grep: fix segfault under -P + PCRE2 <=10.30 + (*NO_JIT)

Fix a bug in the compilation of PCRE2 patterns under JIT (the most
common runtime configuration). Any pattern with a (*NO_JIT) verb would
segfault in any currently released PCRE2 version:

    $ git grep -P '(*NO_JIT)hi.*there'
    Segmentation fault

That this segfaulted was a bug in PCRE2 itself, after reporting it[1]
on pcre-dev it's been fixed in a yet-to-be-released version of
PCRE (presumably released first as 10.31). Now it'll die with:

    $ git grep -P '(*NO_JIT)hi.*there'
    fatal: pcre2_jit_match failed with error code -45: bad JIT option

But the cause of the bug is in our own code dating back to my
94da919 ("grep: add support for PCRE v2", 2017-06-01).

As explained at more length in the comment being added here, it isn't
sufficient to just check pcre2_config() to see whether the JIT should
be used, pcre2_pattern_info() also has to be asked.

This is something I discovered myself when fiddling around with PCRE2
verbs in patterns passed to git. I don't expect that any user of git
has encountered this given the obscurity of passing PCRE2 verbs
through to the library, along with the relative obscurity of (*NO_JIT)
itself.

1. "How am I supposed to use PCRE2 JIT in the face of (*NO_JIT) ?"
   (<CACBZZX5mMqDuWuFmi7sRBp3wH6CFyd-ghACukd=v0NN=rBMnJg@mail.gmail.com> &
    https://lists.exim.org/lurker/thread/20171123.101502.7f0d38ca.en.html)
   on the pcre-dev mailing list

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

grep.c

@@ -477,6 +477,8 @@ static void compile_pcre2_pattern(struct grep_pat *p, const struct grep_opt *opt
 	int options = PCRE2_MULTILINE;
 	const uint8_t *character_tables = NULL;
 	int jitret;
+	int patinforet;
+	size_t jitsizearg;
 
 	assert(opt->pcre2);
 
@@ -511,6 +513,30 @@ static void compile_pcre2_pattern(struct grep_pat *p, const struct grep_opt *opt
 		jitret = pcre2_jit_compile(p->pcre2_pattern, PCRE2_JIT_COMPLETE);
 		if (jitret)
 			die("Couldn't JIT the PCRE2 pattern '%s', got '%d'\n", p->pattern, jitret);
+
+		/*
+		 * The pcre2_config(PCRE2_CONFIG_JIT, ...) call just
+		 * tells us whether the library itself supports JIT,
+		 * but to see whether we're going to be actually using
+		 * JIT we need to extract PCRE2_INFO_JITSIZE from the
+		 * pattern *after* we do pcre2_jit_compile() above.
+		 *
+		 * This is because if the pattern contains the
+		 * (*NO_JIT) verb (see pcre2syntax(3))
+		 * pcre2_jit_compile() will exit early with 0. If we
+		 * then proceed to call pcre2_jit_match() further down
+		 * the line instead of pcre2_match() we'll either
+		 * segfault (pre PCRE 10.31) or run into a fatal error
+		 * (post PCRE2 10.31)
+		 */
+		patinforet = pcre2_pattern_info(p->pcre2_pattern, PCRE2_INFO_JITSIZE, &jitsizearg);
+		if (patinforet)
+			BUG("pcre2_pattern_info() failed: %d", patinforet);
+		if (jitsizearg == 0) {
+			p->pcre2_jit_on = 0;
+			return;
+		}
+
 		p->pcre2_jit_stack = pcre2_jit_stack_create(1, 1024 * 1024, NULL);
 		if (!p->pcre2_jit_stack)
 			die("Couldn't allocate PCRE2 JIT stack");

t/t7810-grep.sh

@@ -1110,6 +1110,12 @@ test_expect_success PCRE 'grep -P pattern' '
 	test_cmp expected actual
 '
 
+test_expect_success LIBPCRE2 "grep -P with (*NO_JIT) doesn't error out" '
+	git grep -P "(*NO_JIT)\p{Ps}.*?\p{Pe}" hello.c >actual &&
+	test_cmp expected actual
+
+'
+
 test_expect_success !PCRE 'grep -P pattern errors without PCRE' '
 	test_must_fail git grep -P "foo.*bar"
 '