Re: git grep -P fatal: pcre_exec failed with error code -8

["Ævar Arnfjörð Bjarmason" <avarab@gmail.com>]

On Mon, Nov 06 2017, Jeff King jotted:

> On Sun, Nov 05, 2017 at 10:41:17AM +0100, Дилян Палаузов wrote:
>
>> I understand that the PCRE's stack can get exhausted for some files, but in
>> such cases, git grep shall proceed with the other files, and print at the
>> end/stderr for which files the pattern was not applied.  Such behaviour
>> would be more usefull than the current one.
>
> Yes, I had a similar thought. It does feel a little funny for us to
> basically treat an error as "no match" for non-interactive use, but then
> the current behavior works out to be more or less the same (we return an
> error code which most shell scripts would interpret as failure).
>
> IOW, I think something like this is probably the right direction:
>
> diff --git a/grep.c b/grep.c
> index ce6a48e634..2c152e5908 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -427,7 +427,7 @@ static int pcre1match(struct grep_pat *p, const char *line, const char *eol,
>  	}
>
>  	if (ret < 0 && ret != PCRE_ERROR_NOMATCH)
> -		die("pcre_exec failed with error code %d", ret);
> +		warning("pcre_exec failed with error code %d", ret);
>  	if (ret > 0) {
>  		ret = 0;
>  		match->rm_so = ovector[0];
>
> but possibly:
>
>   1. It would be nice to report the filename that we couldn't match on.
>      But we don't know it at this level of the code (and it might not be
>      a file at all that we are matching). So probably we'd want to pass
>      the error much further up the call stack. This is tricky as there
>      are multiple regex libraries we can use, and the return value gets
>      normalized to 1/0 for hit/not-hit long before we get as far as
>      something that knows the filename.
>
>      We might need to do something invasive like adding an extra
>      parameter to hold the error message, and passing it through the
>      whole stack.
>
>   2. We should still try to exit with an exit code other than "1" to
>      indicate we hit an error besides "no lines were found".
>
>   3. Other regex libraries might need similar treatment. Probably
>      pcre2match() needs it. It doesn't look like regexec() can ever
>      return an error besides REG_NOMATCH.
>
> -Peff

Some replies to the thread in general, didn't want to spread this out
into different replies.

 * Yes this sucks.

 * Just emitting a warning without an appropriate exit code would suck
   more, would break batch jobs & whatnot that expcept certain results
   from grep.

 * As you point out it would be nice to print out the file name we
   didn't match on, we'd need to pass the grep_source struct down
   further, it goes as far as grep_source_1 but stops there and isn't
   passed to e.g. look_ahead(), which calls patmatch() which calls the
   engine-specific matcher and would need to report the error. We could
   just do this, would slow down things a bit (probably trivally) but we
   could emit better error messages in genreal.

 * You can adjust these limts in PCRE in Git, although it doesn't help
   in this case, you just add (*LIMIT_MATCH=NUM) or
   (*LIMIT_RECURSION=NUM) (or both) to the start of the pattern. See
   pcresyntax(3) or pcre2syntax(3) man pages depending on what version
   you have installed.

 * While regexec() won't return an error its version of dealing with
   this is (at least under glibc) to balloon CPU/memory use until the
   OOMkiller kills git (although not on this particular pattern).