grep/pcre2: move back to thread-only PCREv2 structures

Change the setup of the "pcre2_general_context" to happen per-thread
in compile_pcre2_pattern() instead of in grep_init().

This change brings it in line with how the rest of the pcre2_* members
in the grep_pat structure are set up.

As noted in the preceding commit the approach 513f2b0bbd4 (grep: make
PCRE2 aware of custom allocator, 2019-10-16) took to allocate the
pcre2_general_context seems to have been initially based on a
misunderstanding of how PCREv2 memory allocation works.

The approach of creating a global context in grep_init() is just added
complexity for almost zero gain. On my system it's 24 bytes saved
per-thread. For comparison PCREv2 will then go on to allocate at least
a kilobyte for its own thread-local state.

As noted in 6d423dd542f (grep: don't redundantly compile throwaway
patterns under threading, 2017-05-25) the grep code is intentionally
not trying to micro-optimize allocations by e.g. sharing some PCREv2
structures globally, while making others thread-local.

So let's remove this special case and make all of them thread-local
again for simplicity. With this change we could move the
pcre2_{malloc,free} functions around to live closer to their current
use. I'm not doing that here to keep this change small, that cleanup
will be done in a follow-up commit.

See also the discussion in 94da9193a6 (grep: add support for PCRE v2,
2017-06-01) about thread safety, and Johannes's comments[1] to the
effect that we should be doing what this patch is doing.

1. https://lore.kernel.org/git/nycvar.QRO.7.76.6.1908052120302.46@tvgsbejvaqbjf.bet/

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

1 files changed, 0 insertions, 1 deletions

diff --git a/builtin/grep.c b/builtin/grep.c
index 55d06c9513..c69fe99340 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -1175,6 +1175,5 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 		run_pager(&opt, prefix);
 	clear_pathspec(&pathspec);
 	free_grep_patterns(&opt);
-	grep_destroy();
 	return !hit;
 }