From 027cc647373b05718710831c76177d17ca6e8a14 Mon Sep 17 00:00:00 2001
From: Michael Eischer <michael.eischer@fau.de>
Date: Sun, 19 May 2024 23:24:18 +0200
Subject: [PATCH] repository: fix prune heuristic to allow resuming interrupted
 runs

Pack files created by interrupted prune runs, appear to consist only of
duplicate blobs on the next run. This caused the previous heuristic to
ignore those pack files. Now, a duplicate blob in a specific pack file
is also selected if that pack file only contains duplicate blobs. This
allows prune to select the already rewritten pack files.
---
 internal/repository/prune.go | 19 +++++++++++++------
 1 file changed, 13 insertions(+), 6 deletions(-)

diff --git a/internal/repository/prune.go b/internal/repository/prune.go
index 895b07994..49869fcac 100644
--- a/internal/repository/prune.go
+++ b/internal/repository/prune.go
@@ -72,10 +72,12 @@ type PrunePlan struct {
 }
 
 type packInfo struct {
-	usedBlobs    uint
-	unusedBlobs  uint
-	usedSize     uint64
-	unusedSize   uint64
+	usedBlobs      uint
+	unusedBlobs    uint
+	duplicateBlobs uint
+	usedSize       uint64
+	unusedSize     uint64
+
 	tpe          restic.BlobType
 	uncompressed bool
 }
@@ -226,6 +228,7 @@ func packInfoFromIndex(ctx context.Context, idx restic.ListBlobser, usedBlobs re
 			// mark as unused for now, we will later on select one copy
 			ip.unusedSize += size
 			ip.unusedBlobs++
+			ip.duplicateBlobs++
 
 			// count as duplicate, will later on change one copy to be counted as used
 			stats.Size.Duplicate += size
@@ -256,6 +259,8 @@ func packInfoFromIndex(ctx context.Context, idx restic.ListBlobser, usedBlobs re
 	// if duplicate blobs exist, those will be set to either "used" or "unused":
 	// - mark only one occurrence of duplicate blobs as used
 	// - if there are already some used blobs in a pack, possibly mark duplicates in this pack as "used"
+	// - if a pack only consists of duplicates (which by definition are used blobs), mark it as "used". This
+	//   ensures that already rewritten packs are kept.
 	// - if there are no used blobs in a pack, possibly mark duplicates as "unused"
 	if hasDuplicates {
 		// iterate again over all blobs in index (this is pretty cheap, all in-mem)
@@ -271,8 +276,10 @@ func packInfoFromIndex(ctx context.Context, idx restic.ListBlobser, usedBlobs re
 			ip := indexPack[blob.PackID]
 			size := uint64(blob.Length)
 			switch {
-			case ip.usedBlobs > 0, count == 0:
-				// other used blobs in pack or "last" occurrence ->  transition to used
+			case ip.usedBlobs > 0, (ip.duplicateBlobs == ip.unusedBlobs), count == 0:
+				// other used blobs in pack, only duplicate blobs or "last" occurrence ->  transition to used
+				// a pack file created by an interrupted prune run will consist of only duplicate blobs
+				// thus select such already repacked pack files
 				ip.usedSize += size
 				ip.usedBlobs++
 				ip.unusedSize -= size