Return an error, not a panic, on a malformed snapshot database (closes #231)

Restore cut chunk hashes from the snapshot database to 16 characters
for its error messages, so a shorter hash panicked. Those messages now
use shortHash. Under --verify, a file_chunks row with no chunks row was
dereferenced, and the chunk size from the database was allocated in
one piece, so a negative or huge size panicked. A missing row is now an
error, a negative size is rejected, and each chunk is hashed by
streaming it from the restored file.

A restored file shorter than its chunks now fails verify as a short
read instead of an unexpected EOF.

Model: opus-5-5

internal/vaultik/restore.go

@@ -42,6 +42,7 @@ var (
 	errChunkNotInAnyBlob        = errors.New("chunk not found in any blob")
 	errBlobIDNotInHashIndex     = errors.New("blob id missing from hash index")
 	errShortChunkRead           = errors.New("short read")
+	errChunkRowMissing          = errors.New("chunk has no row in the chunks table")
 	errRestorePathEscapesTarget = errors.New(
 		"refusing to restore path outside the target directory")
 	errTrailingRestoreData = errors.New(
@@ -1267,7 +1268,7 @@ func (s *restoreSession) writeFileChunks(
 		blobChunk, ok := s.chunkToBlobMap[chunkHashStr]
 		if !ok {
 			return bytesWritten, timings, fmt.Errorf(
-				"%w: %s", errChunkNotInAnyBlob, chunkHashStr[:16])
+				"%w: %s", errChunkNotInAnyBlob, shortHash(chunkHashStr))
 		}
 
 		blobHash, ok := s.blobIDToHash[blobChunk.BlobID.String()]
@@ -1284,7 +1285,7 @@ func (s *restoreSession) writeFileChunks(
 		if err != nil {
 			return bytesWritten, timings, fmt.Errorf(
 				"reading chunk %s from cached blob %s: %w",
-				fc.ChunkHash[:16], blobHash[:16], err)
+				shortHash(chunkHashStr), shortHash(blobHash), err)
 		}
 
 		t0 = time.Now()
@@ -1482,33 +1483,44 @@ func (v *Vaultik) verifyFile(
 		chunk, err := repos.Chunks.GetByHash(ctx, fc.ChunkHash.String())
 		if err != nil {
 			return bytesVerified, fmt.Errorf("getting chunk %s: %w",
-				fc.ChunkHash.String()[:16], err)
+				shortHash(fc.ChunkHash.String()), err)
 		}
 
-		// Read chunk data from file
-		chunkData := make([]byte, chunk.Size)
-
-		n, err := io.ReadFull(f, chunkData)
-		if err != nil {
-			return bytesVerified, fmt.Errorf("reading chunk data: %w", err)
+		if chunk == nil {
+			return bytesVerified, fmt.Errorf("%w: %s",
+				errChunkRowMissing, shortHash(fc.ChunkHash.String()))
 		}
 
-		if int64(n) != chunk.Size {
+		// chunk.Size comes from the snapshot database, which is not
+		// trusted: reject a negative size, and hash the chunk by
+		// streaming it rather than allocating that many bytes.
+		if chunk.Size < 0 {
+			return bytesVerified, fmt.Errorf("%w: chunk %d size %d",
+				errNegativeChunkLength, fc.Idx, chunk.Size)
+		}
+
+		hasher := sha256.New()
+
+		n, err := io.CopyN(hasher, f, chunk.Size)
+		if errors.Is(err, io.EOF) {
 			return bytesVerified, fmt.Errorf("%w: expected %d bytes, got %d",
 				errShortChunkRead, chunk.Size, n)
 		}
 
-		// Calculate hash and compare
-		hash := sha256.Sum256(chunkData)
-		actualHash := hex.EncodeToString(hash[:])
+		if err != nil {
+			return bytesVerified, fmt.Errorf("reading chunk data: %w", err)
+		}
+
+		actualHash := hex.EncodeToString(hasher.Sum(nil))
 		expectedHash := fc.ChunkHash.String()
 
 		if actualHash != expectedHash {
 			return bytesVerified, fmt.Errorf("%w: chunk %d: expected %s, got %s",
-				errChunkHashMismatch, fc.Idx, expectedHash[:16], actualHash[:16])
+				errChunkHashMismatch, fc.Idx,
+				shortHash(expectedHash), shortHash(actualHash))
 		}
 
-		bytesVerified += int64(n)
+		bytesVerified += n
 	}
 
 	// The stored chunks account for the whole file, so the reader must