Conditionally write tiles.

experimental/gcp-log/internal/storage/storage.go

@@ -16,6 +16,7 @@
 package storage
 
 import (
+	"bytes"
 	"context"
 	"errors"
 	"fmt"
@@ -376,14 +377,38 @@ func (c *Client) Sequence(ctx context.Context, leafhash []byte, leaf []byte) (ui
 			return 0, fmt.Errorf("couldn't create leafhash object: %w", err)
 		}
 		if err := wLeaf.Close(); err != nil {
-			return 0, fmt.Errorf("couldn't close writer for object %q", leafPath)
+			return 0, fmt.Errorf("couldn't close writer for object %q, %w", leafPath, err)
 		}
 
 		// All done!
 		return seq, nil
 	}
 }
 
+// assertContent checks that the content at `gcsPath` matches the passed in `data`.
+func (c *Client) assertContent(ctx context.Context, gcsPath string, data []byte) (equal bool, err error) {
+	bkt := c.gcsClient.Bucket(c.bucket)
+
+	obj := bkt.Object(gcsPath)
+	r, err := obj.NewReader(ctx)
+	if err != nil {
+		klog.V(2).Infof("assertContent: failed to create reader for object %q in bucket %q: %v",
+			gcsPath, c.bucket, err)
+		return false, err
+	}
+	defer r.Close()
+
+	gcsData, err := io.ReadAll(r)
+	if err != nil {
+		return false, err
+	}
+
+	if bytes.Equal(gcsData, data) {
+		return true, nil
+	}
+	return false, nil
+}
+
 // StoreTile writes a tile out to GCS.
 // Fully populated tiles are stored at the path corresponding to the level &
 // index parameters, partially populated (i.e. right-hand edge) tiles are
@@ -405,12 +430,34 @@ func (c *Client) StoreTile(ctx context.Context, level, index uint64, tile *api.T
 	tPath := filepath.Join(layout.TilePath("", level, index, tileSize%256))
 	obj := bkt.Object(tPath)
 
-	w := obj.NewWriter(ctx)
+	// Tiles, partial or full, should only be written once.
+	w := obj.If(gcs.Conditions{DoesNotExist: true}).NewWriter(ctx)
 	if c.otherCacheControl != "" {
 		w.ObjectAttrs.CacheControl = c.otherCacheControl
 	}
 	if _, err := w.Write(t); err != nil {
 		return fmt.Errorf("failed to write tile object %q to bucket %q: %w", tPath, c.bucket, err)
 	}
-	return w.Close()
+
+	if err := w.Close(); err != nil {
+		switch ee := err.(type) {
+		case *googleapi.Error:
+			// If we run into a precondition failure error, check that the object
+			// which exists contains the same content that we want to write.
+			if ee.Code == http.StatusPreconditionFailed {
+				if equal, err := c.assertContent(ctx, tPath, t); err != nil {
+					return fmt.Errorf("failed to read content of %q: %w", tPath, err)
+				} else if !equal {
+					return fmt.Errorf("assertion that tile content for %q has not changed failed", tPath)
+				} else if equal {
+					klog.V(2).Infof("StoreTile: Tile already exists for level %d index %x ts: %x", level, index, tileSize)
+					return nil
+				}
+			}
+		default:
+			return err
+		}
+	}
+
+	return nil
 }