fix(wal): Use varint encoding for ts_2_dod in WAL format

pkg/storage/wal/README.md

@@ -153,7 +153,7 @@ Unlike the current Loki chunk format, this format does not use smaller blocks be
 ├──────────────────────────────────────────────────────────────────────────┤
 │ len_line_1 <uvarint>                                                     │
 ├──────────────────────────────────────────────────────────────────────────┤
-│ ts_2_dod <uvarint>                                                       │
+│ ts_2_dod <varint>                                                        │
 ├──────────────────────────────────────────────────────────────────────────┤
 │ len_line_2 <uvarint>                                                     │
 ├──────────────────────────────────────────────────────────────────────────┤
@@ -175,7 +175,7 @@ Unlike the current Loki chunk format, this format does not use smaller blocks be
 - **len_line_0 <uvarint>**: The length of the first log line.
 - **ts_1_delta <uvarint>**: The delta from the initial timestamp to the second timestamp.
 - **len_line_1 <uvarint>**: The length of the second log line.
-- **ts_2_dod <uvarint>**: The delta of deltas, representing the difference from the previous delta (i.e., double-delta encoding).
+- **ts_2_dod <varint>**: The delta of deltas, representing the difference from the previous delta (i.e., double-delta encoding). Can be negative if the spacing between points is decreasing.
 - **len_line_2 <uvarint>**: The length of the third log line.
 - **compressed logs <bytes>**: The actual log data, compressed according to the specified encoding.
 - **compressed logs offset <4 bytes>**: The offset of the compressed log data.

pkg/storage/wal/chunk.go

@@ -114,8 +114,8 @@ func writeChunk(w io.Writer, entries []*logproto.Entry, encoding EncodingType) (
 			written += int64(n)
 		default:
 			delta = t - prevT
-			dod := delta - prevDelta
-			n = binary.PutUvarint(buf, dod)
+			dod := int64(delta - prevDelta)
+			n = binary.PutVarint(buf, dod)
 			if _, err := w.Write(buf[:n]); err != nil {
 				return written, err
 			}
@@ -256,19 +256,21 @@ func (r *ChunkReader) Next() bool {
 
 	// Read timestamp
 	var timestamp int64
-	if r.entryIdx == 0 {
+	switch r.entryIdx {
+	case 0:
 		ts, n := binary.Uvarint(r.b[r.pos:])
 		r.pos += n
 		timestamp = int64(ts)
-	} else {
+	case 1:
 		delta, n := binary.Uvarint(r.b[r.pos:])
 		r.pos += n
-		if r.entryIdx == 1 {
-			timestamp = r.prevT + int64(delta)
-		} else {
-			timestamp = r.prevT + r.prevDelta + int64(delta)
-		}
+		timestamp = r.prevT + int64(delta)
 		r.prevDelta = int64(delta)
+	default:
+		dod, n := binary.Varint(r.b[r.pos:])
+		r.pos += n
+		r.prevDelta += dod
+		timestamp = r.prevT + r.prevDelta
 	}
 
 	r.entry.Timestamp = time.Unix(0, timestamp)

pkg/storage/wal/chunk_test.go

@@ -30,6 +30,15 @@ func TestChunkReaderWriter(t *testing.T) {
 				{Timestamp: time.Now().Add(2 * time.Second), Line: "Log entry 3"},
 			},
 		},
+		{
+			name: "Different spacing",
+			entries: []*logproto.Entry{
+				{Timestamp: time.Unix(0, 1), Line: "Log entry 1"},
+				{Timestamp: time.Unix(0, 2), Line: "Log entry 2"},
+				{Timestamp: time.Unix(0, 4), Line: "Log entry 3"},
+				{Timestamp: time.Unix(0, 5), Line: "Log entry 4"},
+			},
+		},
 		{
 			// todo: fix dod for variable timestamp delta causing negative dod
 			name: "Many entries",