eval,fileio: Omit additional fsync() call

Fixes #6420

3 files changed, 43 insertions, 9 deletions

diff --git a/src/nvim/eval.c b/src/nvim/eval.c
index 15b712e7de..a6774a3a0b 100644
--- a/src/nvim/eval.c
+++ b/src/nvim/eval.c
@@ -17289,7 +17289,7 @@ static bool write_list(FileDescriptor *const fp, const list_T *const list,
       }
     }
   }
-  if ((error = file_fsync(fp)) != 0) {
+  if ((error = file_flush(fp)) != 0) {
     goto write_list_error;
   }
   return true;
diff --git a/src/nvim/os/fileio.c b/src/nvim/os/fileio.c
index 775f2bd449..3c928363cc 100644
--- a/src/nvim/os/fileio.c
+++ b/src/nvim/os/fileio.c
@@ -143,21 +143,36 @@ int file_free(FileDescriptor *const fp) FUNC_ATTR_NONNULL_ALL
 /// @param[in,out]  fp  File to work with.
 ///
 /// @return 0 or error code.
-int file_fsync(FileDescriptor *const fp)
+int file_flush(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;
+  const int error = fp->_error;
+  fp->_error = 0;
+  return error;
+}
+
+/// Flush file modifications to disk and run fsync()
+///
+/// @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;
   }
-  const int error = os_fsync(fp->fd);
-  if (error != UV_EINVAL && error != UV_EROFS) {
-    return error;
+  const int flush_error = file_flush(fp);
+  if (flush_error != 0) {
+    return flush_error;
+  }
+  const int fsync_error = os_fsync(fp->fd);
+  if (fsync_error != UV_EINVAL && fsync_error != UV_EROFS) {
+    return fsync_error;
   }
   return 0;
 }
diff --git a/test/unit/os/fileio_spec.lua b/test/unit/os/fileio_spec.lua
index 7a738ce85c..5e1b2523fa 100644
--- a/test/unit/os/fileio_spec.lua
+++ b/test/unit/os/fileio_spec.lua
@@ -80,6 +80,10 @@ local function file_read(fp, size)
   return ret1, ret2
 end
 
+local function file_flush(fp)
+  return m.file_flush(fp)
+end
+
 local function file_fsync(fp)
   return m.file_fsync(fp)
 end
@@ -244,6 +248,21 @@ describe('file_fsync', function()
   end)
 end)
 
+describe('file_flush', function()
+  itp('can flush writes to disk', function()
+    local err, fp = file_open(filec, m.kFileCreateOnly, 384)
+    eq(0, file_flush(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_flush(fp))
+    eq(wsize, lfs.attributes(filec).size)
+    eq(0, m.file_close(fp))
+  end)
+end)
+
 describe('file_read', function()
   itp('can read small chunks of input until eof', function()
     local err, fp = file_open(file1, 0, 384)