[FIXED] Don't remove snapshot if truncate to applied (#6055)

After installing a snapshot for `n.applied=5`, and then truncating back
to its `pterm` and the same `pindex` as `applied` it would remove the
snapshot as well, even though it should be kept.

Signed-off-by: Maurice van Veen <github@mauricevanveen.com>

server/raft.go

@@ -3191,7 +3191,7 @@ func (n *raft) truncateWAL(term, index uint64) {
 	defer func() {
 		// Check to see if we invalidated any snapshots that might have held state
 		// from the entries we are truncating.
-		if snap, _ := n.loadLastSnapshot(); snap != nil && snap.lastIndex >= index {
+		if snap, _ := n.loadLastSnapshot(); snap != nil && snap.lastIndex > index {
 			os.Remove(n.snapfile)
 			n.snapfile = _EMPTY_
 		}

server/raft_test.go

@@ -20,6 +20,7 @@ import (
 	"math"
 	"math/rand"
 	"os"
+	"path"
 	"path/filepath"
 	"testing"
 	"time"
@@ -1451,5 +1452,50 @@ func TestNRGCatchupFromNewLeaderWithIncorrectPtermDoesNotTruncateIfCommitted(t *
 	n.processAppendEntry(aeHeartbeat2, n.aesub)
 	require_Equal(t, n.commit, 2)
 	require_True(t, n.catchup == nil)
+}
+
+func TestNRGDontRemoveSnapshotIfTruncateToApplied(t *testing.T) {
+	n, cleanup := initSingleMemRaftNode(t)
+	defer cleanup()
+
+	// Create a sample entry, the content doesn't matter, just that it's stored.
+	esm := encodeStreamMsgAllowCompress("foo", "_INBOX.foo", nil, nil, 0, 0, true)
+	entries := []*Entry{newEntry(EntryNormal, esm)}
 
+	nats0 := "S1Nunr6R" // "nats-0"
+
+	// Timeline.
+	aeMsg := encode(t, &appendEntry{leader: nats0, term: 1, commit: 0, pterm: 0, pindex: 0, entries: entries})
+	aeHeartbeat := encode(t, &appendEntry{leader: nats0, term: 1, commit: 1, pterm: 1, pindex: 1, entries: nil})
+
+	// Initial case is simple, just store the entry.
+	n.processAppendEntry(aeMsg, n.aesub)
+	require_Equal(t, n.wal.State().Msgs, 1)
+	entry, err := n.loadEntry(1)
+	require_NoError(t, err)
+	require_Equal(t, entry.leader, nats0)
+
+	// Heartbeat, makes sure commit moves up.
+	n.processAppendEntry(aeHeartbeat, n.aesub)
+	require_Equal(t, n.commit, 1)
+	require_Equal(t, n.pterm, 1)
+
+	// Simulate upper layer calling down to apply.
+	n.Applied(1)
+
+	// Install snapshot and check it exists.
+	err = n.InstallSnapshot(nil)
+	require_NoError(t, err)
+
+	snapshots := path.Join(n.sd, snapshotsDir)
+	files, err := os.ReadDir(snapshots)
+	require_NoError(t, err)
+	require_Equal(t, len(files), 1)
+
+	// Truncate and check snapshot is kept.
+	n.truncateWAL(n.pterm, n.applied)
+
+	files, err = os.ReadDir(snapshots)
+	require_NoError(t, err)
+	require_Equal(t, len(files), 1)
 }