[PATCH/RFC v2 01/16] Modify cache_header to prepare for other index formats

[Thomas Gummerer <t.gummerer@gmail.com>]

Modify the cache_header such that other index file formats
can be added and reusing the common part of each index format.

The signature and version have to be present in every
version of the index file format, to check if it can be read
by a specific version of git, while other entries (eg. number
of entries for index v2/3/4) can be different from one file
format to another. Therefore it is split to its own struct.

The structs are also moved to read-cache.c, since they are
not used in any other place except test-index-version, where
cache_version_header is redefined.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
---
 cache.h              |    8 --------
 read-cache.c         |   32 +++++++++++++++++++++++++-------
 test-index-version.c |    7 ++++++-
 3 files changed, 31 insertions(+), 16 deletions(-)

diff --git a/cache.h b/cache.h
index 67f28b4..9bfc9f3 100644
--- a/cache.h
+++ b/cache.h
@@ -94,16 +94,8 @@ unsigned long git_deflate_bound(git_zstream *, unsigned long);
  */
 #define DEFAULT_GIT_PORT 9418
 
-/*
- * Basic data structures for the directory cache
- */
 
 #define CACHE_SIGNATURE 0x44495243	/* "DIRC" */
-struct cache_header {
-	unsigned int hdr_signature;
-	unsigned int hdr_version;
-	unsigned int hdr_entries;
-};
 
 #define INDEX_FORMAT_LB 2
 #define INDEX_FORMAT_UB 4
diff --git a/read-cache.c b/read-cache.c
index 2f8159f..5d61d92 100644
--- a/read-cache.c
+++ b/read-cache.c
@@ -1198,6 +1198,18 @@ static struct cache_entry *refresh_cache_entry(struct cache_entry *ce, int reall
 #define INDEX_FORMAT_DEFAULT 3
 
 /*
+ * Basic data structures for the directory cache
+ */
+struct cache_version_header {
+	unsigned int hdr_signature;
+	unsigned int hdr_version;
+};
+
+struct cache_header_v2 {
+	unsigned int hdr_entries;
+};
+
+/*
  * dev/ino/uid/gid/size are also just tracked to the low 32 bits
  * Again - this is just a (very strong in practice) heuristic that
  * the inode hasn't changed.
@@ -1247,7 +1259,7 @@ struct ondisk_cache_entry_extended {
 			    ondisk_cache_entry_extended_size(ce_namelen(ce)) : \
 			    ondisk_cache_entry_size(ce_namelen(ce)))
 
-static int verify_hdr(struct cache_header *hdr, unsigned long size)
+static int verify_hdr(struct cache_version_header *hdr, unsigned long size)
 {
 	git_SHA_CTX c;
 	unsigned char sha1[20];
@@ -1409,7 +1421,8 @@ int read_index_from(struct index_state *istate, const char *path)
 	int fd, i;
 	struct stat st;
 	unsigned long src_offset;
-	struct cache_header *hdr;
+	struct cache_version_header *hdr;
+	struct cache_header_v2 *hdr_v2;
 	void *mmap;
 	size_t mmap_size;
 	struct strbuf previous_name_buf = STRBUF_INIT, *previous_name;
@@ -1433,7 +1446,7 @@ int read_index_from(struct index_state *istate, const char *path)
 
 	errno = EINVAL;
 	mmap_size = xsize_t(st.st_size);
-	if (mmap_size < sizeof(struct cache_header) + 20)
+	if (mmap_size < sizeof(struct cache_version_header) + 20)
 		die("index file smaller than expected");
 
 	mmap = xmmap(NULL, mmap_size, PROT_READ | PROT_WRITE, MAP_PRIVATE, fd, 0);
@@ -1442,11 +1455,13 @@ int read_index_from(struct index_state *istate, const char *path)
 		die_errno("unable to map index file");
 
 	hdr = mmap;
+	hdr_v2 =  mmap + sizeof(*hdr);
 	if (verify_hdr(hdr, mmap_size) < 0)
 		goto unmap;
 
+	hdr_v2 = mmap + sizeof(*hdr);
 	istate->version = ntohl(hdr->hdr_version);
-	istate->cache_nr = ntohl(hdr->hdr_entries);
+	istate->cache_nr = ntohl(hdr_v2->hdr_entries);
 	istate->cache_alloc = alloc_nr(istate->cache_nr);
 	istate->cache = xcalloc(istate->cache_alloc, sizeof(struct cache_entry *));
 	istate->initialized = 1;
@@ -1456,7 +1471,7 @@ int read_index_from(struct index_state *istate, const char *path)
 	else
 		previous_name = NULL;
 
-	src_offset = sizeof(*hdr);
+	src_offset = sizeof(*hdr) + sizeof(*hdr_v2);
 	for (i = 0; i < istate->cache_nr; i++) {
 		struct ondisk_cache_entry *disk_ce;
 		struct cache_entry *ce;
@@ -1757,7 +1772,8 @@ void update_index_if_able(struct index_state *istate, struct lock_file *lockfile
 int write_index(struct index_state *istate, int newfd)
 {
 	git_SHA_CTX c;
-	struct cache_header hdr;
+	struct cache_version_header hdr;
+	struct cache_header_v2 hdr_v2;
 	int i, err, removed, extended, hdr_version;
 	struct cache_entry **cache = istate->cache;
 	int entries = istate->cache_nr;
@@ -1787,11 +1803,13 @@ int write_index(struct index_state *istate, int newfd)
 
 	hdr.hdr_signature = htonl(CACHE_SIGNATURE);
 	hdr.hdr_version = htonl(hdr_version);
-	hdr.hdr_entries = htonl(entries - removed);
+	hdr_v2.hdr_entries = htonl(entries - removed);
 
 	git_SHA1_Init(&c);
 	if (ce_write(&c, newfd, &hdr, sizeof(hdr)) < 0)
 		return -1;
+	if (ce_write(&c, newfd, &hdr_v2, sizeof(hdr_v2)) < 0)
+		return -1;
 
 	previous_name = (hdr_version == 4) ? &previous_name_buf : NULL;
 	for (i = 0; i < entries; i++) {
diff --git a/test-index-version.c b/test-index-version.c
index bfaad9e..3899a2f 100644
--- a/test-index-version.c
+++ b/test-index-version.c
@@ -1,8 +1,13 @@
 #include "cache.h"
 
+struct cache_version_header {
+	unsigned int hdr_signature;
+	unsigned int hdr_version;
+};
+
 int main(int argc, const char **argv)
 {
-	struct cache_header hdr;
+	struct cache_version_header hdr;
 	int version;
 
 	memset(&hdr,0,sizeof(hdr));
-- 
1.7.10.GIT