Keep enough entries for the previous snapshot

This commit fixes the logic so that the computed trailing does not
remove too many entries in the log which would make the previous
snapshot unusable.

src/raft/replication.c

@@ -1678,31 +1678,35 @@ static void takeSnapshotClose(struct raft *r, struct raft_snapshot *s)
 
 static unsigned dynamicTrailingIndex(struct raft *r, struct raft_snapshot *snapshot) {
 	struct raft_log *l = r->log;
-	unsigned threshold = (unsigned)(l->front - l->back);
-	size_t size = 0;
+	size_t snapshot_size = 0;
+	unsigned trailing = 0;
 	unsigned i;
-
 	/** 
 	 * This should never happen as it would mean that the snapshot
 	 * contains no new entry (i.e. either the snapshot is empty
 	 * or equal to the previous one).
 	 */
-	assert(threshold > 0);
-	threshold = min(r->snapshot.threshold, threshold);
+	assert(l->front > l->back);
 
 	for (i = 0; i < snapshot->n_bufs; i++) {
-		size += snapshot->bufs[i].len;
+		snapshot_size += snapshot->bufs[i].len;
 	}
 
-	for (i = 1; i <= threshold; i++) {
-		struct raft_entry* entry = &(l->entries[l->back - i]);
-		if (entry->buf.len > size) {
-			return i;
+	for (trailing = 1; l->back - trailing >= l->front; i++) {
+		struct raft_entry* entry = &(l->entries[l->back - trailing]);
+		if (entry->buf.len > snapshot_size) {
+			break;
 		}
-		size -= entry->buf.len;
+		snapshot_size -= entry->buf.len;
+	}
+
+	if (trailing < r->snapshot.threshold) {
+		trailing = r->snapshot.threshold;
+	} else if (trailing > r->snapshot.trailing) {
+		trailing = r->snapshot.trailing;
 	}
 
-	return threshold;
+	return trailing;
 }