Horton

Re: BUG #18708: regex problem: (?:[^\d\D]){0} asserts with "lp->nouts == 0 && rp->nins == 0"

Tom Lane <tgl@sss.pgh.pa.us>

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From: Tom Lane <tgl@sss.pgh.pa.us>
To: Alexander Lakhin <exclusion@gmail.com>
Cc: pgsql-bugs@lists.postgresql.org, Aleksander Alekseev <aleksander@timescale.com>, dhyan@nataraj.su
Date: 2024-11-17T06:26:38Z
Lists: pgsql-bugs 
I wrote:
> Alexander Lakhin <exclusion@gmail.com> writes:
>> (Un)fortunately, tern (which is also a ppc animal) has produced the same
>> failure:
>> https://buildfarm.postgresql.org/cgi-bin/show_log.pl?nm=tern&dt=2024-11-16%2022%3A00%3A12

> Yeah, I saw that.  Even more confused now about what it could be.

After testing on hornet's host, it seems that this is a pre-existing
issue that we didn't happen to hit before.  Since the regex
'[^\d\D]' is unsatisfiable, it collapses to nothing (start state,
end state, and no arcs) in the first cleanup() call in optimize().
Then fixempties() counts the number of in-arcs and gets zero,
and then it does

	arcarray = (struct arc **) MALLOC(totalinarcs * sizeof(struct arc *));
	if (arcarray == NULL)
	{
		NERR(REG_ESPACE);
		...

On a machine where malloc(0) returns NULL, this mistakenly
thinks that's an error.

I verified that

-	if (arcarray == NULL)
+	if (arcarray == NULL && totalinarcs != 0)

makes the failure go away, but I wonder if any other places in
backend/regex/ are at the same hazard.  Maybe the smartest fix
would be to put in a wrapper layer that does what pg_malloc
does:

	/* Avoid unportable behavior of malloc(0) */
	if (size == 0)
		size = 1;

One other point is that this theory fails to explain why
hornet didn't fail in the v16 branch ... oh, wait:
v15 has 

#define MALLOC(n)		malloc(n)

where later branches have

#define MALLOC(n)		palloc_extended((n), MCXT_ALLOC_NO_OOM)

So the right answer seems to be to figure out why we didn't
back-patch that change.

			regards, tom lane

Commits

  1. Fix recently-exposed portability issue in regex optimization.

  2. Avoid assertion due to disconnected NFA sub-graphs in regex parsing.

  3. Fix recovery conflict SIGUSR1 handling.

  4. Redesign interrupt/cancel API for regex engine.

  5. Use MemoryContext API for regex memory management.

  6. Invent "rainbow" arcs within the regex engine.