[PATCH v3 1/2] read-cache: core.checksumindex

[git@jeffhostetler.com]

From: Jeff Hostetler <jeffhost@microsoft.com>

Teach git to skip verification of the index SHA in verify_hdr()
in read_index().

This is a performance optimization.  The index file SHA verification
can be considered an ancient relic from the early days of git and only
useful for detecting disk corruption.  For small repositories, this
SHA calculation is not that significant, but for gigantic repositories
this calculation adds significant time to every command.

Added "core.checksumindex" to enable/disable the SHA verification.

Signed-off-by: Jeff Hostetler <jeffhost@microsoft.com>
---
 read-cache.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/read-cache.c b/read-cache.c
index 9054369..2ab4b74 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1376,12 +1376,24 @@ static int verify_hdr(struct cache_header *hdr, unsigned long size)
 	git_SHA_CTX c;
 	unsigned char sha1[20];
 	int hdr_version;
+	int do_checksum = 0;
 
 	if (hdr->hdr_signature != htonl(CACHE_SIGNATURE))
 		return error("bad signature");
 	hdr_version = ntohl(hdr->hdr_version);
 	if (hdr_version < INDEX_FORMAT_LB || INDEX_FORMAT_UB < hdr_version)
 		return error("bad index version %d", hdr_version);
+
+	/*
+	 * Since we run very early in command startup, git_config()
+	 * may not have been called yet and the various "core_*"
+	 * global variables haven't been set.  So look it up
+	 * explicitly.
+	 */
+	git_config_get_bool("core.checksumindex", &do_checksum);
+	if (!do_checksum)
+		return 0;
+
 	git_SHA1_Init(&c);
 	git_SHA1_Update(&c, hdr, size - 20);
 	git_SHA1_Final(sha1, &c);
-- 
2.9.3