Add options for create_snapshot API (#565)

* The only current option is to serialize commits to ensure that
snapshots are created using the latest log index.

include/libnuraft/raft_server.hxx

@@ -814,16 +814,37 @@ public:
      */
     void notify_log_append_completion(bool ok);
 
+    /**
+     * Options for manual snapshot creation.
+     */
+    struct create_snapshot_options {
+        create_snapshot_options()
+            : serialize_commit_(false)
+            {}
+        /**
+         * If `true`, the background commit will be blocked until `create_snapshot`
+         * returns. However, it will not block the commit for the entire duration
+         * of the snapshot creation process, as long as your state machine creates
+         * the snapshot asynchronously.
+         *
+         * The purpose of this flag is to ensure that the log index used for
+         * the snapshot creation is the most recent one.
+         */
+        bool serialize_commit_;
+    };
+
     /**
      * Manually create a snapshot based on the latest committed
      * log index of the state machine.
      *
      * Note that snapshot creation will fail immediately if the previous
      * snapshot task is still running.
      *
+     * @params options Options for snapshot creation.
      * @return Log index number of the created snapshot or`0` if failed.
      */
-    ulong create_snapshot();
+    ulong create_snapshot(const create_snapshot_options& options =
+                              create_snapshot_options());
 
     /**
      * Manually and asynchronously create a snapshot on the next earliest

src/handle_commit.cxx

@@ -464,10 +464,19 @@ bool raft_server::apply_config_log_entry(ptr<log_entry>& le,
     return true;
 }
 
-ulong raft_server::create_snapshot() {
-    uint64_t committed_idx = sm_commit_index_;
-    p_in("manually create a snapshot on %" PRIu64 "", committed_idx);
-    return snapshot_and_compact(committed_idx, true) ? committed_idx : 0;
+ulong raft_server::create_snapshot(const create_snapshot_options& options) {
+    auto exec_internal = [&]() {
+        uint64_t committed_idx = sm_commit_index_;
+        p_in("manually create a snapshot on %" PRIu64 "", committed_idx);
+        return snapshot_and_compact(committed_idx, true) ? committed_idx : 0;
+    };
+
+    if (options.serialize_commit_) {
+        auto_lock(commit_lock_);
+        return exec_internal();
+    } else {
+        return exec_internal();
+    }
 }
 
 ptr< cmd_result<uint64_t> > raft_server::schedule_snapshot_creation() {