[PATCH v3] send-pack.c: add config push.useBitmaps

From: Kyle Zhao <kylezhao@tencent.com>

This allows you to disable bitmaps for "git push". Default is true.

Reachability bitmaps are designed to speed up the "counting objects" phase of generating a pack during a clone or fetch. They are not optimized for Git clients sending a small topic branch via "git push". In some cases (see [1]), using reachability bitmaps during "git push" can cause significant performance regressions.

Add a new "push.useBitmaps" config option to disable reachability bitmaps during "git push". This allows reachability bitmaps to still be used in other areas, such as "git rev-list --use-bitmap-index".

[1]: https://lore.kernel.org/git/87zhoz8b9o.fsf@evledraar.gmail.com/

Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
---
    send-pack.c: add config push.useBitmaps
    
    This patch add config push.useBitmaps to prevent git push using bitmap.
    
    The origin discussion is here:
    https://lore.kernel.org/git/b940e705fbe9454685757f2e3055e2ce@tencent.com/
    
    Thanks, -Kyle
    
    Changes since v1:
    
     * changed the commit message
     * modified and added missing \n to push.txt
     * used test_subcommand for test
     * modified "if" statement for "git_config_get_bool()" in send-pack.c
    
    Changes since v2:
    
     * enable 'push.useBitmaps" by default
     * fix nit in t/t5516-fetch-push.sh

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1263%2Fkeyu98%2Fkz%2Fpush-usebitmps-config-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1263/keyu98/kz/push-usebitmps-config-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1263

Range-diff vs v2:

 1:  42e0b4845b2 ! 1:  a523cb52542 send-pack.c: add config push.useBitmaps
     @@ Metadata
       ## Commit message ##
          send-pack.c: add config push.useBitmaps
      
     -    This allows you to disable bitmaps for "git push". Default is false.
     +    This allows you to disable bitmaps for "git push". Default is true.
      
          Reachability bitmaps are designed to speed up the "counting objects"
          phase of generating a pack during a clone or fetch. They are not
     @@ Documentation/config/push.txt: push.negotiate::
       	in common.
      +
      +push.useBitmaps::
     -+    If this config and `pack.useBitmaps` are both `true`, then Git will
     -+    use reachability bitmaps during `git push`, if available (disabled
     -+    by default).
     ++	If this config and `pack.useBitmaps` are both `true`, then Git will
     ++	use reachability bitmaps during `git push`, if available. If set to
     ++	`false`, may improve push performance without affecting use of the
     ++	reachability bitmaps for other operations. Default is true.
      
       ## send-pack.c ##
      @@ send-pack.c: static int pack_objects(int fd, struct ref *refs, struct oid_array *advertised,
       		strvec_push(&po.args, "--progress");
       	if (is_repository_shallow(the_repository))
       		strvec_push(&po.args, "--shallow");
     -+	if (!args->use_bitmaps)
     ++	if (args->no_use_bitmaps)
      +		strvec_push(&po.args, "--no-use-bitmap-index");
       	po.in = -1;
       	po.out = args->stateless_rpc ? -1 : fd;
     @@ send-pack.c: int send_pack(struct send_pack_args *args,
       		get_commons_through_negotiation(args->url, remote_refs, &commons);
       
      +	if (!git_config_get_bool("push.usebitmaps", &use_bitmaps))
     -+		args->use_bitmaps = use_bitmaps;
     ++		args->no_use_bitmaps = !use_bitmaps;
      +
       	git_config_get_bool("transfer.advertisesid", &advertise_sid);
       
     @@ send-pack.h: struct send_pack_args {
       		stateless_rpc:1,
      -		atomic:1;
      +		atomic:1,
     -+		use_bitmaps:1;
     ++		no_use_bitmaps:1;
       	const struct string_list *push_options;
       };
       
     @@ t/t5516-fetch-push.sh: test_expect_success 'push warns or fails when using usern
      +	GIT_TRACE2_EVENT="$PWD/default" \
      +	git push testrepo main:test &&
      +	test_subcommand git pack-objects --all-progress-implied --revs --stdout \
     -+	--thin --delta-base-offset -q --no-use-bitmap-index <default &&
     ++		--thin --delta-base-offset -q <default &&
      +
      +	git config push.useBitmaps true &&
      +	GIT_TRACE2_EVENT="$PWD/true" \
      +	git push testrepo main:test2 &&
      +	test_subcommand git pack-objects --all-progress-implied --revs --stdout \
     -+	--thin --delta-base-offset -q <true &&
     ++		--thin --delta-base-offset -q <true &&
      +
      +	git config push.useBitmaps false &&
      +	GIT_TRACE2_EVENT="$PWD/false" \
      +	git push testrepo main:test3 &&
      +	test_subcommand git pack-objects --all-progress-implied --revs --stdout \
     -+	--thin --delta-base-offset -q --no-use-bitmap-index <false
     ++		--thin --delta-base-offset -q --no-use-bitmap-index <false
      +'
      +
       test_done

 Documentation/config/push.txt |  6 ++++++
 send-pack.c                   |  6 ++++++
 send-pack.h                   |  3 ++-
 t/t5516-fetch-push.sh         | 21 +++++++++++++++++++++
 4 files changed, 35 insertions(+), 1 deletion(-)