pack-objects: use of version 3 delta is now optional.

This introduces a new configuration item, pack.deltaversion, to
control whether pack-objects is allowed to use version 3 delta.
By default, we keep generating version 2 delta (and version 2
packfile format) to be compatible with git earlier than v1.2.0.

This configuration affects the command in the following ways:

 - the resulting packfile will have the specified version;

 - when generating delta, larger copies are allowed only when
   deltaversion is 3;

 - the logic to reuse delta from existing packs refuses to reuse
   delta from packs that uses delta version 3 when the
   configuration is set to 2.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-pack-objects.c

@@ -456,7 +456,7 @@ static void write_pack_file(void)
 		fprintf(stderr, "Writing %d objects.\n", nr_result);
 
 	hdr.hdr_signature = htonl(PACK_SIGNATURE);
-	hdr.hdr_version = htonl(PACK_VERSION);
+	hdr.hdr_version = htonl(delta_version);
 	hdr.hdr_entries = htonl(nr_result);
 	sha1write(f, &hdr, sizeof(hdr));
 	offset = sizeof(hdr);
@@ -914,12 +914,15 @@ static void check_object(struct object_entry *entry)
 		/* Check if it is delta, and the base is also an object
 		 * we are going to pack.  If so we will reuse the existing
 		 * delta.
+		 *
+		 * Also make sure that we do not reuse delta from an existing
+		 * pack that uses higher delta version than allowed.
 		 */
 		if (!no_reuse_delta &&
 		    entry->in_pack_type == OBJ_DELTA &&
 		    (base_entry = locate_object_entry(base)) &&
-		    (!base_entry->preferred_base)) {
-
+		    (!base_entry->preferred_base) &&
+		    entry->in_pack->pack_version <= delta_version) {
 			/* Depth value does not matter - find_deltas()
 			 * will never consider reused delta as the
 			 * base object to deltify other objects
@@ -1101,6 +1104,7 @@ static int try_delta(struct unpacked *trg, struct unpacked *src,
 	 * on an earlier try, but only when reusing delta data.
 	 */
 	if (!no_reuse_delta && trg_entry->in_pack &&
+	    trg_entry->in_pack->pack_version <= delta_version &&
 	    trg_entry->in_pack == src_entry->in_pack)
 		return 0;
 
@@ -1326,10 +1330,15 @@ static void setup_progress_signal(void)
 
 static int git_pack_config(const char *k, const char *v)
 {
-	if(!strcmp(k, "pack.window")) {
+	if (!strcmp(k, "pack.window")) {
 		window = git_config_int(k, v);
 		return 0;
 	}
+	if (!strcmp(k, "pack.deltaversion")) {
+		delta_version = git_config_int(k, v);
+		if (!pack_version_ok(htonl(delta_version)))
+			die("value %s for '%s' not allowed", v, k);
+	}
 	return git_default_config(k, v);
 }
 

cache.h

@@ -337,6 +337,7 @@ extern struct packed_git {
 	unsigned int pack_last_used;
 	unsigned int pack_use_cnt;
 	int pack_local;
+	int pack_version;
 	unsigned char sha1[20];
 	/* something like ".git/objects/pack/xxxxx.pack" */
 	char pack_name[FLEX_ARRAY]; /* more */

delta.h

@@ -1,6 +1,8 @@
 #ifndef DELTA_H
 #define DELTA_H
 
+extern int delta_version;
+
 /* opaque object for delta index */
 struct delta_index;
 

diff-delta.c

@@ -253,10 +253,13 @@ create_delta(const struct delta_index *index,
 	int inscnt;
 	const unsigned char *ref_data, *ref_top, *data, *top;
 	unsigned char *out;
+	unsigned int ref_size_limit;
 
 	if (!trg_buf || !trg_size)
 		return NULL;
 
+	ref_size_limit = (delta_version > 2) ? 0xffffff : 0x10000;
+
 	outpos = 0;
 	outsize = 8192;
 	if (max_size && outsize >= max_size)
@@ -308,8 +311,8 @@ create_delta(const struct delta_index *index,
 				continue;
 			if (ref_size > top - src)
 				ref_size = top - src;
-			if (ref_size > 0x10000)
-				ref_size = 0x10000;
+			if (ref_size > ref_size_limit)
+				ref_size = ref_size_limit;
 			if (ref_size <= msize)
 				break;
 			while (ref_size-- && *src++ == *ref)
@@ -318,6 +321,8 @@ create_delta(const struct delta_index *index,
 				/* this is our best match so far */
 				msize = ref - entry->ptr;
 				moff = entry->ptr - ref_data;
+				if (delta_version > 2 && msize >= 0x10000)
+					break; /* this is good enough */
 			}
 		}
 
@@ -381,6 +386,12 @@ create_delta(const struct delta_index *index,
 			if (msize & 0xff) { out[outpos++] = msize; i |= 0x10; }
 			msize >>= 8;
 			if (msize & 0xff) { out[outpos++] = msize; i |= 0x20; }
+			if (delta_version > 2) {
+				msize >>= 8;
+				if (msize & 0xff) {
+					out[outpos++] = msize; i |= 0x40;
+				}
+			}
 
 			*op = i;
 		}

environment.c

@@ -8,6 +8,7 @@
  * are.
  */
 #include "cache.h"
+#include "pack.h"
 
 char git_default_email[MAX_GITNAME];
 char git_default_name[MAX_GITNAME];
@@ -25,6 +26,8 @@ const char *apply_default_whitespace;
 int zlib_compression_level = Z_DEFAULT_COMPRESSION;
 int pager_in_use;
 int pager_use_color = 1;
+/* by default we allow 2 but up to PACK_VERSION is allowed */
+int delta_version = 2;
 
 static const char *git_dir;
 static char *git_object_dir, *git_index_file, *git_refs_dir, *git_graft_file;

sha1_file.c

@@ -527,6 +527,7 @@ int use_packed_git(struct packed_git *p)
 			    p->pack_size - 20)) {
 			die("packfile %s does not match index.", p->pack_name);
 		}
+		p->pack_version = ntohl(hdr->hdr_version);
 	}
 	p->pack_last_used = pack_used_ctr++;
 	p->pack_use_cnt++;