pack-objects: warn on split packs disabling bitmaps

It can be tempting for a server admin to want a stable set of
long-lived packs for dumb clients; but also want to enable bitmaps
to serve smart clients more quickly.

Unfortunately, such a configuration is impossible; so at least warn
users of this incompatibility since commit 21134714 (pack-objects:
turn off bitmaps when we split packs, 2014-10-16).

Tested the warning by inspecting the output of:

	make -C t t5310-pack-bitmaps.sh GIT_TEST_OPTS=-v

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config.txt

@@ -2147,8 +2147,11 @@ pack.packSizeLimit::
 	The maximum size of a pack.  This setting only affects
 	packing to a file when repacking, i.e. the git:// protocol
 	is unaffected.  It can be overridden by the `--max-pack-size`
-	option of linkgit:git-repack[1]. The minimum size allowed is
-	limited to 1 MiB. The default is unlimited.
+	option of linkgit:git-repack[1].  Reaching this limit results
+	in the creation of multiple packfiles; which in turn prevents
+	bitmaps from being created.
+	The minimum size allowed is limited to 1 MiB.
+	The default is unlimited.
 	Common unit suffixes of 'k', 'm', or 'g' are
 	supported.
 
@@ -2548,8 +2551,9 @@ repack.writeBitmaps::
 	objects to disk (e.g., when `git repack -a` is run).  This
 	index can speed up the "counting objects" phase of subsequent
 	packs created for clones and fetches, at the cost of some disk
-	space and extra time spent on the initial repack.  Defaults to
-	false.
+	space and extra time spent on the initial repack.  This has
+	no effect if multiple packfiles are created.
+	Defaults to false.
 
 rerere.autoUpdate::
 	When set to true, `git-rerere` updates the index with the

Documentation/git-pack-objects.txt

@@ -110,7 +110,8 @@ base-name::
 --max-pack-size=<n>::
 	Maximum size of each output pack file. The size can be suffixed with
 	"k", "m", or "g". The minimum size allowed is limited to 1 MiB.
-	If specified,  multiple packfiles may be created.
+	If specified, multiple packfiles may be created, which also
+	prevents the creation of a bitmap index.
 	The default is unlimited, unless the config variable
 	`pack.packSizeLimit` is set.
 

Documentation/git-repack.txt

@@ -106,7 +106,8 @@ other objects in that pack they already have locally.
 --max-pack-size=<n>::
 	Maximum size of each output pack file. The size can be suffixed with
 	"k", "m", or "g". The minimum size allowed is limited to 1 MiB.
-	If specified,  multiple packfiles may be created.
+	If specified, multiple packfiles may be created, which also
+	prevents the creation of a bitmap index.
 	The default is unlimited, unless the config variable
 	`pack.packSizeLimit` is set.
 
@@ -115,7 +116,8 @@ other objects in that pack they already have locally.
 	Write a reachability bitmap index as part of the repack. This
 	only makes sense when used with `-a` or `-A`, as the bitmaps
 	must be able to refer to all reachable objects. This option
-	overrides the setting of `pack.writeBitmaps`.
+	overrides the setting of `repack.writeBitmaps`.  This option
+	has no effect if multiple packfiles are created.
 
 --pack-kept-objects::
 	Include objects in `.keep` files when repacking.  Note that we
@@ -123,7 +125,7 @@ other objects in that pack they already have locally.
 	This means that we may duplicate objects, but this makes the
 	option safe to use when there are concurrent pushes or fetches.
 	This option is generally only useful if you are writing bitmaps
-	with `-b` or `pack.writeBitmaps`, as it ensures that the
+	with `-b` or `repack.writeBitmaps`, as it ensures that the
 	bitmapped packfile has the necessary objects.
 
 Configuration

builtin/pack-objects.c

@@ -759,6 +759,10 @@ static off_t write_reused_pack(struct sha1file *f)
 	return reuse_packfile_offset - sizeof(struct pack_header);
 }
 
+static const char no_split_warning[] = N_(
+"disabling bitmap writing, packs are split due to pack.packSizeLimit"
+);
+
 static void write_pack_file(void)
 {
 	uint32_t i = 0, j;
@@ -813,8 +817,11 @@ static void write_pack_file(void)
 			fixup_pack_header_footer(fd, sha1, pack_tmp_name,
 						 nr_written, sha1, offset);
 			close(fd);
+			if (write_bitmap_index) {
+				warning(_(no_split_warning));
 				write_bitmap_index = 0;
 			}
+		}
 
 		if (!pack_to_stdout) {
 			struct stat st;