Function AbsorbSyncRequests

src/backend/postmaster/checkpointer.c:1273–1323

* AbsorbSyncRequests * Retrieve queued sync requests and pass them to sync mechanism. * * This is exported because it must be called during CreateCheckPoint; * we have to be sure we have accepted all pending requests just before * we start fsync'ing. Since CreateCheckPoint sometimes runs in * non-checkpointer processes, do nothing if not checkpointer. */

 * non-checkpointer processes, do nothing if not checkpointer.
 */
void
AbsorbSyncRequests(void)
{
	CheckpointerRequest *requests = NULL;
	CheckpointerRequest *request;
	int			n;

	if (IsUnderPostmaster && !AmStartupProcess() && !AmCheckpointerProcess())
		elog(ERROR, "AbsorbFsyncRequests() called in process %d (type %d)",
			 MyProcPid, MyAuxProcType);

	LWLockAcquire(CheckpointerCommLock, LW_EXCLUSIVE);

	/* Transfer stats counts into pending pgstats message */
	BgWriterStats.m_buf_written_backend += CheckpointerShmem->num_backend_writes;
	BgWriterStats.m_buf_fsync_backend += CheckpointerShmem->num_backend_fsync;

	CheckpointerShmem->num_backend_writes = 0;
	CheckpointerShmem->num_backend_fsync = 0;

	/*
	 * We try to avoid holding the lock for a long time by copying the request
	 * array, and processing the requests after releasing the lock.
	 *
	 * Once we have cleared the requests from shared memory, we have to PANIC
	 * if we then fail to absorb them (eg, because our hashtable runs out of
	 * memory).  This is because the system cannot run safely if we are unable
	 * to fsync what we have been told to fsync.  Fortunately, the hashtable
	 * is so small that the problem is quite unlikely to arise in practice.
	 */
	n = CheckpointerShmem->num_requests;
	if (n > 0)
	{
		requests = (CheckpointerRequest *) palloc(n * sizeof(CheckpointerRequest));
		memcpy(requests, CheckpointerShmem->requests, n * sizeof(CheckpointerRequest));
	}

	START_CRIT_SECTION();

	CheckpointerShmem->num_requests = 0;

	LWLockRelease(CheckpointerCommLock);

	for (request = requests; n > 0; request++, n--)
		RememberSyncRequest(&request->ftag, request->type);

	END_CRIT_SECTION();

	if (requests)
		pfree(requests);
}

/*
 * Update any shared memory configurations based on config parameters