaDa file \"%s\""), fname);
+    verbose_leave();
+  }
 
   convert_setup(&sd_writer.sd_conv, p_enc, "utf-8");
 
@@ -3066,15 +2994,11 @@ shada_write_file_nomerge: {}
                                                            ? NULL
                                                            : &sd_reader));
   assert(sw_ret != kSDWriteIgnError);
-#ifndef UNIX
-  sd_writer.close(&sd_writer);
-#endif
   if (!nomerge) {
     sd_reader.close(&sd_reader);
     bool did_remove = false;
     if (sw_ret == kSDWriteSuccessfull) {
 #ifdef UNIX
-      bool closed = false;
       // For Unix we check the owner of the file.  It's not very nice to
       // overwrite a user’s viminfo file after a "su root", with a
       // viminfo file that the user can't read.
@@ -3083,16 +3007,15 @@ shada_write_file_nomerge: {}
         if (getuid() == ROOT_UID) {
           if (old_info.stat.st_uid != ROOT_UID
               || old_info.stat.st_gid != getgid()) {
-            const uv_uid_t old_uid = (uv_uid_t) old_info.stat.st_uid;
-            const uv_gid_t old_gid = (uv_gid_t) old_info.stat.st_gid;
-            const int fchown_ret = os_fchown((int) fd, old_uid, old_gid);
-            sd_writer.close(&sd_writer);
+            const uv_uid_t old_uid = (uv_uid_t)old_info.stat.st_uid;
+            const uv_gid_t old_gid = (uv_gid_t)old_info.stat.st_gid;
+            const int fchown_ret = os_fchown(file_fd(sd_writer.cookie),
+                                             old_uid, old_gid);
             if (fchown_ret != 0) {
               EMSG3(_(RNERR "Failed setting uid and gid for file %s: %s"),
                     tempname, os_strerror(fchown_ret));
               goto shada_write_file_did_not_remove;
             }
-            closed = true;
           }
         } else if (!(old_info.stat.st_uid == getuid()
                      ? (old_info.stat.st_mode & 0200)
@@ -3100,13 +3023,9 @@ shada_write_file_nomerge: {}
                         ? (old_info.stat.st_mode & 0020)
                         : (old_info.stat.st_mode & 0002)))) {
           EMSG2(_("E137: ShaDa file is not writable: %s"), fname);
-          sd_writer.close(&sd_writer);
           goto shada_write_file_did_not_remove;
         }
       }
-      if (!closed) {
-        sd_writer.close(&sd_writer);
-      }
 #endif
       if (vim_rename(tempname, fname) == -1) {
         EMSG3(_(RNERR "Can't rename ShaDa file from %s to %s!"),
@@ -3133,6 +3052,7 @@ shada_write_file_did_not_remove:
     }
     xfree(tempname);
   }
+  sd_writer.close(&sd_writer);
 
   xfree(fname);
   return OK;
@@ -3262,20 +3182,20 @@ static ShaDaReadResult fread_len(ShaDaReadDef *const sd_reader,
   FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
 {
   const ptrdiff_t read_bytes = sd_reader->read(sd_reader, buffer, length);
-  (void) read_bytes;
 
-  if (sd_reader->error != NULL) {
-    emsgf(_(SERR "System error while reading ShaDa file: %s"),
-          sd_reader->error);
-    return kSDReadStatusReadError;
-  } else if (sd_reader->eof) {
-    emsgf(_(RCERR "Error while reading ShaDa file: "
-            "last entry specified that it occupies %" PRIu64 " bytes, "
-            "but file ended earlier"),
-          (uint64_t) length);
-    return kSDReadStatusNotShaDa;
+  if (read_bytes != (ptrdiff_t)length) {
+    if (sd_reader->error != NULL) {
+      emsgf(_(SERR "System error while reading ShaDa file: %s"),
+            sd_reader->error);
+      return kSDReadStatusReadError;
+    } else {
+      emsgf(_(RCERR "Error while reading ShaDa file: "
+              "last entry specified that it occupies %" PRIu64 " bytes, "
+              "but file ended earlier"),
+            (uint64_t)length);
+      return kSDReadStatusNotShaDa;
+    }
   }
-  assert(read_bytes >= 0 && (size_t) read_bytes == length);
   return kSDReadStatusSuccess;
 }
 
diff --git a/test/unit/helpers.lua b/test/unit/helpers.lua
index f0c193884e..91da459393 100644
--- a/test/unit/helpers.lua
+++ b/test/unit/helpers.lua
@@ -7,6 +7,7 @@ local global_helpers = require('test.helpers')
 
 local neq = global_helpers.neq
 local eq = global_helpers.eq
+local ok = global_helpers.ok
 
 -- add some standard header locations
 for _, p in ipairs(Paths.include_paths) do
@@ -34,7 +35,8 @@ local function filter_complex_blocks(body)
     if not (string.find(line, "(^)", 1, true) ~= nil
             or string.find(line, "_ISwupper", 1, true)
             or string.find(line, "msgpack_zone_push_finalizer")
-            or string.find(line, "msgpack_unpacker_reserve_buffer")) then
+            or string.find(line, "msgpack_unpacker_reserve_buffer")
+            or string.find(line, "inline _Bool")) then
       result[#result + 1] = line
     end
   end
@@ -156,6 +158,7 @@ return {
   cimport = cimport,
   cppimport = cppimport,
   internalize = internalize,
+  ok = ok,
   eq = eq,
   neq = neq,
   ffi = ffi,
diff --git a/test/unit/os/fileio_spec.lua b/test/unit/os/fileio_spec.lua
new file mode 100644
index 0000000000..5358022422
--- /dev/null
+++ b/test/unit/os/fileio_spec.lua
@@ -0,0 +1,365 @@
+local lfs = require('lfs')
+
+local helpers = require('test.unit.helpers')
+
+local eq = helpers.eq
+local ffi = helpers.ffi
+local cimport = helpers.cimport
+
+local m = cimport('./src/nvim/os/fileio.h')
+
+local fcontents = ''
+for i = 0, 255 do
+  fcontents = fcontents .. (i == 0 and '\0' or ('%c'):format(i))
+end
+fcontents = fcontents:rep(16)
+
+local dir = 'Xtest-unit-file_spec.d'
+local file1 = dir .. '/file1.dat'
+local file2 = dir .. '/file2.dat'
+local linkf = dir .. '/file.lnk'
+local linkb = dir .. '/broken.lnk'
+local filec = dir .. '/created-file.dat'
+
+before_each(function()
+  lfs.mkdir(dir);
+
+  local f1 = io.open(file1, 'w')
+  f1:write(fcontents)
+  f1:close()
+
+  local f2 = io.open(file2, 'w')
+  f2:write(fcontents)
+  f2:close()
+
+  lfs.link('file1.dat', linkf, true)
+  lfs.link('broken.dat', linkb, true)
+end)
+
+after_each(function()
+  os.remove(file1)
+  os.remove(file2)
+  os.remove(linkf)
+  os.remove(linkb)
+  os.remove(filec)
+  lfs.rmdir(dir)
+end)
+
+local function file_open(fname, flags, mode)
+  local ret2 = ffi.new('FileDescriptor')
+  local ret1 = m.file_open(ret2, fname, flags, mode)
+  return ret1, ret2
+end
+
+local function file_open_new(fname, flags, mode)
+  local ret1 = ffi.new('int[?]', 1, {0})
+  local ret2 = ffi.gc(m.file_open_new(ret1, fname, flags, mode), nil)
+  return ret1[0], ret2
+end
+
+local function file_write(fp, buf)
+  return m.file_write(fp, buf, #buf)
+end
+
+local function file_read(fp, size)
+  local buf = nil
+  if size == nil then
+    size = 0
+  else
+    -- For some reason if length of NUL-bytes-string is the same as `char[?]`
+    -- size luajit garbage collector crashes. But it does not do so in
+    -- os_read[v] tests in os/fs_spec.lua.
+    buf = ffi.new('char[?]', size + 1, ('\0'):rep(size))
+  end
+  local ret1 = m.file_read(fp, buf, size)
+  local ret2 = ''
+  if buf ~= nil then
+    ret2 = ffi.string(buf, size)
+  end
+  return ret1, ret2
+end
+
+local function file_fsync(fp)
+  return m.file_fsync(fp)
+end
+
+local function file_skip(fp, size)
+  return m.file_skip(fp, size)
+end
+
+describe('file_open', function()
+  it('can create a rwx------ file with kFileCreate', function()
+    local err, fp = file_open(filec, m.kFileCreate, 448)
+    eq(0, err)
+    local attrs = lfs.attributes(filec)
+    eq('rwx------', attrs.permissions)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can create a rw------- file with kFileCreate', function()
+    local err, fp = file_open(filec, m.kFileCreate, 384)
+    eq(0, err)
+    local attrs = lfs.attributes(filec)
+    eq('rw-------', attrs.permissions)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can create a rwx------ file with kFileCreateOnly', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 448)
+    eq(0, err)
+    local attrs = lfs.attributes(filec)
+    eq('rwx------', attrs.permissions)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can create a rw------- file with kFileCreateOnly', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 384)
+    eq(0, err)
+    local attrs = lfs.attributes(filec)
+    eq('rw-------', attrs.permissions)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('fails to open an existing file with kFileCreateOnly', function()
+    local err, _ = file_open(file1, m.kFileCreateOnly, 384)
+    eq(m.UV_EEXIST, err)
+  end)
+
+  it('fails to open an symlink with kFileNoSymlink', function()
+    local err, _ = file_open(linkf, m.kFileNoSymlink, 384)
+    -- err is UV_EMLINK in FreeBSD, but if I use `ok(err == m.UV_ELOOP or err ==
+    -- m.UV_EMLINK)`, then I loose the ability to see actual `err` value.
+    if err ~= m.UV_ELOOP then eq(m.UV_EMLINK, err) end
+  end)
+
+  it('can open an existing file write-only with kFileCreate', function()
+    local err, fp = file_open(file1, m.kFileCreate, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can open an existing file read-only with zero', function()
+    local err, fp = file_open(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can open an existing file read-only with kFileReadOnly', function()
+    local err, fp = file_open(file1, m.kFileReadOnly, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can open an existing file read-only with kFileNoSymlink', function()
+    local err, fp = file_open(file1, m.kFileNoSymlink, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can truncate an existing file with kFileTruncate', function()
+    local err, fp = file_open(file1, m.kFileTruncate, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    eq(0, m.file_close(fp))
+    local attrs = lfs.attributes(file1)
+    eq(0, attrs.size)
+  end)
+
+  it('can open an existing file write-only with kFileWriteOnly', function()
+    local err, fp = file_open(file1, m.kFileWriteOnly, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    eq(0, m.file_close(fp))
+    local attrs = lfs.attributes(file1)
+    eq(4096, attrs.size)
+  end)
+
+  it('fails to create a file with just kFileWriteOnly', function()
+    local err, _ = file_open(filec, m.kFileWriteOnly, 384)
+    eq(m.UV_ENOENT, err)
+    local attrs = lfs.attributes(filec)
+    eq(nil, attrs)
+  end)
+
+  it('can truncate an existing file with kFileTruncate when opening a symlink',
+  function()
+    local err, fp = file_open(linkf, m.kFileTruncate, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    eq(0, m.file_close(fp))
+    local attrs = lfs.attributes(file1)
+    eq(0, attrs.size)
+  end)
+
+  it('fails to open a directory write-only', function()
+    local err, _ = file_open(dir, m.kFileWriteOnly, 384)
+    eq(m.UV_EISDIR, err)
+  end)
+
+  it('fails to open a broken symbolic link write-only', function()
+    local err, _ = file_open(linkb, m.kFileWriteOnly, 384)
+    eq(m.UV_ENOENT, err)
+  end)
+
+  it('fails to open a broken symbolic link read-only', function()
+    local err, _ = file_open(linkb, m.kFileReadOnly, 384)
+    eq(m.UV_ENOENT, err)
+  end)
+end)
+
+describe('file_open_new', function()
+  it('can open a file read-only', function()
+    local err, fp = file_open_new(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq(0, m.file_free(fp))
+  end)
+
+  it('fails to open an existing file with kFileCreateOnly', function()
+    local err, fp = file_open_new(file1, m.kFileCreateOnly, 384)
+    eq(m.UV_EEXIST, err)
+    eq(nil, fp)
+  end)
+end)
+
+-- file_close is called above, so it is not tested directly
+
+describe('file_fsync', function()
+  it('can flush writes to disk', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 384)
+    eq(0, file_fsync(fp))
+    eq(0, err)
+    eq(0, lfs.attributes(filec).size)
+    local wsize = file_write(fp, 'test')
+    eq(4, wsize)
+    eq(0, lfs.attributes(filec).size)
+    eq(0, file_fsync(fp))
+    eq(wsize, lfs.attributes(filec).size)
+    eq(0, m.file_close(fp))
+  end)
+end)
+
+describe('file_read', function()
+  it('can read small chunks of input until eof', function()
+    local err, fp = file_open(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    local shift = 0
+    while shift < #fcontents do
+      local size = 3
+      local exp_err = size
+      local exp_s = fcontents:sub(shift + 1, shift + size)
+      if shift + size >= #fcontents then
+        exp_err = #fcontents - shift
+        exp_s = (fcontents:sub(shift + 1, shift + size)
+                 .. (('\0'):rep(size - exp_err)))
+      end
+      eq({exp_err, exp_s}, {file_read(fp, size)})
+      shift = shift + size
+    end
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can read the whole file at once', function()
+    local err, fp = file_open(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq({#fcontents, fcontents}, {file_read(fp, #fcontents)})
+    eq({0, ('\0'):rep(#fcontents)}, {file_read(fp, #fcontents)})
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can read more then 1024 bytes after reading a small chunk', function()
+    local err, fp = file_open(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq({5, fcontents:sub(1, 5)}, {file_read(fp, 5)})
+    eq({#fcontents - 5, fcontents:sub(6) .. (('\0'):rep(5))},
+       {file_read(fp, #fcontents)})
+    eq(0, m.file_close(fp))
+  end)
+
+  it('can read file by 768-byte-chunks', function()
+    local err, fp = file_open(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    local shift = 0
+    while shift < #fcontents do
+      local size = 768
+      local exp_err = size
+      local exp_s = fcontents:sub(shift + 1, shift + size)
+      if shift + size >= #fcontents then
+        exp_err = #fcontents - shift
+        exp_s = (fcontents:sub(shift + 1, shift + size)
+                 .. (('\0'):rep(size - exp_err)))
+      end
+      eq({exp_err, exp_s}, {file_read(fp, size)})
+      shift = shift + size
+    end
+    eq(0, m.file_close(fp))
+  end)
+end)
+
+describe('file_write', function()
+  it('can write the whole file at once', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    local wr = file_write(fp, fcontents)
+    eq(#fcontents, wr)
+    eq(0, m.file_close(fp))
+    eq(wr, lfs.attributes(filec).size)
+    eq(fcontents, io.open(filec):read('*a'))
+  end)
+
+  it('can write the whole file by small chunks', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    local shift = 0
+    while shift < #fcontents do
+      local size = 3
+      local s = fcontents:sub(shift + 1, shift + size)
+      local wr = file_write(fp, s)
+      eq(wr, #s)
+      shift = shift + size
+    end
+    eq(0, m.file_close(fp))
+    eq(#fcontents, lfs.attributes(filec).size)
+    eq(fcontents, io.open(filec):read('*a'))
+  end)
+
+  it('can write the whole file by 768-byte-chunks', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 384)
+    eq(0, err)
+    eq(true, fp.wr)
+    local shift = 0
+    while shift < #fcontents do
+      local size = 768
+      local s = fcontents:sub(shift + 1, shift + size)
+      local wr = file_write(fp, s)
+      eq(wr, #s)
+      shift = shift + size
+    end
+    eq(0, m.file_close(fp))
+    eq(#fcontents, lfs.attributes(filec).size)
+    eq(fcontents, io.open(filec):read('*a'))
+  end)
+end)
+
+describe('file_skip', function()
+  it('can skip 3 bytes', function()
+    local err, fp = file_open(file1, 0, 384)
+    eq(0, err)
+    eq(false, fp.wr)
+    eq(3, file_skip(fp, 3))
+    local rd, s = file_read(fp, 3)
+    eq(3, rd)
+    eq(fcontents:sub(4, 6), s)
+    eq(0, m.file_close(fp))
+  end)
+end)
diff --git a/test/unit/os/fs_spec.lua b/test/unit/os/fs_spec.lua
index 857a5001f1..cc10b0cfa4 100644
--- a/test/unit/os/fs_spec.lua
+++ b/test/unit/os/fs_spec.lua
@@ -6,6 +6,7 @@ local helpers = require('test.unit.helpers')
 local cimport = helpers.cimport
 local cppimport = helpers.cppimport
 local internalize = helpers.internalize
+local ok = helpers.ok
 local eq = helpers.eq
 local neq = helpers.neq
 local ffi = helpers.ffi
@@ -27,6 +28,12 @@ cppimport('sys/stat.h')
 cppimport('fcntl.h')
 cppimport('uv-errno.h')
 
+local s = ''
+for i = 0, 255 do
+  s = s .. (i == 0 and '\0' or ('%c'):format(i))
+end
+local fcontents = s:rep(16)
+
 local buffer = ""
 local directory = nil
 local absolute_executable = nil
@@ -150,11 +157,11 @@ describe('fs function', function()
     local function os_can_exe(name)
       local buf = ffi.new('char *[1]')
       buf[0] = NULL
-      local ok = fs.os_can_exe(to_cstr(name), buf, true)
+      local ce_ret = fs.os_can_exe(to_cstr(name), buf, true)
 
       -- When os_can_exe returns true, it must set the path.
       -- When it returns false, the path must be NULL.
-      if ok then
+      if ce_ret then
         neq(NULL, buf[0])
         return internalize(buf[0])
       else
@@ -368,6 +375,51 @@ describe('fs function', function()
     local function os_open(path, flags, mode)
       return fs.os_open((to_cstr(path)), flags, mode)
     end
+    local function os_close(fd)
+      return fs.os_close(fd)
+    end
+    -- For some reason if length of NUL-bytes-string is the same as `char[?]`
+    -- size luajit crashes. Though it does not do so in this test suite, better
+    -- be cautios and allocate more elements then needed. I only did this to
+    -- strings.
+    local function os_read(fd, size)
+      local buf = nil
+      if size == nil then
+        size = 0
+      else
+        buf = ffi.new('char[?]', size + 1, ('\0'):rep(size))
+      end
+      local eof = ffi.new('bool[?]', 1, {true})
+      local ret2 = fs.os_read(fd, eof, buf, size)
+      local ret1 = eof[0]
+      local ret3 = ''
+      if buf ~= nil then
+        ret3 = ffi.string(buf, size)
+      end
+      return ret1, ret2, ret3
+    end
+    local function os_readv(fd, sizes)
+      local bufs = {}
+      for i, size in ipairs(sizes) do
+        bufs[i] = {
+          iov_base=ffi.new('char[?]', size + 1, ('\0'):rep(size)),
+          iov_len=size,
+        }
+      end
+      local iov = ffi.new('struct iovec[?]', #sizes, bufs)
+      local eof = ffi.new('bool[?]', 1, {true})
+      local ret2 = fs.os_readv(fd, eof, iov, #sizes)
+      local ret1 = eof[0]
+      local ret3 = {}
+      for i = 1,#sizes do
+        -- Warning: iov may not be used.
+        ret3[i] = ffi.string(bufs[i].iov_base, bufs[i].iov_len)
+      end
+      return ret1, ret2, ret3
+    end
+    local function os_write(fd, data)
+      return fs.os_write(fd, data, data and #data or 0)
+    end
 
     describe('os_file_exists', function()
       it('returns false when given a non-existing file', function()
@@ -432,30 +484,34 @@ describe('fs function', function()
     end)
 
     describe('os_open', function()
+      local new_file = 'test_new_file'
+      local existing_file = 'unit-test-directory/test_existing.file'
+
       before_each(function()
-        io.open('unit-test-directory/test_existing.file', 'w').close()
+        (io.open(existing_file, 'w')):close()
       end)
 
       after_each(function()
-        os.remove('unit-test-directory/test_existing.file')
-        os.remove('test_new_file')
+        os.remove(existing_file)
+        os.remove(new_file)
       end)
 
-      local new_file = 'test_new_file'
-      local existing_file = 'unit-test-directory/test_existing.file'
-
       it('returns UV_ENOENT for O_RDWR on a non-existing file', function()
         eq(ffi.C.UV_ENOENT, (os_open('non-existing-file', ffi.C.kO_RDWR, 0)))
       end)
 
-      it('returns non-negative for O_CREAT on a non-existing file', function()
+      it('returns non-negative for O_CREAT on a non-existing file which then can be closed', function()
         assert_file_does_not_exist(new_file)
-        assert.is_true(0 <= (os_open(new_file, ffi.C.kO_CREAT, 0)))
+        local fd = os_open(new_file, ffi.C.kO_CREAT, 0)
+        assert.is_true(0 <= fd)
+        eq(0, os_close(fd))
       end)
 
-      it('returns non-negative for O_CREAT on a existing file', function()
+      it('returns non-negative for O_CREAT on a existing file which then can be closed', function()
         assert_file_exists(existing_file)
-        assert.is_true(0 <= (os_open(existing_file, ffi.C.kO_CREAT, 0)))
+        local fd = os_open(existing_file, ffi.C.kO_CREAT, 0)
+        assert.is_true(0 <= fd)
+        eq(0, os_close(fd))
       end)
 
       it('returns UV_EEXIST for O_CREAT|O_EXCL on a existing file', function()
@@ -463,24 +519,181 @@ describe('fs function', function()
         eq(ffi.C.kUV_EEXIST, (os_open(existing_file, (bit.bor(ffi.C.kO_CREAT, ffi.C.kO_EXCL)), 0)))
       end)
 
-      it('sets `rwx` permissions for O_CREAT 700', function()
+      it('sets `rwx` permissions for O_CREAT 700 which then can be closed', function()
         assert_file_does_not_exist(new_file)
         --create the file
-        os_open(new_file, ffi.C.kO_CREAT, tonumber("700", 8))
+        local fd = os_open(new_file, ffi.C.kO_CREAT, tonumber("700", 8))
         --verify permissions
         eq('rwx------', lfs.attributes(new_file)['permissions'])
+        eq(0, os_close(fd))
       end)
 
-      it('sets `rw` permissions for O_CREAT 600', function()
+      it('sets `rw` permissions for O_CREAT 600 which then can be closed', function()
         assert_file_does_not_exist(new_file)
         --create the file
-        os_open(new_file, ffi.C.kO_CREAT, tonumber("600", 8))
+        local fd = os_open(new_file, ffi.C.kO_CREAT, tonumber("600", 8))
         --verify permissions
         eq('rw-------', lfs.attributes(new_file)['permissions'])
+        eq(0, os_close(fd))
+      end)
+
+      it('returns a non-negative file descriptor for an existing file which then can be closed', function()
+        local fd = os_open(existing_file, ffi.C.kO_RDWR, 0)
+        assert.is_true(0 <= fd)
+        eq(0, os_close(fd))
+      end)
+    end)
+
+    describe('os_close', function()
+      it('returns EBADF for negative file descriptors', function()
+        eq(ffi.C.UV_EBADF, os_close(-1))
+        eq(ffi.C.UV_EBADF, os_close(-1000))
+      end)
+    end)
+
+    describe('os_read', function()
+      local file = 'test-unit-os-fs_spec-os_read.dat'
+
+      before_each(function()
+        local f = io.open(file, 'w')
+        f:write(fcontents)
+        f:close()
+      end)
+
+      after_each(function()
+        os.remove(file)
+      end)
+
+      it('can read zero bytes from a file', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, 0, ''}, {os_read(fd, nil)})
+        eq({false, 0, ''}, {os_read(fd, 0)})
+        eq(0, os_close(fd))
+      end)
+
+      it('can read from a file multiple times', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, 2, '\000\001'}, {os_read(fd, 2)})
+        eq({false, 2, '\002\003'}, {os_read(fd, 2)})
+        eq(0, os_close(fd))
+      end)
+
+      it('can read the whole file at once and then report eof', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, #fcontents, fcontents}, {os_read(fd, #fcontents)})
+        eq({true, 0, ('\0'):rep(#fcontents)}, {os_read(fd, #fcontents)})
+        eq(0, os_close(fd))
+      end)
+
+      it('can read the whole file in two calls, one partially', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, #fcontents * 3/4, fcontents:sub(1, #fcontents * 3/4)},
+           {os_read(fd, #fcontents * 3/4)})
+        eq({true,
+            (#fcontents * 1/4),
+            fcontents:sub(#fcontents * 3/4 + 1) .. ('\0'):rep(#fcontents * 2/4)},
+           {os_read(fd, #fcontents * 3/4)})
+        eq(0, os_close(fd))
+      end)
+    end)
+
+    describe('os_readv', function()
+      -- Function may be absent
+      if not pcall(function() return fs.os_readv end) then
+        return
+      end
+      local file = 'test-unit-os-fs_spec-os_readv.dat'
+
+      before_each(function()
+        local f = io.open(file, 'w')
+        f:write(fcontents)
+        f:close()
+      end)
+
+      after_each(function()
+        os.remove(file)
+      end)
+
+      it('can read zero bytes from a file', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, 0, {}}, {os_readv(fd, {})})
+        eq({false, 0, {'', '', ''}}, {os_readv(fd, {0, 0, 0})})
+        eq(0, os_close(fd))
+      end)
+
+      it('can read from a file multiple times to a differently-sized buffers', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, 2, {'\000\001'}}, {os_readv(fd, {2})})
+        eq({false, 5, {'\002\003', '\004\005\006'}}, {os_readv(fd, {2, 3})})
+        eq(0, os_close(fd))
+      end)
+
+      it('can read the whole file at once and then report eof', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false,
+            #fcontents,
+            {fcontents:sub(1, #fcontents * 1/4),
+             fcontents:sub(#fcontents * 1/4 + 1, #fcontents * 3/4),
+             fcontents:sub(#fcontents * 3/4 + 1, #fcontents * 15/16),
+             fcontents:sub(#fcontents * 15/16 + 1, #fcontents)}},
+           {os_readv(fd, {#fcontents * 1/4,
+                          #fcontents * 2/4,
+                          #fcontents * 3/16,
+                          #fcontents * 1/16})})
+        eq({true, 0, {'\0'}}, {os_readv(fd, {1})})
+        eq(0, os_close(fd))
+      end)
+
+      it('can read the whole file in two calls, one partially', function()
+        local fd = os_open(file, ffi.C.kO_RDONLY, 0)
+        ok(fd >= 0)
+        eq({false, #fcontents * 3/4, {fcontents:sub(1, #fcontents * 3/4)}},
+           {os_readv(fd, {#fcontents * 3/4})})
+        eq({true,
+            (#fcontents * 1/4),
+            {fcontents:sub(#fcontents * 3/4 + 1) .. ('\0'):rep(#fcontents * 2/4)}},
+           {os_readv(fd, {#fcontents * 3/4})})
+        eq(0, os_close(fd))
+      end)
+    end)
+
+    describe('os_write', function()
+      -- Function may be absent
+      local file = 'test-unit-os-fs_spec-os_write.dat'
+
+      before_each(function()
+        local f = io.open(file, 'w')
+        f:write(fcontents)
+        f:close()
+      end)
+
+      after_each(function()
+        os.remove(file)
+      end)
+
+      it('can write zero bytes to a file', function()
+        local fd = os_open(file, ffi.C.kO_WRONLY, 0)
+        ok(fd >= 0)
+        eq(0, os_write(fd, ''))
+        eq(0, os_write(fd, nil))
+        eq(fcontents, io.open(file, 'r'):read('*a'))
+        eq(0, os_close(fd))
       end)
 
-      it('returns a non-negative file descriptor for an existing file', function()
-        assert.is_true(0 <= (os_open(existing_file, ffi.C.kO_RDWR, 0)))
+      it('can write some data to a file', function()
+        local fd = os_open(file, ffi.C.kO_WRONLY, 0)
+        ok(fd >= 0)
+        eq(3, os_write(fd, 'abc'))
+        eq(4, os_write(fd, ' def'))
+        eq('abc def' .. fcontents:sub(8), io.open(file, 'r'):read('*a'))
+        eq(0, os_close(fd))
       end)
     end)