grep: stess test PCRE v2 on invalid UTF-8 data

Since my b65abcafc7 ("grep: use PCRE v2 for optimized fixed-string
search", 2019-07-01) we've been dying on invalid UTF-8 data when
grepping for fixed strings if the following are all true:

    * The subject string is non-ASCII (e.g. "ævar")
    * We're under a is_utf8_locale(), e.g. "en_US.UTF-8", not "C"
    * We compiled with PCRE v2
    * That PCRE v2 did not have JIT support

The last of those is why this wasn't caught earlier, per pcre2jit(3):

    "unless PCRE2_NO_UTF_CHECK is set, a UTF subject string is tested
    for validity. In the interests of speed, these checks do not
    happen on the JIT fast path, and if invalid data is passed, the
    result is undefined."

I.e. the subject being matched against our pattern was invalid, but we
were lucky and getting away with it on the JIT path, but the non-JIT
one is stricter.

This patch does nothing to fix that, instead we sneak in support for
fixed patterns starting with "(*NO_JIT)", this disables the PCRE v2
jit with implicit fixed-string matching for testing, see
pcre2syntax(3) the syntax.

This is technically a change in behavior, but it's so obscure that I
figured it was OK. We'd previously consider this an invalid regular
expression as regcomp() would die on it, now we feed it to the PCRE v2
fixed-string path. I thought this was better than introducing yet
another GIT_TEST_* environment variable.

We're also relying on a behavior of PCRE v2 that technically could
change, but I think the test coverage is worth dipping our toe into
some somewhat undefined behavior.

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

grep.c

@@ -615,6 +615,16 @@ static void compile_regexp(struct grep_pat *p, struct grep_opt *opt)
 		die(_("given pattern contains NULL byte (via -f <file>). This is only supported with -P under PCRE v2"));
 
 	p->is_fixed = is_fixed(p->pattern, p->patternlen);
+#ifdef USE_LIBPCRE2
+       if (!p->fixed && !p->is_fixed) {
+	       const char *no_jit = "(*NO_JIT)";
+	       const int no_jit_len = strlen(no_jit);
+	       if (starts_with(p->pattern, no_jit) &&
+		   is_fixed(p->pattern + no_jit_len,
+			    p->patternlen - no_jit_len))
+		       p->is_fixed = 1;
+       }
+#endif
 	if (p->fixed || p->is_fixed) {
 #ifdef USE_LIBPCRE2
 		opt->pcre2 = 1;

t/t7812-grep-icase-non-ascii.sh

@@ -53,4 +53,32 @@ test_expect_success REGEX_LOCALE 'pickaxe -i on non-ascii' '
 	test_cmp expected actual
 '
 
+test_expect_success GETTEXT_LOCALE,LIBPCRE2 'PCRE v2: setup invalid UTF-8 data' '
+	printf "\\200\\n" >invalid-0x80 &&
+	echo "ævar" >expected &&
+	cat expected >>invalid-0x80 &&
+	git add invalid-0x80
+'
+
+test_expect_success GETTEXT_LOCALE,LIBPCRE2 'PCRE v2: grep ASCII from invalid UTF-8 data' '
+	git grep -h "var" invalid-0x80 >actual &&
+	test_cmp expected actual &&
+	git grep -h "(*NO_JIT)var" invalid-0x80 >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success GETTEXT_LOCALE,LIBPCRE2 'PCRE v2: grep non-ASCII from invalid UTF-8 data' '
+	test_might_fail git grep -h "æ" invalid-0x80 >actual &&
+	test_cmp expected actual &&
+	test_must_fail git grep -h "(*NO_JIT)æ" invalid-0x80 &&
+	test_cmp expected actual
+'
+
+test_expect_success GETTEXT_LOCALE,LIBPCRE2 'PCRE v2: grep non-ASCII from invalid UTF-8 data with -i' '
+	test_might_fail git grep -hi "Æ" invalid-0x80 >actual &&
+	test_cmp expected actual &&
+	test_must_fail git grep -hi "(*NO_JIT)Æ" invalid-0x80 &&
+	test_cmp expected actual
+'
+
 test_done