[RFC PATCH 05/12] box: retrieve instance uuid before starting local recovery

[Vladimir Davydov <vdavydov.dev@gmail.com>]

In order to find out if the current instance fell too much behind its
peers in the cluster and so needs to be rebootstrapped, we need to
connect it to remote peers before proceeding to local recovery. The
problem is box.cfg.replication may have an entry corresponding to the
instance itself so before connecting we have to start listening to
incoming connections. Since an instance is supposed to sent its uuid in
the greeting message, we also have to initialize INSTANCE_UUID early,
before we start local recovery. So this patch makes memtx engine
constructor not only scan the snapshot directory, but also read the
header of the most recent snapshot to initialize INSTANCE_UUID.

Needed for #461
---
 src/box/box.cc         | 18 ++++++++++--------
 src/box/memtx_engine.c | 11 ++++++++++-
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/src/box/box.cc b/src/box/box.cc
index 61bfa117..e1bf3934 100644
--- a/src/box/box.cc
+++ b/src/box/box.cc
@@ -1839,6 +1839,15 @@ box_cfg_xc(void)
 	}
 	bool is_bootstrap_leader = false;
 	if (last_checkpoint_lsn >= 0) {
+		/* Check instance UUID. */
+		assert(!tt_uuid_is_nil(&INSTANCE_UUID));
+		if (!tt_uuid_is_nil(&instance_uuid) &&
+		    !tt_uuid_is_equal(&instance_uuid, &INSTANCE_UUID)) {
+			tnt_raise(ClientError, ER_INSTANCE_UUID_MISMATCH,
+				  tt_uuid_str(&instance_uuid),
+				  tt_uuid_str(&INSTANCE_UUID));
+		}
+
 		struct wal_stream wal_stream;
 		wal_stream_create(&wal_stream, cfg_geti64("rows_per_wal"));
 
@@ -1882,7 +1891,6 @@ box_cfg_xc(void)
 				      cfg_getd("wal_dir_rescan_delay"));
 		title("hot_standby");
 
-		assert(!tt_uuid_is_nil(&INSTANCE_UUID));
 		/*
 		 * Leave hot standby mode, if any, only
 		 * after acquiring the lock.
@@ -1902,13 +1910,7 @@ box_cfg_xc(void)
 		recovery_finalize(recovery, &wal_stream.base);
 		engine_end_recovery_xc();
 
-		/* Check replica set and instance UUID. */
-		if (!tt_uuid_is_nil(&instance_uuid) &&
-		    !tt_uuid_is_equal(&instance_uuid, &INSTANCE_UUID)) {
-			tnt_raise(ClientError, ER_INSTANCE_UUID_MISMATCH,
-				  tt_uuid_str(&instance_uuid),
-				  tt_uuid_str(&INSTANCE_UUID));
-		}
+		/* Check replica set UUID. */
 		if (!tt_uuid_is_nil(&replicaset_uuid) &&
 		    !tt_uuid_is_equal(&replicaset_uuid, &REPLICASET_UUID)) {
 			tnt_raise(ClientError, ER_REPLICASET_UUID_MISMATCH,
diff --git a/src/box/memtx_engine.c b/src/box/memtx_engine.c
index fac84ce1..df0c6c28 100644
--- a/src/box/memtx_engine.c
+++ b/src/box/memtx_engine.c
@@ -164,7 +164,6 @@ memtx_engine_recover_snapshot(struct memtx_engine *memtx,
 	struct xlog_cursor cursor;
 	if (xlog_cursor_open(&cursor, filename) < 0)
 		return -1;
-	INSTANCE_UUID = cursor.meta.instance_uuid;
 
 	int rc;
 	struct xrow_header row;
@@ -1001,6 +1000,16 @@ memtx_engine_new(const char *snap_dirname, bool force_recovery,
 	if (xdir_scan(&memtx->snap_dir) != 0)
 		goto fail;
 
+	int64_t snap_signature = xdir_last_vclock(&memtx->snap_dir, NULL);
+	if (snap_signature >= 0) {
+		struct xlog_cursor cursor;
+		if (xdir_open_cursor(&memtx->snap_dir,
+				     snap_signature, &cursor) != 0)
+			goto fail;
+		INSTANCE_UUID = cursor.meta.instance_uuid;
+		xlog_cursor_close(&cursor, false);
+	}
+
 	stailq_create(&memtx->gc_queue);
 	memtx->gc_fiber = fiber_new("memtx.gc", memtx_engine_gc_f);
 	if (memtx->gc_fiber == NULL)
-- 
2.11.0