fs: Adds fd_sync (#1026)

Signed-off-by: Nuno Cruces <ncruces@users.noreply.github.com>

imports/wasi_snapshot_preview1/fs.go

@@ -950,7 +950,24 @@ func fdSeekFn(_ context.Context, mod api.Module, params []uint64) Errno {
 // and metadata of a file to disk.
 //
 // See https://github.com/WebAssembly/WASI/blob/snapshot-01/phases/snapshot/docs.md#-fd_syncfd-fd---errno
-var fdSync = stubFunction(FdSyncName, []api.ValueType{i32}, "fd")
+var fdSync = newHostFunc(FdSyncName, fdSyncFn, []api.ValueType{i32}, "fd")
+
+func fdSyncFn(_ context.Context, mod api.Module, params []uint64) Errno {
+	fsc := mod.(*wasm.CallContext).Sys.FS()
+	fd := uint32(params[0])
+
+	type syncer interface{ Sync() error }
+	// Check to see if the file descriptor is available
+	if f, ok := fsc.LookupFile(fd); !ok {
+		return ErrnoBadf
+		// fs.FS doesn't declare Sync, but implementations such as os.File implement it.
+	} else if syncer, ok := f.File.(syncer); !ok {
+		return ErrnoBadf
+	} else if err := syncer.Sync(); err != nil {
+		return ErrnoIo
+	}
+	return ErrnoSuccess
+}
 
 // fdTell is the WASI function named FdTellName which returns the current
 // offset of a file descriptor.

imports/wasi_snapshot_preview1/fs_test.go

@@ -1666,13 +1666,48 @@ func Test_fdSeek_Errors(t *testing.T) {
 	}
 }
 
-// Test_fdSync only tests it is stubbed for GrainLang per #271
+// Test_fdSync only tests that the call succeeds; it's hard to test its effectiveness.
 func Test_fdSync(t *testing.T) {
-	log := requireErrnoNosys(t, FdSyncName, 0)
-	require.Equal(t, `
---> wasi_snapshot_preview1.fd_sync(fd=0)
-<-- errno=ENOSYS
-`, log)
+	tmpDir := t.TempDir() // open before loop to ensure no locking problems.
+	pathName := "test_path"
+	mod, fd, log, r := requireOpenFile(t, tmpDir, pathName, []byte{}, false)
+	defer r.Close(testCtx)
+
+	tests := []struct {
+		name          string
+		fd            uint32
+		expectedErrno Errno
+		expectedLog   string
+	}{
+		{
+			name:          "invalid fd",
+			fd:            42, // arbitrary invalid fd
+			expectedErrno: ErrnoBadf,
+			expectedLog: `
+==> wasi_snapshot_preview1.fd_sync(fd=42)
+<== errno=EBADF
+`,
+		},
+		{
+			name:          "valid fd",
+			fd:            fd,
+			expectedErrno: ErrnoSuccess,
+			expectedLog: `
+==> wasi_snapshot_preview1.fd_sync(fd=4)
+<== errno=ESUCCESS
+`,
+		},
+	}
+
+	for _, tt := range tests {
+		tc := tt
+		t.Run(tc.name, func(t *testing.T) {
+			defer log.Reset()
+
+			requireErrno(t, tc.expectedErrno, mod, FdSyncName, uint64(tc.fd))
+			require.Equal(t, tc.expectedLog, "\n"+log.String())
+		})
+	}
 }
 
 // Test_fdTell only tests it is stubbed for GrainLang per #271

internal/syscallfs/syscall_windows.go

@@ -97,7 +97,8 @@ func maybeWrapFile(f file) file {
 	return struct {
 		readFile
 		io.Writer
-	}{f, &windowsWriter{f}}
+		syncer
+	}{f, &windowsWriter{f}, f}
 }
 
 // windowsWriter translates error codes not mapped properly by Go.

internal/syscallfs/syscallfs.go

@@ -142,4 +142,7 @@ type readFile interface {
 type file interface {
 	readFile
 	io.Writer
+	syncer
 }
+
+type syncer interface{ Sync() error }