Improve the support for invalid UTF-8 haystacks given a non-ASCII
needle when using the PCREv2 backend.

This is a more complete fix for a bug I started to fix in
870eea8166 (grep: do not enter PCRE2_UTF mode on fixed matching,
2019-07-26), now that PCREv2 has the PCRE2_MATCH_INVALID_UTF mode we
can make use of it.

This fixes the sort of case described in 8a5999838e (grep: stess test
PCRE v2 on invalid UTF-8 data, 2019-07-26), i.e.:

    - 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 are using --ignore-case, or we're a non-fixed pattern

If those conditions were satisfied and we matched found non-valid
UTF-8 data PCREv2 might bark on it, in practice this only happened
under the JIT backend (turned on by default on most platforms).

Ultimately this fixes a "regression" in b65abcafc7 ("grep: use PCRE v2
for optimized fixed-string search", 2019-07-01), I'm putting that in
scare-quotes because before then we wouldn't properly support these
complex case-folding, locale etc. cases either, it just broke in
different ways.

There was a bug related to this the PCRE2_NO_START_OPTIMIZE flag fixed
in PCREv2 10.36. It can be worked around by setting the
PCRE2_NO_START_OPTIMIZE flag. Let's do that in those cases, and add
tests for the bug.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
---
 Makefile                        |  1 +
 grep.c                          |  8 +++++-
 grep.h                          |  4 +++
 t/helper/test-pcre2-config.c    | 12 +++++++++
 t/helper/test-tool.c            |  1 +
 t/helper/test-tool.h            |  1 +
 t/t7812-grep-icase-non-ascii.sh | 46 ++++++++++++++++++++++++++++++++-
 7 files changed, 71 insertions(+), 2 deletions(-)
 create mode 100644 t/helper/test-pcre2-config.c

diff --git a/Makefile b/Makefile
index 4edfda3e00..42a7ed96e2 100644
--- a/Makefile
+++ b/Makefile

@@ -722,6 +722,7 @@ TEST_BUILTINS_OBJS += test-online-cpus.o
 TEST_BUILTINS_OBJS += test-parse-options.o
 TEST_BUILTINS_OBJS += test-parse-pathspec-file.o
 TEST_BUILTINS_OBJS += test-path-utils.o
+TEST_BUILTINS_OBJS += test-pcre2-config.o
 TEST_BUILTINS_OBJS += test-pkt-line.o
 TEST_BUILTINS_OBJS += test-prio-queue.o
 TEST_BUILTINS_OBJS += test-proc-receive.o

diff --git a/grep.c b/grep.c
index efeb6dc58d..e329f19877 100644
--- a/grep.c
+++ b/grep.c

@@ -492,7 +492,13 @@ static void compile_pcre2_pattern(struct grep_pat *p, const struct grep_opt *opt
 	}
 	if (!opt->ignore_locale && is_utf8_locale() && has_non_ascii(p->pattern) &&
 	    !(!opt->ignore_case && (p->fixed || p->is_fixed)))
-		options |= PCRE2_UTF;
+		options |= (PCRE2_UTF | PCRE2_MATCH_INVALID_UTF);
+
+	if (PCRE2_MATCH_INVALID_UTF &&
+	    options & (PCRE2_UTF | PCRE2_CASELESS) &&
+	    !(PCRE2_MAJOR >= 10 && PCRE2_MAJOR >= 36))