[PATCH] grep: Fix race condition in delta_base_cache

[Nicolas Morey-Chaisemartin <devel-git@morey-chaisemartin.com>]

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When running large git grep (ie: git grep regexp $(git rev-list --all)), glibc error sometimes occur:
*** glibc detected *** git: double free or corruption (!prev): 0x00000000010abdf0 ***

According to gdb the problem originate from release_delta_cash (sha1_file.c:1703)
		free(ent->data);

>From my analysis it seems that git grep threads do acquire lock before calling read_sha1_file but not before calling
read_object_with_reference who ends up calling read_sha1_file too.

Adding the lock around read_object_with_reference seems to fix the issue for me.
I've ran git grep about a dozen time and seen no more error while
it usually happened half the time before.

Signed-off-by: Nicolas Morey-Chaisemartin <nicolas@morey-chaisemartin.com>
---
 builtin/grep.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)



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diff --git a/builtin/grep.c b/builtin/grep.c
index 1c359c2..56398d5 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -598,8 +598,10 @@ static int grep_object(struct grep_opt *opt, const struct pathspec *pathspec,
 		struct strbuf base;
 		int hit, len;
 
+		read_sha1_lock();
 		data = read_object_with_reference(obj->sha1, tree_type,
 						  &size, NULL);
+		read_sha1_unlock();
 		if (!data)
 			die(_("unable to read tree (%s)"), sha1_to_hex(obj->sha1));