From 5b39ad861e09af728e18dc42017f76c67a21922f Mon Sep 17 00:00:00 2001
From: Michael Eischer <michael.eischer@fau.de>
Date: Sat, 30 May 2026 20:08:18 +0200
Subject: [PATCH] repository: repair index: correctly handle split index
 entries

In restic <0.10.0, it was possible that the blobs of a pack file were
split across multiple indexes. `MasterIndex.Rewrite` however assumed
that each an index always contains the full description of a pack file.
Therefore, further index entries for a pack were filtered out as
duplicates. Now, the code also checks the blobs contained in the index
entry while filtering out duplicates.
---
 internal/repository/index/index.go        | 21 +++++++++++++++
 internal/repository/index/master_index.go | 33 +++++++++++++++++------
 2 files changed, 46 insertions(+), 8 deletions(-)

diff --git a/internal/repository/index/index.go b/internal/repository/index/index.go
index e96530ec1..abd8eda54 100644
--- a/internal/repository/index/index.go
+++ b/internal/repository/index/index.go
@@ -3,10 +3,13 @@ package index
 import (
 	"bytes"
 	"context"
+	"crypto/sha256"
+	"encoding/binary"
 	"encoding/json"
 	"io"
 	"iter"
 	"math"
+	"slices"
 	"sync"
 	"time"
 
@@ -546,3 +549,21 @@ func (idx *Index) Len(t restic.BlobType) uint {
 
 	return idx.byType[t].len()
 }
+
+func PackBlobsHash(pbs restic.PackBlobs) restic.ID {
+	h := sha256.New()
+	h.Write(pbs.PackID[:])
+
+	sortedBlobs := slices.Clone(pbs.Blobs)
+	sortedBlobs.Sort()
+	for _, blob := range sortedBlobs {
+		h.Write(blob.ID[:])
+		buf := make([]byte, 0, 16)
+		buf = binary.LittleEndian.AppendUint32(buf, uint32(blob.Type))
+		buf = binary.LittleEndian.AppendUint32(buf, uint32(blob.Offset))
+		buf = binary.LittleEndian.AppendUint32(buf, uint32(blob.Length))
+		buf = binary.LittleEndian.AppendUint32(buf, uint32(blob.UncompressedLength))
+		h.Write(buf)
+	}
+	return restic.ID(h.Sum(nil))
+}
diff --git a/internal/repository/index/master_index.go b/internal/repository/index/master_index.go
index 70afff0fb..84db0d962 100644
--- a/internal/repository/index/master_index.go
+++ b/internal/repository/index/master_index.go
@@ -467,15 +467,27 @@ func (mi *MasterIndex) Rewrite(ctx context.Context, repo restic.Unpacked[restic.
 
 	wg.Go(func() error {
 		defer close(saveCh)
+		// duplicate packs must be tracked separately to allow the `EachByPack` loop to check
+		// for duplicate index entries with different blobs.
+		// this is necessary to work around a bug in restic < 0.10.0 where the blobs of
+		// a pack file could be split over multiple indexes.
+		packBlobsIDSet := restic.NewIDSet()
 		newIndex := NewIndex()
 		for task := range rewriteCh {
-			// always rewrite indexes that include a pack that must be removed or that are not full
+			// always rewrite indexes that include a pack that must be removed or is a duplicate or that are not full
 			if len(task.idx.Packs().Intersect(excludePacks)) == 0 && Full(task.idx) && !Oversized(task.idx) {
-				// make sure that each pack is only stored exactly once in the index
-				excludePacks.Merge(task.idx.Packs())
-				// index is already up to date
-				p.Add(1)
-				continue
+				// check that no pack index entry is a duplicate of an already processed one
+				idxPackBlobsIDSet := restic.NewIDSet()
+				for pbs := range task.idx.EachByPack(wgCtx, excludePacks) {
+					idxPackBlobsIDSet.Insert(PackBlobsHash(pbs))
+				}
+				if len(idxPackBlobsIDSet.Intersect(packBlobsIDSet)) == 0 {
+					// index is already up to date
+					// make sure that each pack is only stored exactly once in the index
+					packBlobsIDSet.Merge(idxPackBlobsIDSet)
+					p.Add(1)
+					continue
+				}
 			}
 
 			ids, err := task.idx.IDs()
@@ -485,6 +497,13 @@ func (mi *MasterIndex) Rewrite(ctx context.Context, repo restic.Unpacked[restic.
 			obsolete.Merge(restic.NewIDSet(ids...))
 
 			for pbs := range task.idx.EachByPack(wgCtx, excludePacks) {
+				// only filter pack blobs with matching packID and blobs
+				packBlobsID := PackBlobsHash(pbs)
+				if packBlobsIDSet.Has(packBlobsID) {
+					continue
+				}
+				packBlobsIDSet.Insert(packBlobsID)
+
 				newIndex.StorePack(pbs.PackID, pbs.Blobs)
 				if Full(newIndex) {
 					select {
@@ -498,8 +517,6 @@ func (mi *MasterIndex) Rewrite(ctx context.Context, repo restic.Unpacked[restic.
 			if wgCtx.Err() != nil {
 				return wgCtx.Err()
 			}
-			// make sure that each pack is only stored exactly once in the index
-			excludePacks.Merge(task.idx.Packs())
 			p.Add(1)
 		}