From bcfbfb8b5a75d38e24a2d5313299be91340110db Mon Sep 17 00:00:00 2001
From: Maurice van Veen <github@mauricevanveen.com>
Date: Mon, 6 Oct 2025 19:31:16 +0200
Subject: [PATCH] [FIXED] Filestore detect delete gap with last SkipMsg

Signed-off-by: Maurice van Veen <github@mauricevanveen.com>
---
 server/filestore.go      | 32 ++++++++++++++++++------------
 server/filestore_test.go | 43 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 62 insertions(+), 13 deletions(-)

diff --git a/server/filestore.go b/server/filestore.go
index c607617e453..3096bdadc62 100644
--- a/server/filestore.go
+++ b/server/filestore.go
@@ -1527,6 +1527,23 @@ func (mb *msgBlock) rebuildStateLocked() (*LostStreamData, []uint64, error) {
 	var last uint64
 	var hb [highwayhash.Size64]byte
 
+	updateLast := func(seq uint64, ts int64) {
+		// The sequence needs to only ever move up.
+		if seq <= last {
+			return
+		}
+
+		// Check for any gaps from compaction, meaning no ebit entry.
+		if last > 0 && seq != last+1 {
+			for dseq := last + 1; dseq < seq; dseq++ {
+				addToDmap(dseq)
+			}
+		}
+		last = seq
+		atomic.StoreUint64(&mb.last.seq, last)
+		mb.last.ts = ts
+	}
+
 	for index, lbuf := uint32(0), uint32(len(buf)); index < lbuf; {
 		if index+msgHdrSize > lbuf {
 			truncate(index)
@@ -1591,8 +1608,7 @@ func (mb *msgBlock) rebuildStateLocked() (*LostStreamData, []uint64, error) {
 		if seq == 0 || seq&ebit != 0 || seq < fseq {
 			seq = seq &^ ebit
 			if seq >= fseq {
-				atomic.StoreUint64(&mb.last.seq, seq)
-				mb.last.ts = ts
+				updateLast(seq, ts)
 				if mb.msgs == 0 {
 					atomic.StoreUint64(&mb.first.seq, seq+1)
 					mb.first.ts = 0
@@ -1640,17 +1656,7 @@ func (mb *msgBlock) rebuildStateLocked() (*LostStreamData, []uint64, error) {
 			}
 		}
 
-		// Check for any gaps from compaction, meaning no ebit entry.
-		if last > 0 && seq != last+1 {
-			for dseq := last + 1; dseq < seq; dseq++ {
-				addToDmap(dseq)
-			}
-		}
-
-		// Always set last
-		last = seq
-		atomic.StoreUint64(&mb.last.seq, last)
-		mb.last.ts = ts
+		updateLast(seq, ts)
 
 		// Advance to next record.
 		index += rl
diff --git a/server/filestore_test.go b/server/filestore_test.go
index 7bbeef720e9..29bb77a9693 100644
--- a/server/filestore_test.go
+++ b/server/filestore_test.go
@@ -10852,3 +10852,46 @@ func TestFileStoreTombstonesSelectNextFirstCleanupOnRecovery(t *testing.T) {
 		}
 	})
 }
+
+func TestFileStoreDetectDeleteGapWithLastSkipMsg(t *testing.T) {
+	testFileStoreAllPermutations(t, func(t *testing.T, fcfg FileStoreConfig) {
+		cfg := StreamConfig{Name: "zzz", Subjects: []string{"foo"}, Storage: FileStorage}
+		created := time.Now()
+		fs, err := newFileStoreWithCreated(fcfg, cfg, created, prf(&fcfg), nil)
+		require_NoError(t, err)
+		defer fs.Stop()
+
+		_, _, err = fs.StoreMsg("foo", nil, nil, 0)
+		require_NoError(t, err)
+
+		// Skip a message at a sequence such that a gap is created.
+		// The gap should be detected later on as deleted messages.
+		require_NoError(t, fs.SkipMsgs(2, 3))
+
+		// We should have 3 deletes, one is the skip msg, the other two is the gap.
+		before := fs.State()
+		require_Equal(t, before.Msgs, 1)
+		require_Equal(t, before.FirstSeq, 1)
+		require_Equal(t, before.LastSeq, 4)
+		require_Equal(t, before.NumDeleted, 3)
+
+		// Make sure we can recover properly with no index.db present.
+		fs.Stop()
+		os.Remove(filepath.Join(fs.fcfg.StoreDir, msgDir, streamStreamStateFile))
+
+		fs, err = newFileStoreWithCreated(fcfg, cfg, created, prf(&fcfg), nil)
+		require_NoError(t, err)
+		defer fs.Stop()
+
+		if state := fs.State(); !reflect.DeepEqual(state, before) {
+			t.Fatalf("Expected state of %+v, got %+v", before, state)
+		}
+
+		mb := fs.getFirstBlock()
+		mb.mu.RLock()
+		defer mb.mu.RUnlock()
+		require_Equal(t, atomic.LoadUint64(&mb.first.seq), 1)
+		require_Equal(t, atomic.LoadUint64(&mb.last.seq), 4)
+		require_Len(t, mb.dmap.Size(), 3)
+	})
+}