Function StartBufferIO

src/backend/storage/buffer/bufmgr.c:4675–4712

* StartBufferIO: begin I/O on this buffer * (Assumptions) * My process is executing no IO * The buffer is Pinned * * In some scenarios there are race conditions in which multiple backends * could attempt the same I/O operation concurrently. If someone else * has already started I/O on this buffer then we will block on the * I/O condition variable until he's done. * * Input operations ar

 * false if someone else already did the work.
 */
static bool
StartBufferIO(BufferDesc *buf, bool forInput)
{
	uint32		buf_state;

	Assert(!InProgressBuf);

	for (;;)
	{
		buf_state = LockBufHdr(buf);

		if (!(buf_state & BM_IO_IN_PROGRESS))
			break;
		UnlockBufHdr(buf, buf_state);
		WaitIO(buf);
	}

	/* Once we get here, there is definitely no I/O active on this buffer */

	if (forInput ? (buf_state & BM_VALID) : !(buf_state & BM_DIRTY))
	{
		/* someone else already did the I/O */
		UnlockBufHdr(buf, buf_state);
		return false;
	}

	buf_state |= BM_IO_IN_PROGRESS;
	UnlockBufHdr(buf, buf_state);

#ifdef MPROTECT_BUFFERS
    BufferMProtect(buf, forInput ? PROT_WRITE|PROT_READ : PROT_READ);
#endif

	InProgressBuf = buf;
	IsForInput = forInput;

	return true;
}

/*
 * TerminateBufferIO: release a buffer we were doing I/O on