prune: correctly account for duplicates in max-unused check

The size comparison for `--max-unused` only accounted for unused but not
for duplicate data. For repositories with a large amount of duplicates
this can result in a situation where no data gets pruned even though
the amount of unused data is much higher than specified.

changelog/unreleased/pull-5212

@@ -0,0 +1,10 @@
+Bugfix: Correctly account for duplicate data in `prune --max-unused`
+
+`prune --max-unused size` did not correctly account for duplicate data.
+If a repository contained a large amount of duplicate data, this could
+previously result in pruning too little data.
+
+This issue is now resolved.
+
+https://github.com/restic/restic/pull/5212
+https://forum.restic.net/t/restic-not-obeying-max-unused-parameter-on-prune/8879

internal/repository/prune.go

@@ -478,7 +478,8 @@ func decidePackAction(ctx context.Context, opts PruneOptions, repo *Repository,
 	maxUnusedSizeAfter := opts.MaxUnusedBytes(stats.Size.Used)
 
 	for _, p := range repackCandidates {
-		reachedUnusedSizeAfter := (stats.Size.Unused-stats.Size.Remove-stats.Size.Repackrm < maxUnusedSizeAfter)
+		remainingUnusedSize := stats.Size.Duplicate + stats.Size.Unused - stats.Size.Remove - stats.Size.Repackrm
+		reachedUnusedSizeAfter := remainingUnusedSize < maxUnusedSizeAfter
 		reachedRepackSize := stats.Size.Repack+p.unusedSize+p.usedSize >= opts.MaxRepackBytes
 		packIsLargeEnough := p.unusedSize+p.usedSize >= uint64(targetPackSize)
 

internal/repository/prune_test.go

@@ -112,3 +112,82 @@ func TestPrune(t *testing.T) {
 		})
 	}
 }
+
+// TestPruneMaxUnusedDuplicate checks that MaxUnused correctly accounts for duplicates.
+//
+// Create a repository containing blobs a to d that are stored in packs as follows:
+// - a, d
+// - b, d
+// - c, d
+// All blobs should be kept during prune, but the duplicates should be gone afterwards.
+// The special construction ensures that each pack contains a used, non-duplicate blob.
+// This ensures that special cases that delete completely duplicate packs files do not
+// apply.
+func TestPruneMaxUnusedDuplicate(t *testing.T) {
+	seed := time.Now().UnixNano()
+	random := rand.New(rand.NewSource(seed))
+	t.Logf("rand initialized with seed %d", seed)
+
+	repo, _, _ := repository.TestRepositoryWithVersion(t, 0)
+	// large blobs to prevent repacking due to too small packsize
+	const blobSize = 1024 * 1024
+
+	bufs := [][]byte{}
+	for i := 0; i < 4; i++ {
+		// use uniform length for simpler control via MaxUnusedBytes
+		buf := make([]byte, blobSize)
+		random.Read(buf)
+		bufs = append(bufs, buf)
+	}
+	keep := restic.NewBlobSet()
+
+	for _, blobs := range [][][]byte{
+		{bufs[0], bufs[3]},
+		{bufs[1], bufs[3]},
+		{bufs[2], bufs[3]},
+	} {
+		var wg errgroup.Group
+		repo.StartPackUploader(context.TODO(), &wg)
+
+		for _, blob := range blobs {
+			id, _, _, err := repo.SaveBlob(context.TODO(), restic.DataBlob, blob, restic.ID{}, true)
+			keep.Insert(restic.BlobHandle{Type: restic.DataBlob, ID: id})
+			rtest.OK(t, err)
+		}
+
+		rtest.OK(t, repo.Flush(context.Background()))
+	}
+
+	opts := repository.PruneOptions{
+		MaxRepackBytes: math.MaxUint64,
+		// non-zero number of unused bytes, that is nevertheless smaller than a single blob
+		// setting this to zero would bypass the unused/duplicate size accounting that should
+		// be tested here
+		MaxUnusedBytes: func(used uint64) (unused uint64) { return blobSize / 2 },
+	}
+
+	plan, err := repository.PlanPrune(context.TODO(), opts, repo, func(ctx context.Context, repo restic.Repository, usedBlobs restic.FindBlobSet) error {
+		for blob := range keep {
+			usedBlobs.Insert(blob)
+		}
+		return nil
+	}, &progress.NoopPrinter{})
+	rtest.OK(t, err)
+
+	rtest.OK(t, plan.Execute(context.TODO(), &progress.NoopPrinter{}))
+
+	rsize := plan.Stats().Size
+	remainingUnusedSize := rsize.Duplicate + rsize.Unused - rsize.Remove - rsize.Repackrm
+	maxUnusedSize := opts.MaxUnusedBytes(rsize.Used)
+	rtest.Assert(t, remainingUnusedSize <= maxUnusedSize, "too much unused data remains got %v, expected less than %v", remainingUnusedSize, maxUnusedSize)
+
+	// divide by blobSize to ignore pack file overhead
+	rtest.Equals(t, rsize.Used/blobSize, uint64(4))
+	rtest.Equals(t, rsize.Duplicate/blobSize, uint64(2))
+	rtest.Equals(t, rsize.Unused, uint64(0))
+	rtest.Equals(t, rsize.Remove, uint64(0))
+	rtest.Equals(t, rsize.Repack/blobSize, uint64(4))
+	rtest.Equals(t, rsize.Repackrm/blobSize, uint64(2))
+	rtest.Equals(t, rsize.Unref, uint64(0))
+	rtest.Equals(t, rsize.Uncompressed, uint64(0))
+}