[PATCH 4/6] Introduce index version 4 with global flags

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

v4 adds 32-bit field to cache header after 32-bit number of entries.
If this field is zero, fall back to v3.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Documentation/technical/index-format.txt |    4 ++-
 cache.h                                  |    6 +++++
 read-cache.c                             |   31 ++++++++++++++++++++++-------
 3 files changed, 32 insertions(+), 9 deletions(-)

diff --git a/Documentation/technical/index-format.txt b/Documentation/technical/index-format.txt
index 8930b3f..2b6a38e 100644
--- a/Documentation/technical/index-format.txt
+++ b/Documentation/technical/index-format.txt
@@ -12,10 +12,12 @@ GIT index format
        The signature is { 'D', 'I', 'R', 'C' } (stands for "dircache")
 
      4-byte version number:
-       The current supported versions are 2 and 3.
+       The current supported versions are 2, 3 and 4.
 
      32-bit number of index entries.
 
+     32-bit flags (version 4 only).
+
    - A number of sorted index entries (see below).
 
    - Extensions
diff --git a/cache.h b/cache.h
index 9bd8c2d..c2e884a 100644
--- a/cache.h
+++ b/cache.h
@@ -105,6 +105,11 @@ struct cache_header {
 	unsigned int hdr_entries;
 };
 
+struct ext_cache_header {
+	struct cache_header h;
+	unsigned int hdr_flags;
+};
+
 /*
  * The "cache_time" is just the low 32 bits of the
  * time. It doesn't matter if it overflows - we only
@@ -314,6 +319,7 @@ static inline unsigned int canon_mode(unsigned int mode)
 struct index_state {
 	struct cache_entry **cache;
 	unsigned int cache_nr, cache_alloc, cache_changed;
+	unsigned int hdr_flags;
 	struct string_list *resolve_undo;
 	struct cache_tree *cache_tree;
 	struct cache_time timestamp;
diff --git a/read-cache.c b/read-cache.c
index fe6b0e0..fd21af6 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1190,7 +1190,9 @@ static int verify_hdr(struct cache_header *hdr, unsigned long size)
 
 	if (hdr->hdr_signature != htonl(CACHE_SIGNATURE))
 		return error("bad signature");
-	if (hdr->hdr_version != htonl(2) && hdr->hdr_version != htonl(3))
+	if (hdr->hdr_version != htonl(2) &&
+	    hdr->hdr_version != htonl(3) &&
+	    hdr->hdr_version != htonl(4))
 		return error("bad index version");
 	git_SHA1_Init(&c);
 	git_SHA1_Update(&c, hdr, size - 20);
@@ -1320,7 +1322,12 @@ int read_index_from(struct index_state *istate, const char *path)
 	istate->cache = xcalloc(istate->cache_alloc, sizeof(struct cache_entry *));
 	istate->initialized = 1;
 
-	src_offset = sizeof(*hdr);
+	if (ntohl(hdr->hdr_version) >= 4) {
+		struct ext_cache_header *ehdr = mmap;
+		istate->hdr_flags = ntohl(ehdr->hdr_flags);
+		src_offset = sizeof(*ehdr);
+	} else
+		src_offset = sizeof(*hdr);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct ondisk_cache_entry *disk_ce;
 		struct cache_entry *ce;
@@ -1375,6 +1382,7 @@ int discard_index(struct index_state *istate)
 	resolve_undo_clear_index(istate);
 	istate->cache_nr = 0;
 	istate->cache_changed = 0;
+	istate->hdr_flags = 0;
 	istate->timestamp.sec = 0;
 	istate->timestamp.nsec = 0;
 	istate->name_hash_initialized = 0;
@@ -1531,8 +1539,8 @@ void update_index_if_able(struct index_state *istate, struct lock_file *lockfile
 int write_index(struct index_state *istate, int newfd)
 {
 	struct sha1file *f;
-	struct cache_header hdr;
-	int i, err, removed;
+	struct ext_cache_header hdr;
+	int i, err, removed, hdr_size;
 	struct cache_entry **cache = istate->cache;
 	int entries = istate->cache_nr;
 	struct stat st;
@@ -1548,12 +1556,19 @@ int write_index(struct index_state *istate, int newfd)
 		}
 	}
 
-	hdr.hdr_signature = htonl(CACHE_SIGNATURE);
-	hdr.hdr_version = htonl(3);
-	hdr.hdr_entries = htonl(entries - removed);
+	hdr.h.hdr_signature = htonl(CACHE_SIGNATURE);
+	if (istate->hdr_flags) {
+		hdr.h.hdr_version = htonl(4);
+		hdr.hdr_flags = htonl(istate->hdr_flags);
+		hdr_size = sizeof(hdr);
+	} else {
+		hdr.h.hdr_version = htonl(3);
+		hdr_size = sizeof(hdr.h);
+	}
+	hdr.h.hdr_entries = htonl(entries - removed);
 
 	f = sha1fd(newfd, NULL);
-	if (ce_write(f, &hdr, sizeof(hdr)) < 0)
+	if (ce_write(f, &hdr, hdr_size) < 0)
 		return -1;
 
 	for (i = 0; i < entries; i++) {
-- 
1.7.8.36.g69ee2