Greenplum数据库源码学习——FTS HandleFtsWalRepPromote函数

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HandleFtsWalRepPromote函数位于src/backend/fts/ftsmessagehandler.c，用于在postgres后端处理FTS进程发送的Promote请求。后续内容欢迎关注公号或者充值CSDN VIP阅读。

static void HandleFtsWalRepPromote(void) {
	FtsResponse response = {
		false, /* IsMirrorUp */
		false, /* IsInSync */
		false, /* IsSyncRepEnabled */
		am_mirror,  /* IsRoleMirror */ 
		false, /* RequestRetry */
	};
	ereport(LOG, (errmsg("promoting mirror to primary due to FTS request")));

FTS 向mirror发送提升消息。mirror可能正在进行提升。 因此，应该以幂等的方式处理提升消息。数据库的状态只有在DB_IN_ARCHIVE_RECOVERY状态下，才能进行promote。在promote时，重置 sync_standby_names。 这是为了避免提交(commits)挂起/等待复制（hanging/waiting for replication）直到下一次 FTS 探测。 下一个 FTS 探测将检测到该节点不同步并重置相同的节点，这可能需要一分钟。 由于我们知道在mirror promotion中它在gp_segment_configuration 中被标记为不同步，因此最好立即清理 sync_standby_names。

	/* FTS sends promote message to a mirror.  The mirror may be undergoing
	 * promotion.  Promote messages should therefore be handled in an
	 * idempotent way. */
	DBState state = GetCurrentDBState(); // 取出ControlFile->state，也就是数据库的状态
	if (state == DB_IN_ARCHIVE_RECOVERY) {
		/* Reset sync_standby_names on promotion. This is to avoid commits
		 * hanging/waiting for replication till next FTS probe. Next FTS probe
		 * will detect this node to be not in sync and reset the same which
		 * can take a min. Since we know on mirror promotion its marked as not
		 * in sync in gp_segment_configuration, best to right away clean the
		 * sync_standby_names. */
		UnsetSyncStandbysDefined();
		CreateReplicationSlotOnPromote(INTERNAL_WAL_REPLICATION_SLOT_NAME);
		SignalPromote();
	} else {
		elog(LOG, "ignoring promote request, walreceiver not running, DBState = %d", state);
	}
	SendFtsResponse(&response, FTS_MSG_PROMOTE);
}
DBState GetCurrentDBState(void) {
	Assert(ControlFile);
	return ControlFile->state;
}

CreateReplicationSlotOnPromote(“internal_wal_replication_slot”)函数，先从ReplicationSlotCtl->replication_slots数组中找出名为"internal_wal_replication_slot"的槽位，如果没有则需要创建复制槽（mirror本应该没有，使用ReplicationSlotCreate(name, false, RS_PERSISTENT)函数创建）。只有在提升时信号复制槽(signal replication slot)才会在mirror上创建。 如果节点充当mirror，则其上不应存在复制槽。 因此，restart_lsn不为零意味着是由先前提升信号尝试设置的，因此不需要覆盖它。关于restart_lsn为零情况下，所做的工作后续分析复制槽时再进行讲解（to do）。

static void CreateReplicationSlotOnPromote(const char *name) {
	int             i;
	/* Check for name collision, and identify an allocatable slot.  We need to
	 * hold ReplicationSlotControlLock in shared mode for this, so that nobody
	 * else can change the in_use flags while we're looking at them. */
	LWLockAcquire(ReplicationSlotControlLock, LW_SHARED);
	for (i = 0; i < max_replication_slots; i++) {
		ReplicationSlot *s = &ReplicationSlotCtl->replication_slots[i];
		if (s->in_use && strcmp(name, NameStr(s->data.name)) == 0)
			MyReplicationSlot = s;
	}
	LWLockRelease(ReplicationSlotControlLock);
	if (MyReplicationSlot == NULL) {
		ereport(LOG, (errmsg("creating replication slot %s", name)));
		ReplicationSlotCreate(name, false, RS_PERSISTENT);
	} else
		ereport(LOG, (errmsg("replication slot %s exists", name)));
	/* Only on promote signal replication slot is created on mirror. If
	 * node was acting as mirror, no replication slot should exists on it.
	 * Hence, no-zero restart_lsn means was set by previous attempt on promote
	 * signal and hence no need to overwrite the same. */
	if (MyReplicationSlot->data.restart_lsn == 0) {
		/* Starting reserving WAL right away for pg_rewind to work later */
		ReplicationSlotReserveWal();
		/* Write this slot to disk */
		ReplicationSlotMarkDirty();
		ReplicationSlotSave();
		if (MyReplicationSlot->active)
			ReplicationSlotRelease();
	}
	MyReplicationSlot = NULL;
}

SignalPromote函数写PROMOTE_SIGNAL_FILE文件，并向postmaster发送SIGUSR1信号。

/* GPDB_90_MERGE_FIXME: This function should be removed once hot standby can and will be enabled for mirrors. */
void SignalPromote(void) {
	FILE *fd;
	if ((fd = fopen(PROMOTE_SIGNAL_FILE, "w"))) {
		fclose(fd);
		kill(PostmasterPid, SIGUSR1);
	}
}

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