Handle null preconditions correctly and simplify gcs lib again (#8400)

server/backends/blobstore/gcs/gcs.go

@@ -200,35 +200,37 @@ func (g *GCSBlobStore) BlobExists(ctx context.Context, blobName string) (bool, e
 	}
 }
 
+// isEmpty returns a boolean indicating if the set of preconditions is empty
+// or not. If any have been set, false is returned, otherwise true.
+func isEmpty(conds storage.Conditions) bool {
+	switch {
+	case conds.GenerationMatch != 0:
+		return false
+	case conds.GenerationNotMatch != 0:
+		return false
+	case conds.DoesNotExist:
+		return false
+	case conds.MetagenerationMatch != 0:
+		return false
+	case conds.MetagenerationNotMatch != 0:
+		return false
+	default:
+		return true
+	}
+}
+
 func (g *GCSBlobStore) ConditionalWriter(ctx context.Context, blobName string, conds storage.Conditions) (interfaces.CommittedWriteCloser, error) {
 	ctx, cancel := context.WithCancel(ctx)
-	bw := g.bucketHandle.Object(blobName).If(conds).NewWriter(ctx)
-
-	var zw io.WriteCloser
-	if g.compress {
-		zw = util.NewCompressWriter(bw)
+	var bw *storage.Writer
+	if isEmpty(conds) {
+		bw = g.bucketHandle.Object(blobName).NewWriter(ctx)
 	} else {
-		zw = bw
-	}
-	cwc := ioutil.NewCustomCommitWriteCloser(zw)
-	cwc.CommitFn = func(int64) error {
-		if compresserCloseErr := zw.Close(); compresserCloseErr != nil {
-			cancel() // Don't try to finish the commit op if Close() failed.
-			// Canceling the context closes the Writer, so don't call bw.Close().
-			return compresserCloseErr
-		}
-		return bw.Close()
-	}
-	cwc.CloseFn = func() error {
-		cancel()
-		return nil
+		bw = g.bucketHandle.Object(blobName).If(conds).NewWriter(ctx)
 	}
-	return cwc, nil
-}
 
-func (g *GCSBlobStore) Writer(ctx context.Context, blobName string) (interfaces.CommittedWriteCloser, error) {
-	ctx, cancel := context.WithCancel(ctx)
-	bw := g.bucketHandle.Object(blobName).NewWriter(ctx)
+	// See https://pkg.go.dev/cloud.google.com/go/storage#Writer
+	// Always disable buffering in the client for these writes.
+	bw.ChunkSize = 0
 
 	var zw io.WriteCloser
 	if g.compress {
@@ -252,6 +254,10 @@ func (g *GCSBlobStore) Writer(ctx context.Context, blobName string) (interfaces.
 	return cwc, nil
 }
 
+func (g *GCSBlobStore) Writer(ctx context.Context, blobName string) (interfaces.CommittedWriteCloser, error) {
+	return g.ConditionalWriter(ctx, blobName, storage.Conditions{})
+}
+
 type decompressingCloser struct {
 	io.ReadCloser
 	alwaysClose func() error