BUG #17903: There is a bug in the KeepLogSeg()

The following bug has been logged on the website:

Bug reference:      17903
Logged by:          xu xingwang
Email address:      xu.xw2008@163.com
PostgreSQL version: 13.3
Operating system:   openEuler
Description:        

Hi,

I found that KeepLogSeg() has a piece of code that is not correctly.

segno may be larger than currSegNo, since the slot_keep_segs variable is of
type "uint64", in this case the code "if (currSegNo - segno >
slot_keep_segs)" is incorrect. 

"if (currSegNo - segno < keep_segs)" is also the same.

Checkpoint calls the KeepLogSeg function, and there are many operations
between recptr and XLogGetReplicationSlotMinimumLSN, including updating the
pg_control file, so segno may be larger than currSegNo.

KeepLogSeg(XLogRecPtr recptr, XLogSegNo *logSegNo)
{
	XLogSegNo	currSegNo;
	XLogSegNo	segno;
	XLogRecPtr	keep;

	XLByteToSeg(recptr, currSegNo, wal_segment_size);
	segno = currSegNo;

	/*
	 * Calculate how many segments are kept by slots first, adjusting for
	 * max_slot_wal_keep_size.
	 */
	keep = XLogGetReplicationSlotMinimumLSN();
	if (keep != InvalidXLogRecPtr)
	{
		XLByteToSeg(keep, segno, wal_segment_size);

		/* Cap by max_slot_wal_keep_size ... */
		if (max_slot_wal_keep_size_mb >= 0)
		{
			uint64		slot_keep_segs;

			slot_keep_segs =
				ConvertToXSegs(max_slot_wal_keep_size_mb, wal_segment_size);

			if (currSegNo - segno > slot_keep_segs)
				segno = currSegNo - slot_keep_segs;
		}
	}

	/* but, keep at least wal_keep_size if that's set */
	if (wal_keep_size_mb > 0)
	{
		uint64		keep_segs;

		keep_segs = ConvertToXSegs(wal_keep_size_mb, wal_segment_size);
		if (currSegNo - segno < keep_segs)
		{
			/* avoid underflow, don't go below 1 */
			if (currSegNo <= keep_segs)
				segno = 1;
			else
				segno = currSegNo - keep_segs;
		}
	}


regards.

-- 
xu xingwang