midx.c: respect 'pack.writeBitmapHashcache' when writing bitmaps

In the previous commit, the bitmap writing code learned to propagate
values from an existing hash-cache extension into the bitmap that it is
writing.

Now that that functionality exists, let's expose it by teaching the 'git
multi-pack-index' builtin to respect the `pack.writeBitmapHashCache`
option so that the hash-cache may be written at all.

Two minor points worth noting here:

  - The 'git multi-pack-index write' sub-command didn't previously read
    any configuration (instead this is handled in the base command). A
    separate handler is added here to respect this write-specific
    config option.

  - I briefly considered adding a 'bitmap_flags' field to the static
    options struct, but decided against it since it would require
    plumbing through a new parameter to the write_midx_file() function.

    Instead, a new MIDX-specific flag is added, which is translated to
    the corresponding bitmap one.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/pack.txt

@@ -159,6 +159,10 @@ pack.writeBitmapHashCache::
 	between an older, bitmapped pack and objects that have been
 	pushed since the last gc). The downside is that it consumes 4
 	bytes per object of disk space. Defaults to true.
++
+When writing a multi-pack reachability bitmap, no new namehashes are
+computed; instead, any namehashes stored in an existing bitmap are
+permuted into their appropriate location when writing a new bitmap.
 
 pack.writeReverseIndex::
 	When true, git will write a corresponding .rev file (see:

builtin/multi-pack-index.c

@@ -61,6 +61,23 @@ static struct option *add_common_options(struct option *prev)
 	return parse_options_concat(common_opts, prev);
 }
 
+static int git_multi_pack_index_write_config(const char *var, const char *value,
+					     void *cb)
+{
+	if (!strcmp(var, "pack.writebitmaphashcache")) {
+		if (git_config_bool(var, value))
+			opts.flags |= MIDX_WRITE_BITMAP_HASH_CACHE;
+		else
+			opts.flags &= ~MIDX_WRITE_BITMAP_HASH_CACHE;
+	}
+
+	/*
+	 * We should never make a fall-back call to 'git_default_config', since
+	 * this was already called in 'cmd_multi_pack_index()'.
+	 */
+	return 0;
+}
+
 static int cmd_multi_pack_index_write(int argc, const char **argv)
 {
 	struct option *options;
@@ -73,6 +90,10 @@ static int cmd_multi_pack_index_write(int argc, const char **argv)
 		OPT_END(),
 	};
 
+	opts.flags |= MIDX_WRITE_BITMAP_HASH_CACHE;
+
+	git_config(git_multi_pack_index_write_config, NULL);
+
 	options = add_common_options(builtin_multi_pack_index_write_options);
 
 	trace2_cmd_mode(argv[0]);

midx.c

@@ -1008,9 +1008,13 @@ static int write_midx_bitmap(char *midx_name, unsigned char *midx_hash,
 	struct pack_idx_entry **index;
 	struct commit **commits = NULL;
 	uint32_t i, commits_nr;
+	uint16_t options = 0;
 	char *bitmap_name = xstrfmt("%s-%s.bitmap", midx_name, hash_to_hex(midx_hash));
 	int ret;
 
+	if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
+		options |= BITMAP_OPT_HASH_CACHE;
+
 	prepare_midx_packing_data(&pdata, ctx);
 
 	commits = find_commits_for_midx_bitmap(&commits_nr, ctx);
@@ -1049,7 +1053,7 @@ static int write_midx_bitmap(char *midx_name, unsigned char *midx_hash,
 		goto cleanup;
 
 	bitmap_writer_set_checksum(midx_hash);
-	bitmap_writer_finish(index, pdata.nr_objects, bitmap_name, 0);
+	bitmap_writer_finish(index, pdata.nr_objects, bitmap_name, options);
 
 cleanup:
 	free(index);

midx.h

@@ -44,6 +44,7 @@ struct multi_pack_index {
 #define MIDX_PROGRESS     (1 << 0)
 #define MIDX_WRITE_REV_INDEX (1 << 1)
 #define MIDX_WRITE_BITMAP (1 << 2)
+#define MIDX_WRITE_BITMAP_HASH_CACHE (1 << 3)
 
 const unsigned char *get_midx_checksum(struct multi_pack_index *m);
 char *get_midx_filename(const char *object_dir);