read-cache: force_verify_index_checksum

Teach git to skip verification of the SHA1-1 checksum at the end of
the index file in verify_hdr() which is called from read_index()
unless the "force_verify_index_checksum" global variable is set.

Teach fsck to force this verification.

The checksum verification is for detecting disk corruption, and for
small projects, the time it takes to compute SHA-1 is not that
significant, but for gigantic repositories this calculation adds
significant time to every command.

These effect can be seen using t/perf/p0002-read-cache.sh:

Test                                          HEAD~1            HEAD
--------------------------------------------------------------------------------------
0002.1: read_cache/discard_cache 1000 times   0.66(0.44+0.20)   0.30(0.27+0.02) -54.5%

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/cache.h b/cache.h
index fbdf7a8..ed3c592 100644
--- a/cache.h
+++ b/cache.h
@@ -705,6 +705,8 @@ extern void update_index_if_able(struct index_state *, struct lock_file *);
 extern int hold_locked_index(struct lock_file *, int);
 extern void set_alternate_index_output(const char *);
 
+extern int verify_index_checksum;
+
 /* Environment bits from configuration mechanism */
 extern int trust_executable_bit;
 extern int trust_ctime;