operations: add operations.ReadFile to read the contents of a file into memory

fs/operations/operations.go

@@ -628,6 +628,25 @@ func DeleteFiles(ctx context.Context, toBeDeleted fs.ObjectsChan) error {
 	return DeleteFilesWithBackupDir(ctx, toBeDeleted, nil)
 }
 
+// ReadFile reads the object into memory and accounts it
+func ReadFile(ctx context.Context, o fs.Object) (b []byte, err error) {
+	tr := accounting.Stats(ctx).NewTransfer(o, nil)
+	defer func() {
+		tr.Done(ctx, err)
+	}()
+	in0, err := Open(ctx, o)
+	if err != nil {
+		return nil, fmt.Errorf("failed to open %v: %w", o, err)
+	}
+	in := tr.Account(ctx, in0).WithBuffer() // account and buffer the transfer
+	defer fs.CheckClose(in, &err)           // closes in0 also
+	b, err = io.ReadAll(in)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read %v: %w", o, err)
+	}
+	return b, nil
+}
+
 // SameRemoteType returns true if fdst and fsrc are the same type
 func SameRemoteType(fdst, fsrc fs.Info) bool {
 	return fmt.Sprintf("%T", fdst) == fmt.Sprintf("%T", fsrc)

fs/operations/operations_test.go

@@ -492,6 +492,23 @@ func TestMaxDeleteSize(t *testing.T) {
 	assert.Equal(t, int64(1), objects) // 10 or 100 bytes
 }
 
+func TestReadFile(t *testing.T) {
+	ctx := context.Background()
+	r := fstest.NewRun(t)
+	defer r.Finalise()
+
+	contents := "A file to read the contents."
+	file := r.WriteObject(ctx, "ReadFile", contents, t1)
+	r.CheckRemoteItems(t, file)
+
+	o, err := r.Fremote.NewObject(ctx, file.Path)
+	require.NoError(t, err)
+
+	buf, err := operations.ReadFile(ctx, o)
+	require.NoError(t, err)
+	assert.Equal(t, contents, string(buf))
+}
+
 func TestRetry(t *testing.T) {
 	ctx := context.Background()