[FIXED] Do not panic when re-calculating first sequence for SimpleState when fseq moves ahead of old first. (#4346)

When a lazy simple state has an outdated first that needs to be updated,
if fseq had moved past it would panic.
This was not common but with latest fix prior in can become more common,
hence why it showed up.

Signed-off-by: Derek Collison <derek@nats.io>

Author: derekcollison

server/filestore.go

@@ -5934,6 +5934,8 @@ func (mb *msgBlock) recalculateFirstForSubj(subj string, startSeq uint64, ss *Si
 	if startSlot >= len(mb.cache.idx) {
 		ss.First = ss.Last
 		return
+	} else if startSlot < 0 {
+		startSlot = 0
 	}
 
 	var le = binary.LittleEndian

server/filestore_test.go

@@ -5537,3 +5537,35 @@ func TestFileStoreRestoreEncryptedWithNoKeyFuncFails(t *testing.T) {
 	)
 	require_Error(t, err, errNoMainKey)
 }
+
+func TestFileStoreRecaluclateFirstForSubjBug(t *testing.T) {
+	fs, err := newFileStore(FileStoreConfig{StoreDir: t.TempDir()}, StreamConfig{Name: "zzz", Subjects: []string{"*"}, Storage: FileStorage})
+	require_NoError(t, err)
+
+	fs.StoreMsg("foo", nil, nil) // 1
+	fs.StoreMsg("bar", nil, nil) // 2
+	fs.StoreMsg("foo", nil, nil) // 3
+
+	// Now remove first 2..
+	fs.RemoveMsg(1)
+	fs.RemoveMsg(2)
+
+	// Now grab first (and only) block.
+	fs.mu.RLock()
+	mb := fs.blks[0]
+	fs.mu.RUnlock()
+
+	// Since we lazy update the first, simulate that we have not updated it as of yet.
+	ss := &SimpleState{Msgs: 1, First: 1, Last: 3, firstNeedsUpdate: true}
+
+	mb.mu.Lock()
+	defer mb.mu.Unlock()
+
+	// Flush the cache.
+	mb.clearCacheAndOffset()
+	// Now call with start sequence of 1, the old one
+	// This will panic without the fix.
+	mb.recalculateFirstForSubj("foo", 1, ss)
+	// Make sure it was update properly.
+	require_True(t, *ss == SimpleState{Msgs: 1, First: 3, Last: 3, firstNeedsUpdate: false})
+}