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

Use buffered reading/writing for ShaDa files

3 files changed, 600 insertions, 19 deletions

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)