Function LockBufferForCleanup

src/backend/storage/buffer/bufmgr.c:4345–4490

* LockBufferForCleanup - lock a buffer in preparation for deleting items * * Items may be deleted from a disk page only when the caller (a) holds an * exclusive lock on the buffer and (b) has observed that no other backend * holds a pin on the buffer. If there is a pin, then the other backend * might have a pointer into the buffer (for example, a heapscan reference * to an item --- see READ

 * it has successfully observed pin count = 1.
 */
void
LockBufferForCleanup(Buffer buffer)
{
	BufferDesc *bufHdr;
	char	   *new_status = NULL;
	TimestampTz waitStart = 0;
	bool		logged_recovery_conflict = false;

	Assert(BufferIsPinned(buffer));
	Assert(PinCountWaitBuf == NULL);

	if (BufferIsLocal(buffer))
	{
		/* There should be exactly one pin */
		if (LocalRefCount[-buffer - 1] != 1)
			elog(ERROR, "incorrect local pin count: %d",
				 LocalRefCount[-buffer - 1]);
		/* Nobody else to wait for */
		return;
	}

	/* There should be exactly one local pin */
	if (GetPrivateRefCount(buffer) != 1)
		elog(ERROR, "incorrect local pin count: %d",
			 GetPrivateRefCount(buffer));

	bufHdr = GetBufferDescriptor(buffer - 1);

	for (;;)
	{
		uint32		buf_state;

		/* Try to acquire lock */
		LockBuffer(buffer, BUFFER_LOCK_EXCLUSIVE);
		buf_state = LockBufHdr(bufHdr);

		Assert(BUF_STATE_GET_REFCOUNT(buf_state) > 0);
		if (BUF_STATE_GET_REFCOUNT(buf_state) == 1)
		{
			/* Successfully acquired exclusive lock with pincount 1 */
			UnlockBufHdr(bufHdr, buf_state);

			/*
			 * Emit the log message if recovery conflict on buffer pin was
			 * resolved but the startup process waited longer than
			 * deadlock_timeout for it.
			 */
			if (logged_recovery_conflict)
				LogRecoveryConflict(PROCSIG_RECOVERY_CONFLICT_BUFFERPIN,
									waitStart, GetCurrentTimestamp(),
									NULL, false);

			/* Report change to non-waiting status */
			if (new_status)
			{
				set_ps_display(new_status);
				pfree(new_status);
			}