pack-bitmap-write: learn pack.writeBitmapLookupTable and add tests

Teach Git to provide a way for users to enable/disable bitmap lookup
table extension by providing a config option named 'writeBitmapLookupTable'.
Default is false.

Also add test to verify writting of lookup table.

Mentored-by: Taylor Blau <me@ttaylorr.com>
Co-Mentored-by: Kaartic Sivaraam <kaartic.sivaraam@gmail.com>
Co-Authored-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Abhradeep Chakraborty <chakrabortyabhradeep79@gmail.com>
Reviewed-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/config/pack.txt

@@ -164,6 +164,13 @@ 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.writeBitmapLookupTable::
+	When true, Git will include a "lookup table" section in the
+	bitmap index (if one is written). This table is used to defer
+	loading individual bitmaps as late as possible. This can be
+	beneficial in repositories that have relatively large bitmap
+	indexes. Defaults to false.
+
 pack.writeReverseIndex::
 	When true, git will write a corresponding .rev file (see:
 	link:../technical/pack-format.html[Documentation/technical/pack-format.txt])

builtin/multi-pack-index.c

@@ -87,6 +87,13 @@ static int git_multi_pack_index_write_config(const char *var, const char *value,
 			opts.flags &= ~MIDX_WRITE_BITMAP_HASH_CACHE;
 	}
 
+	if (!strcmp(var, "pack.writebitmaplookuptable")) {
+		if (git_config_bool(var, value))
+			opts.flags |= MIDX_WRITE_BITMAP_LOOKUP_TABLE;
+		else
+			opts.flags &= ~MIDX_WRITE_BITMAP_LOOKUP_TABLE;
+	}
+
 	/*
 	 * We should never make a fall-back call to 'git_default_config', since
 	 * this was already called in 'cmd_multi_pack_index()'.

builtin/pack-objects.c

@@ -3148,6 +3148,14 @@ static int git_pack_config(const char *k, const char *v, void *cb)
 		else
 			write_bitmap_options &= ~BITMAP_OPT_HASH_CACHE;
 	}
+
+	if (!strcmp(k, "pack.writebitmaplookuptable")) {
+		if (git_config_bool(k, v))
+			write_bitmap_options |= BITMAP_OPT_LOOKUP_TABLE;
+		else
+			write_bitmap_options &= ~BITMAP_OPT_LOOKUP_TABLE;
+	}
+
 	if (!strcmp(k, "pack.usebitmaps")) {
 		use_bitmap_index_default = git_config_bool(k, v);
 		return 0;

midx.c

@@ -1070,6 +1070,9 @@ static int write_midx_bitmap(const char *midx_name,
 	if (flags & MIDX_WRITE_BITMAP_HASH_CACHE)
 		options |= BITMAP_OPT_HASH_CACHE;
 
+	if (flags & MIDX_WRITE_BITMAP_LOOKUP_TABLE)
+		options |= BITMAP_OPT_LOOKUP_TABLE;
+
 	/*
 	 * Build the MIDX-order index based on pdata.objects (which is already
 	 * in MIDX order; c.f., 'midx_pack_order_cmp()' for the definition of

midx.h

@@ -47,6 +47,7 @@ struct multi_pack_index {
 #define MIDX_WRITE_REV_INDEX (1 << 1)
 #define MIDX_WRITE_BITMAP (1 << 2)
 #define MIDX_WRITE_BITMAP_HASH_CACHE (1 << 3)
+#define MIDX_WRITE_BITMAP_LOOKUP_TABLE (1 << 4)
 
 const unsigned char *get_midx_checksum(struct multi_pack_index *m);
 void get_midx_filename(struct strbuf *out, const char *object_dir);

t/t5310-pack-bitmaps.sh

@@ -26,6 +26,20 @@ has_any () {
 	grep -Ff "$1" "$2"
 }
 
+test_bitmap_cases () {
+	writeLookupTable=false
+	for i in "$@"
+	do
+		case "$i" in
+		"pack.writeBitmapLookupTable") writeLookupTable=true;;
+		esac
+	done
+
+	test_expect_success 'setup test repository' '
+		rm -fr * .git &&
+		git init &&
+		git config pack.writeBitmapLookupTable '"$writeLookupTable"'
+	'
 	setup_bitmap_history
 
 	test_expect_success 'setup writing bitmaps during repack' '
@@ -43,17 +57,6 @@ test_expect_success 'full repack creates bitmaps' '
 
 	basic_bitmap_tests
 
-test_expect_success 'incremental repack fails when bitmaps are requested' '
-	test_commit more-1 &&
-	test_must_fail git repack -d 2>err &&
-	test_i18ngrep "Incremental repacks are incompatible with bitmap" err
-'
-
-test_expect_success 'incremental repack can disable bitmaps' '
-	test_commit more-2 &&
-	git repack -d --no-write-bitmap-index
-'
-
 	test_expect_success 'pack-objects respects --local (non-local loose)' '
 		git init --bare alt.git &&
 		echo $(pwd)/alt.git/objects >.git/objects/info/alternates &&
@@ -200,6 +203,7 @@ test_expect_success JGIT,SHA1 'jgit can read our bitmaps' '
 		git clone --bare . compat-us.git &&
 		(
 			cd compat-us.git &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 			git repack -adb &&
 			# jgit gc will barf if it does not like our bitmaps
 			jgit gc
@@ -364,6 +368,7 @@ test_expect_success 'pack.preferBitmapTips' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			# create enough commits that not all are receive bitmap
 			# coverage even if they are all at the tip of some reference.
@@ -403,6 +408,7 @@ test_expect_success 'complains about multiple pack bitmaps' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit base &&
 
@@ -424,5 +430,27 @@ test_expect_success 'complains about multiple pack bitmaps' '
 			grep "ignoring extra bitmap file" err
 		)
 	'
+}
+
+test_bitmap_cases
+
+test_expect_success 'incremental repack fails when bitmaps are requested' '
+	test_commit more-1 &&
+	test_must_fail git repack -d 2>err &&
+	test_i18ngrep "Incremental repacks are incompatible with bitmap" err
+'
+
+test_expect_success 'incremental repack can disable bitmaps' '
+	test_commit more-2 &&
+	git repack -d --no-write-bitmap-index
+'
+
+test_bitmap_cases "pack.writeBitmapLookupTable"
+
+test_expect_success 'verify writing bitmap lookup table when enabled' '
+	GIT_TRACE2_EVENT="$(pwd)/trace2" \
+		git repack -ad &&
+	grep "\"label\":\"writing_lookup_table\"" trace2
+'
 
 test_done

t/t5311-pack-bitmaps-shallow.sh

@@ -17,7 +17,20 @@ test_description='check bitmap operation with shallow repositories'
 # the tree for A. But in a shallow one, we've grafted away
 # A, and fetching A to B requires that the other side send
 # us the tree for file=1.
+test_shallow_bitmaps () {
+	writeLookupTable=false
+
+	for i in "$@"
+	do
+		case $i in
+		"pack.writeBitmapLookupTable") writeLookupTable=true;;
+		esac
+	done
+
 	test_expect_success 'setup shallow repo' '
+		rm -rf * .git &&
+		git init &&
+		git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 		echo 1 >file &&
 		git add file &&
 		git commit -m orig &&
@@ -35,5 +48,9 @@ test_expect_success 'turn on bitmaps in the parent' '
 	test_expect_success 'shallow fetch from bitmapped repo' '
 		(cd shallow.git && git fetch)
 	'
+}
+
+test_shallow_bitmaps
+test_shallow_bitmaps "pack.writeBitmapLookupTable"
 
 test_done

t/t5326-multi-pack-bitmaps.sh

@@ -15,17 +15,24 @@ GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
 sane_unset GIT_TEST_MIDX_WRITE_REV
 sane_unset GIT_TEST_MIDX_READ_RIDX
 
-midx_bitmap_core
-
 bitmap_reuse_tests() {
 	from=$1
 	to=$2
+	writeLookupTable=false
+
+	for i in $3-${$#}
+	do
+		case $i in
+		"pack.writeBitmapLookupTable") writeLookupTable=true;;
+		esac
+	done
 
 	test_expect_success "setup pack reuse tests ($from -> $to)" '
 		rm -fr repo &&
 		git init repo &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 			test_commit_bulk 16 &&
 			git tag old-tip &&
 
@@ -43,6 +50,7 @@ bitmap_reuse_tests() {
 	test_expect_success "build bitmap from existing ($from -> $to)" '
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 			test_commit_bulk --id=further 16 &&
 			git tag new-tip &&
 
@@ -59,6 +67,7 @@ bitmap_reuse_tests() {
 	test_expect_success "verify resulting bitmaps ($from -> $to)" '
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 			git for-each-ref &&
 			git rev-list --test-bitmap refs/tags/old-tip &&
 			git rev-list --test-bitmap refs/tags/new-tip
@@ -66,9 +75,31 @@ bitmap_reuse_tests() {
 	'
 }
 
-bitmap_reuse_tests 'pack' 'MIDX'
+test_midx_bitmap_cases () {
-bitmap_reuse_tests 'MIDX' 'pack'
+	writeLookupTable=false
-bitmap_reuse_tests 'MIDX' 'MIDX'
+	writeBitmapLookupTable=
+
+	for i in "$@"
+	do
+		case $i in
+		"pack.writeBitmapLookupTable")
+			writeLookupTable=true
+			writeBitmapLookupTable="$i"
+			;;
+		esac
+	done
+
+	test_expect_success 'setup test_repository' '
+		rm -rf * .git &&
+		git init &&
+		git config pack.writeBitmapLookupTable '"$writeLookupTable"'
+	'
+
+	midx_bitmap_core
+
+	bitmap_reuse_tests 'pack' 'MIDX' "$writeBitmapLookupTable"
+	bitmap_reuse_tests 'MIDX' 'pack' "$writeBitmapLookupTable"
+	bitmap_reuse_tests 'MIDX' 'MIDX' "$writeBitmapLookupTable"
 
 	test_expect_success 'missing object closure fails gracefully' '
 		rm -fr repo &&
@@ -76,6 +107,7 @@ test_expect_success 'missing object closure fails gracefully' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit loose &&
 			test_commit packed &&
@@ -100,6 +132,7 @@ test_expect_success 'removing a MIDX clears stale bitmaps' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 			test_commit base &&
 			git repack &&
 			git multi-pack-index write --bitmap &&
@@ -124,6 +157,7 @@ test_expect_success 'pack.preferBitmapTips' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit_bulk --message="%s" 103 &&
 
@@ -161,6 +195,7 @@ test_expect_success 'writing a bitmap with --refs-snapshot' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit one &&
 			test_commit two &&
@@ -202,6 +237,7 @@ test_expect_success 'write a bitmap with --refs-snapshot (preferred tips)' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit_bulk --message="%s" 103 &&
 
@@ -242,6 +278,7 @@ test_expect_success 'hash-cache values are propagated from pack bitmaps' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit base &&
 			test_commit base2 &&
@@ -272,6 +309,7 @@ test_expect_success 'no .bitmap is written without any objects' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			empty="$(git pack-objects $objdir/pack/pack </dev/null)" &&
 			cat >packs <<-EOF &&
@@ -294,6 +332,7 @@ test_expect_success 'graceful fallback when missing reverse index' '
 		test_when_finished "rm -fr repo" &&
 		(
 			cd repo &&
+			git config pack.writeBitmapLookupTable '"$writeLookupTable"' &&
 
 			test_commit base &&
 
@@ -306,5 +345,25 @@ test_expect_success 'graceful fallback when missing reverse index' '
 			! grep "ignoring extra bitmap file" err
 		)
 	'
+}
+
+test_midx_bitmap_cases
+
+test_midx_bitmap_cases "pack.writeBitmapLookupTable"
+
+test_expect_success 'multi-pack-index write writes lookup table if enabled' '
+	rm -fr repo &&
+	git init repo &&
+	test_when_finished "rm -fr repo" &&
+	(
+		cd repo &&
+		test_commit base &&
+		git config pack.writeBitmapLookupTable true &&
+		git repack -ad &&
+		GIT_TRACE2_EVENT="$(pwd)/trace" \
+			git multi-pack-index write --bitmap &&
+		grep "\"label\":\"writing_lookup_table\"" trace
+	)
+'
 
 test_done

t/t5327-multi-pack-bitmaps-rev.sh

@@ -17,7 +17,27 @@ GIT_TEST_MIDX_READ_RIDX=0
 export GIT_TEST_MIDX_WRITE_REV
 export GIT_TEST_MIDX_READ_RIDX
 
+test_midx_bitmap_rev () {
+	writeLookupTable=false
+
+	for i in "$@"
+	do
+		case $i in
+		"pack.writeBitmapLookupTable") writeLookupTable=true;;
+		esac
+	done
+
+	test_expect_success 'setup bitmap config' '
+		rm -rf * .git &&
+		git init &&
+		git config pack.writeBitmapLookupTable '"$writeLookupTable"'
+	'
+
 	midx_bitmap_core rev
 	midx_bitmap_partial_tests rev
+}
+
+test_midx_bitmap_rev
+test_midx_bitmap_rev "pack.writeBitmapLookupTable"
 
 test_done