Write rclone uploads under a temporary name, re-upload short blobs (closes #266)

The rclone backend wrote each object straight to its key, so killing an
upload to a local or sftp remote left a truncated object there. The next
backup found the key, skipped the upload and recorded a snapshot that
could not be restored.

On a remote where rclone says a file can be seen while it is still being
written, an object is now written under a name ending in `.partial` and
moved onto its key with the remote's server-side move, as rclone's own
copy does; listings skip such names. A backup also uploads a blob again
when the stored object's size differs from the blob's.

Judgement call: the temporary name is used only where rclone sets its
PartialUploads feature; other remotes already show an object only once
complete.

Model: opus-5-5

internal/snapshot/scanner.go

@@ -1535,8 +1535,8 @@ func (s *Scanner) handleBlobReady(
 	return nil
 }
 
-// uploadBlobIfNeeded uploads the blob to storage if it doesn't already
-// exist, returns whether it existed
+// uploadBlobIfNeeded uploads the blob to storage unless an object of the
+// blob's size is already there, and returns whether it was there
 func (s *Scanner) uploadBlobIfNeeded(
 	ctx context.Context,
 	blobPath string,
@@ -1546,11 +1546,13 @@ func (s *Scanner) uploadBlobIfNeeded(
 ) (bool, error) {
 	finishedBlob := blobWithReader.FinishedBlob
 
-	// Check if blob already exists (deduplication after restart)
+	// Check if blob already exists (deduplication after restart). An
+	// object of another size was left by an upload cut off part-way and
+	// is uploaded again.
 	destination := s.storage.Info().Location
 
-	_, err := s.storage.Stat(ctx, blobPath)
-	if err == nil {
+	stored, err := s.storage.Stat(ctx, blobPath)
+	if err == nil && stored.Size == finishedBlob.Compressed {
 		log.Info("Blob already exists in storage, skipping upload",
 			"hash", finishedBlob.Hash,
 			"size", humanize.Bytes(safeUint64(finishedBlob.Compressed)))
@@ -1561,6 +1563,11 @@ func (s *Scanner) uploadBlobIfNeeded(
 		return true, nil
 	}
 
+	if err == nil {
+		log.Warn("Blob in storage has the wrong size, uploading it again",
+			"hash", finishedBlob.Hash, "stored_size", stored.Size)
+	}
+
 	s.ui.Beginf("Uploading blob %s (%s) to %s.",
 		s.ui.Hex(finishedBlob.Hash), s.ui.Size(finishedBlob.Compressed),
 		s.ui.Path(destination))