os/fileio: Add msgpack_file_write function

src/nvim/os/fileio.c

@@ -26,6 +26,7 @@
 #include "nvim/globals.h"
 #include "nvim/rbuffer.h"
 #include "nvim/macros.h"
+#include "nvim/message.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "os/fileio.c.generated.h"
@@ -345,3 +346,23 @@ ptrdiff_t file_skip(FileDescriptor *const fp, const size_t size)
 
   return (ptrdiff_t)read_bytes;
 }
+
+/// Msgpack callback for writing to a file
+///
+/// @param  data  File to write to.
+/// @param[in]  buf  Data to write.
+/// @param[in]  len  Length of the data to write.
+///
+/// @return 0 in case of success, -1 in case of error.
+int msgpack_file_write(void *data, const char *buf, size_t len)
+  FUNC_ATTR_NONNULL_ALL FUNC_ATTR_WARN_UNUSED_RESULT
+{
+  assert(len < PTRDIFF_MAX);
+  const ptrdiff_t written_bytes = file_write((FileDescriptor *)data, buf, len);
+  if (written_bytes < 0) {
+    emsgf(_("E5420: Failed to write to file: %s"),
+          os_strerror((int)written_bytes));
+    return -1;
+  }
+  return 0;
+}

test/unit/os/fileio_spec.lua

@@ -62,6 +62,10 @@ local function file_write(fp, buf)
   return m.file_write(fp, buf, #buf)
 end
 
+local function msgpack_file_write(fp, buf)
+  return m.msgpack_file_write(fp, buf, #buf)
+end
+
 local function file_read(fp, size)
   local buf = nil
   if size == nil then
@@ -393,6 +397,18 @@ describe('file_write', function()
   end)
 end)
 
+describe('msgpack_file_write', function()
+  itp('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 = msgpack_file_write(fp, fcontents)
+    eq(0, wr)
+    eq(0, m.file_close(fp, false))
+    eq(fcontents, io.open(filec):read('*a'))
+  end)
+end)
+
 describe('file_skip', function()
   itp('can skip 3 bytes', function()
     local err, fp = file_open(file1, 0, 384)