From 9a5f7fbadf4a9afca1bb66abbed3809d7189a34f Mon Sep 17 00:00:00 2001
From: Simon Frei <freisim93@gmail.com>
Date: Mon, 18 May 2020 10:42:51 +0200
Subject: [PATCH 1/2] lib/db: Don't panic on seq. coruption when debugging
 (#6662)

---
 lib/db/transactions.go | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/lib/db/transactions.go b/lib/db/transactions.go
index 36459d19e..eafb970c7 100644
--- a/lib/db/transactions.go
+++ b/lib/db/transactions.go
@@ -233,8 +233,7 @@ func (t *readOnlyTransaction) withHaveSequence(folder []byte, startSeq int64, fn
 
 		if shouldDebug() {
 			if seq := t.keyer.SequenceFromSequenceKey(dbi.Key()); f.Sequence != seq {
-				l.Warnf("Sequence index corruption (folder %v, file %v): sequence %d != expected %d", string(folder), f.Name, f.Sequence, seq)
-				panic("sequence index corruption")
+				l.Debugf("Sequence index corruption (folder %v, file %v): sequence %d != expected %d", string(folder), f.Name, f.Sequence, seq)
 			}
 		}
 		if !fn(f) {

From 8c741776997fd22e8017c19650e40f22802372bd Mon Sep 17 00:00:00 2001
From: Simon Frei <freisim93@gmail.com>
Date: Wed, 20 May 2020 11:01:27 +0200
Subject: [PATCH 2/2] lib/db: Initialize need meta on first dev occurrence
 (fixes #6668) (#6669)

---
 lib/db/meta.go     | 11 +++++++++--
 lib/db/set_test.go | 18 ++++++++++++++++++
 2 files changed, 27 insertions(+), 2 deletions(-)

diff --git a/lib/db/meta.go b/lib/db/meta.go
index b5632f9b2..b8a462ae2 100644
--- a/lib/db/meta.go
+++ b/lib/db/meta.go
@@ -118,9 +118,16 @@ func (m *metadataTracker) countsPtr(dev protocol.DeviceID, flag uint32) *Counts
 		idx = len(m.counts.Counts)
 		m.counts.Counts = append(m.counts.Counts, Counts{DeviceID: dev[:], LocalFlags: flag})
 		m.indexes[key] = idx
-		if flag == needFlag {
+		// Need bucket must be initialized when a device first occurs in
+		// the metadatatracker, even if there's no change to the need
+		// bucket itself.
+		nkey := metaKey{dev, needFlag}
+		nidx, ok := m.indexes[nkey]
+		if !ok {
 			// Initially a new device needs everything, except deletes
-			m.counts.Counts[idx] = m.allNeededCounts(dev)
+			nidx = len(m.counts.Counts)
+			m.counts.Counts = append(m.counts.Counts, m.allNeededCounts(dev))
+			m.indexes[nkey] = nidx
 		}
 	}
 	return &m.counts.Counts[idx]
diff --git a/lib/db/set_test.go b/lib/db/set_test.go
index b6b2cd777..bd5f2346c 100644
--- a/lib/db/set_test.go
+++ b/lib/db/set_test.go
@@ -1653,6 +1653,24 @@ func TestUpdateWithOneFileTwice(t *testing.T) {
 	}
 }
 
+// https://github.com/syncthing/syncthing/issues/6668
+func TestNeedRemoteOnly(t *testing.T) {
+	ldb := db.NewLowlevel(backend.OpenMemory())
+	defer ldb.Close()
+
+	s := db.NewFileSet("test", fs.NewFilesystem(fs.FilesystemTypeFake, ""), ldb)
+
+	remote0Have := fileList{
+		protocol.FileInfo{Name: "b", Version: protocol.Vector{Counters: []protocol.Counter{{ID: myID, Value: 1001}}}, Blocks: genBlocks(2)},
+	}
+	s.Update(remoteDevice0, remote0Have)
+
+	need := needSize(s, remoteDevice0)
+	if !need.Equal(db.Counts{}) {
+		t.Error("Expected nothing needed, got", need)
+	}
+}
+
 func replace(fs *db.FileSet, device protocol.DeviceID, files []protocol.FileInfo) {
 	fs.Drop(device)
 	fs.Update(device, files)