pack-objects: support --blob-max-bytes

As part of an effort to improve Git support for very large repositories
in which clients typically have only a subset of all version-controlled
blobs, teach pack-objects to support --blob-max-bytes, packing only
blobs not exceeding that size unless the blob corresponds to a file
whose name starts with ".git". upload-pack will eventually be taught to
use this new parameter if needed to exclude certain blobs during a fetch
or clone, potentially drastically reducing network consumption when
serving these very large repositories.

Since any excluded blob should not act as a delta base, I did this by
avoiding such oversized blobs from ever going into the "to_pack" data
structure, which contains both preferred bases (which do not go into the
final packfile but can act as delta bases) and the objects to be packed
(which go into the final packfile and also can act as delta bases). To
that end, add_object_entry() has been modified to exclude the
appropriate non-preferred-base objects. (Preferred bases, regardless of
size, still enter "to_pack" - they are something that the client
indicates that it has, not something that the server needs to serve, so
no exclusion occurs.)

If bitmaps are to be used, we would not know if a blob corresponded to a
file whose name starts with ".git". For this reason, when invoked with
--blob-max-bytes, pack-objects will not use bitmaps.

Signed-off-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

4 files changed, 99 insertions, 3 deletions

diff --git a/Documentation/git-pack-objects.txt b/Documentation/git-pack-objects.txt
index 473a16135a..ddce2d4c17 100644
--- a/Documentation/git-pack-objects.txt
+++ b/Documentation/git-pack-objects.txt
@@ -12,7 +12,8 @@ SYNOPSIS
 'git pack-objects' [-q | --progress | --all-progress] [--all-progress-implied]
 	[--no-reuse-delta] [--delta-base-offset] [--non-empty]
 	[--local] [--incremental] [--window=<n>] [--depth=<n>]
-	[--revs [--unpacked | --all]] [--stdout | base-name]
+	[--revs [--unpacked | --all]]
+	[--stdout [--blob-max-bytes=<n>] | base-name]
 	[--shallow] [--keep-true-parents] < object-list
 
 
@@ -236,6 +237,15 @@ So does `git bundle` (see linkgit:git-bundle[1]) when it creates a bundle.
 	With this option, parents that are hidden by grafts are packed
 	nevertheless.
 
+--blob-max-bytes=<n>::
+	This option can only be used with --stdout. If specified, a blob
+	larger than this will not be packed unless a to-be-packed tree
+	has that blob with a filename beginning with ".git".  The size
+	can be suffixed with "k", "m", or "g", and may be "0".
++
+If specified, after printing the packfile, pack-objects will print, in packet
+format (pkt-line), the names of excluded blobs and their sizes.
+
 SEE ALSO
 --------
 linkgit:git-rev-list[1]
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index ef0b61d5ff..8686b4351b 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -26,6 +26,8 @@
 #include "argv-array.h"
 #include "mru.h"
 #include "packfile.h"
+#include "pkt-line.h"
+#include "varint.h"
 
 static const char *pack_usage[] = {
 	N_("git pack-objects --stdout [<options>...] [< <ref-list> | < <object-list>]"),
@@ -81,6 +83,8 @@ static unsigned long cache_max_small_delta_size = 1000;
 
 static unsigned long window_memory_limit = 0;
 
+static long blob_max_bytes = -1;
+
 /*
  * stats
  */
@@ -1072,6 +1076,27 @@ static const char no_closure_warning[] = N_(
 "disabling bitmap writing, as some objects are not being packed"
 );
 
+/*
+ * Returns 1 if the given blob does not meet any defined blob size
+ * limits.  Blobs that appear as a tree entry whose basename begins with
+ * ".git" are never considered oversized.
+ */
+static int oversized(const unsigned char *sha1, const char *name) {
+	const char *last_slash, *basename;
+	unsigned long size;
+
+	if (blob_max_bytes < 0)
+		return 0;
+
+	last_slash = strrchr(name, '/');
+	basename = last_slash ? last_slash + 1 : name;
+	if (starts_with(basename, ".git"))
+		return 0;
+
+	sha1_object_info(sha1, &size);
+	return size > blob_max_bytes;
+}
+
 static int add_object_entry(const unsigned char *sha1, enum object_type type,
 			    const char *name, int preferred_base)
 {
@@ -1082,7 +1107,8 @@ static int add_object_entry(const unsigned char *sha1, enum object_type type,
 	if (have_duplicate_entry(sha1, preferred_base, &index_pos))
 		return 0;
 
-	if (ignore_object(sha1, preferred_base, &found_pack, &found_offset)) {
+	if (ignore_object(sha1, preferred_base, &found_pack, &found_offset) ||
+	    (!preferred_base && type == OBJ_BLOB && oversized(sha1, name))) {
 		/* The pack is missing an object, so it will not have closure */
 		if (write_bitmap_index) {
 			warning(_(no_closure_warning));
@@ -2956,6 +2982,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 			 N_("write a bitmap index together with the pack index")),
 		OPT_BOOL(0, "exclude-promisor-objects", &exclude_promisor_objects,
 			 N_("do not pack objects in promisor packfiles")),
+		OPT_MAGNITUDE(0, "blob-max-bytes", &blob_max_bytes,
+			      N_("exclude blobs larger than this")),
 		OPT_END(),
 	};
 
@@ -3054,7 +3082,8 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 		use_bitmap_index = use_bitmap_index_default;
 
 	/* "hard" reasons not to use bitmaps; these just won't work at all */
-	if (!use_internal_rev_list || (!pack_to_stdout && write_bitmap_index) || is_repository_shallow())
+	if (!use_internal_rev_list || (!pack_to_stdout && write_bitmap_index) ||
+	    is_repository_shallow() || (blob_max_bytes >= 0))
 		use_bitmap_index = 0;
 
 	if (pack_to_stdout || !rev_list_all)
diff --git a/t/t5300-pack-object.sh b/t/t5300-pack-object.sh
index 9c68b99251..697cc20f2f 100755
--- a/t/t5300-pack-object.sh
+++ b/t/t5300-pack-object.sh
@@ -457,6 +457,51 @@ test_expect_success !PTHREADS,C_LOCALE_OUTPUT 'pack-objects --threads=N or pack.
 	grep -F "no threads support, ignoring pack.threads" err
 '
 
+lcut () {
+	perl -e '$/ = undef; $_ = <>; s/^.{'$1'}//s; print $_'
+}
+
+test_expect_success '--blob-size-limit works with multiple excluded' '
+	rm -rf server &&
+	git init server &&
+	printf a > server/a &&
+	printf b > server/b &&
+	printf c-very-long-file > server/c &&
+	printf d-very-long-file > server/d &&
+	git -C server add a b c d &&
+	git -C server commit -m x &&
+
+	git -C server rev-parse HEAD >objects &&
+	git -C server pack-objects --revs --stdout --blob-max-bytes=10 <objects >my.pack &&
+
+	# Ensure that only the small blobs are in the packfile
+	git index-pack my.pack &&
+	git verify-pack -v my.idx >objectlist &&
+	grep $(git hash-object server/a) objectlist &&
+	grep $(git hash-object server/b) objectlist &&
+	! grep $(git hash-object server/c) objectlist &&
+	! grep $(git hash-object server/d) objectlist
+'
+
+test_expect_success '--blob-size-limit never excludes special files' '
+	rm -rf server &&
+	git init server &&
+	printf a-very-long-file > server/a &&
+	printf a-very-long-file > server/.git-a &&
+	printf b-very-long-file > server/b &&
+	git -C server add a .git-a b &&
+	git -C server commit -m x &&
+
+	git -C server rev-parse HEAD >objects &&
+	git -C server pack-objects --revs --stdout --blob-max-bytes=10 <objects >my.pack &&
+
+	# Ensure that the .git-a blob is in the packfile, despite also
+	# appearing as a non-.git file
+	git index-pack my.pack &&
+	git verify-pack -v my.idx >objectlist &&
+	grep $(git hash-object server/a) objectlist
+'
+
 #
 # WARNING!
 #
diff --git a/t/test-lib-functions.sh b/t/test-lib-functions.sh
index 1701fe2a06..07b79c7add 100644
--- a/t/test-lib-functions.sh
+++ b/t/test-lib-functions.sh
@@ -1020,3 +1020,15 @@ nongit () {
 		"$@"
 	)
 }
+
+# Converts big-endian pairs of hexadecimal digits into bytes. For example,
+# "printf 61620d0a | hex_pack" results in "ab\r\n".
+hex_pack () {
+	perl -e '$/ = undef; $input = <>; print pack("H*", $input)'
+}
+
+# Converts bytes into big-endian pairs of hexadecimal digits. For example,
+# "printf 'ab\r\n' | hex_unpack" results in "61620d0a".
+hex_unpack () {
+	perl -e '$/ = undef; $input = <>; print unpack("H2" x length($input), $input)'
+}