Subject: [PATCH 5/6] Allow to use crc32 as a lighter checksum on index

From: Nguyễn Thái Ngọc Duy <[email-masked]>
Date: Mon, 6 Feb 2012 12:48:38 +0700

Signed-off-by: Nguyễn Thái Ngọc Duy <[email-masked]>
---
 Documentation/git-update-index.txt |   12 +++++++-
 builtin/update-index.c             |   11 +++++++
 cache.h                            |    2 +
 read-cache.c                       |   54 ++++++++++++++++++++++++++++--------
 4 files changed, 66 insertions(+), 13 deletions(-)

diff --git a/Documentation/git-update-index.txt b/Documentation/git-update-index.txt
index a3081f4..2574a4e 100644
--- a/Documentation/git-update-index.txt
+++ b/Documentation/git-update-index.txt
@@ -13,7 +13,7 @@ SYNOPSIS
 	     [--add] [--remove | --force-remove] [--replace]
 	     [--refresh] [-q] [--unmerged] [--ignore-missing]
 	     [(--cacheinfo <mode> <object> <file>)...]
-	     [--chmod=(+|-)x]
+	     [--chmod=(+|-)x] [--[no-]crc32]
 	     [--assume-unchanged | --no-assume-unchanged]
 	     [--skip-worktree | --no-skip-worktree]
 	     [--ignore-submodules]
@@ -109,6 +109,16 @@ you will need to handle the situation manually.
 	set and unset the "skip-worktree" bit for the paths. See
 	section "Skip-worktree bit" below for more information.
 
+--crc32::
+--no-crc32::
+	Normally SHA-1 is used to check for index integrity. When the
+	index is large, SHA-1 computation cost can be significant.
+	--crc32 will convert current index to use (cheaper) crc32
+	instead. Note that later writes to index by other commands can
+	convert the index back to SHA-1. Older git versions may not
+	understand crc32 index, --no-crc32 can be used to convert it
+	back to SHA-1.
+
 -g::
 --again::
 	Runs 'git update-index' itself on the paths whose index
diff --git a/builtin/update-index.c b/builtin/update-index.c
index a6a23fa..6913226 100644
--- a/builtin/update-index.c
+++ b/builtin/update-index.c
@@ -707,6 +707,7 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 {
 	int newfd, entries, has_errors = 0, line_termination = '\n';
 	int read_from_stdin = 0;
+	int do_crc = -1;
 	int prefix_length = prefix ? strlen(prefix) : 0;
 	char set_executable_bit = 0;
 	struct refresh_params refresh_args = {0, &has_errors};
@@ -791,6 +792,8 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 			"(for porcelains) forget saved unresolved conflicts",
 			PARSE_OPT_NOARG | PARSE_OPT_NONEG,
 			resolve_undo_clear_callback},
+		OPT_BOOL(0, "crc32", &do_crc,
+			 "use crc32 as checksum instead of sha1"),
 		OPT_END()
 	};
 
@@ -852,6 +855,14 @@ int cmd_update_index(int argc, const char **argv, const char *prefix)
 	}
 	argc = parse_options_end(&ctx);
 
+	if (do_crc != -1) {
+		if (do_crc)
+			the_index.hdr_flags |= CACHE_F_CRC;
+		else
+			the_index.hdr_flags &= ~CACHE_F_CRC;
+		active_cache_changed = 1;
+	}
+
 	if (read_from_stdin) {
 		struct strbuf buf = STRBUF_INIT, nbuf = STRBUF_INIT;
 
diff --git a/cache.h b/cache.h
index c2e884a..7352402 100644
--- a/cache.h
+++ b/cache.h
@@ -105,6 +105,8 @@ struct cache_header {
 	unsigned int hdr_entries;
 };
 
+#define CACHE_F_CRC	1	/* use crc32 instead of sha1 for index checksum */
+
 struct ext_cache_header {
 	struct cache_header h;
 	unsigned int hdr_flags;
diff --git a/read-cache.c b/read-cache.c
index fd21af6..a34878e 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1185,20 +1185,33 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
 
 static int verify_hdr(struct cache_header *hdr, unsigned long size)
 {
+	int do_crc;
 	git_SHA_CTX c;
 	unsigned char sha1[20];
 
 	if (hdr->hdr_signature != htonl(CACHE_SIGNATURE))
 		return error("bad signature");
-	if (hdr->hdr_version != htonl(2) &&
-	    hdr->hdr_version != htonl(3) &&
-	    hdr->hdr_version != htonl(4))
+	if (hdr->hdr_version == htonl(2) ||
+	    hdr->hdr_version == htonl(3))
+		do_crc = 0;
+	else if (hdr->hdr_version == htonl(4)) {
+		struct ext_cache_header *ehdr = (struct ext_cache_header *)hdr;
+		do_crc = ntohl(ehdr->hdr_flags) & CACHE_F_CRC;
+	}
+	else
 		return error("bad index version");
-	git_SHA1_Init(&c);
-	git_SHA1_Update(&c, hdr, size - 20);
-	git_SHA1_Final(sha1, &c);
-	if (hashcmp(sha1, (unsigned char *)hdr + size - 20))
-		return error("bad index file sha1 signature");
+	if (do_crc) {
+		uint32_t crc = crc32(0, NULL, 0);
+		crc = crc32(crc,(void *) hdr, size - sizeof(uint32_t));
+		if (crc != *(uint32_t*)((unsigned char *)hdr + size - sizeof(uint32_t)))
+			return error("bad index file crc32 signature");
+	} else {
+		git_SHA1_Init(&c);
+		git_SHA1_Update(&c, hdr, size - 20);
+		git_SHA1_Final(sha1, &c);
+		if (hashcmp(sha1, (unsigned char *)hdr + size - 20))
+			return error("bad index file sha1 signature");
+	}
 	return 0;
 }
 
@@ -1421,11 +1434,24 @@ static int write_index_ext_header(struct sha1file *f,
 static int ce_flush(struct sha1file *f)
 {
 	unsigned char sha1[20];
-	int fd = sha1close(f, sha1, 0);
+	int fd;
 
-	if (fd < 0)
-		return -1;
-	return (write_in_full(fd, sha1, 20) != 20) ? -1 : 0;
+	if (f->do_crc) {
+		uint32_t crc;
+
+		assert(f->do_sha1 == 0);
+		sha1flush(f);
+		crc = crc32_end(f);
+		fd = sha1close(f, sha1, 0);
+		if (fd < 0)
+			return -1;
+		return (write_in_full(fd, &crc, sizeof(crc)) != sizeof(crc)) ? -1 : 0;
+	} else {
+		fd = sha1close(f, sha1, 0);
+		if (fd < 0)
+			return -1;
+		return (write_in_full(fd, sha1, 20) != 20) ? -1 : 0;
+	}
 }
 
 static void ce_smudge_racily_clean_entry(struct cache_entry *ce)
@@ -1568,6 +1594,10 @@ int write_index(struct index_state *istate, int newfd)
 	hdr.h.hdr_entries = htonl(entries - removed);
 
 	f = sha1fd(newfd, NULL);
+	if (istate->hdr_flags & CACHE_F_CRC) {
+		crc32_begin(f);
+		f->do_sha1 = 0;
+	}
 	if (ce_write(f, &hdr, hdr_size) < 0)
 		return -1;
 
-- 
1.7.8.36.g69ee2