Subject: Re: [PATCH] repack: disable bitmaps-by-default if .keep files exist

From: Ævar Arnfjörð Bjarmason <[email-masked]>
Date: Fri, 28 Jun 2019 09:21:18 +0200

On Fri, Jun 28 2019, Eric Wong wrote:

> Jeff King <[email-masked]> wrote:
>> On Sun, Jun 23, 2019 at 06:08:25PM +0000, Eric Wong wrote:
>>
>> > > I'm not sure of the right solution. For maximal backwards-compatibility,
>> > > the default for bitmaps could become "if not bare and if there are no
>> > > .keep files". But that would mean bitmaps sometimes not getting
>> > > generated because of the problems that ee34a2bead was trying to solve.
>> > >
>> > > That's probably OK, though; you can always flip the bitmap config to
>> > > "true" yourself if you _must_ have bitmaps.
>> >
>> > What about something like this?  Needs tests but I need to leave, now.
>>
>> Yeah, I think that's the right direction.
>
> OK.  I have a real patch with one additional test, below.
> (don't have a lot of time for hacking)
>
>> Though...
>>
>> > +static int has_pack_keep_file(void)
>> > +{
>> > +	DIR *dir;
>> > +	struct dirent *e;
>> > +	int found = 0;
>> > +
>> > +	if (!(dir = opendir(packdir)))
>> > +		return found;
>> > +
>> > +	while ((e = readdir(dir)) != NULL) {
>> > +		if (ends_with(e->d_name, ".keep")) {
>> > +			found = 1;
>> > +			break;
>> > +		}
>> > +	}
>> > +	closedir(dir);
>> > +	return found;
>> > +}
>>
>> I think this can be replaced with just checking p->pack_keep for each
>> item in the packed_git list.
>
> Good point, I tend to forget git C API internals as soon as I
> learn them :x
>
>> That's racy, but then so is your code here, since it's really the child
>> pack-objects which is going to deal with the .keep. I don't think we
>> need to care much about the race, though. Either:
>
> Agreed.  <snip>
>
> --------8<-------
> Subject: [PATCH] repack: disable bitmaps-by-default if .keep files exist
>
> Bitmaps aren't useful with multiple packs, and users with
> .keep files ended up with redundant packs when bitmaps
> got enabled by default in bare repos.
>
> So detect when .keep files exist and stop enabling bitmaps
> by default in that case.
>
> Wasteful (but otherwise harmless) race conditions with .keep files
> documented by Jeff King still apply and there's a chance we'd
> still end up with redundant data on the FS:
>
>   https://public-inbox.org/git/[email-masked]/
>
> Fixes: 36eba0323d3288a8 ("repack: enable bitmaps by default on bare repos")
> Signed-off-by: Eric Wong <[email-masked]>
> Helped-by: Jeff King <[email-masked]>
> Reported-by: Janos Farkas <[email-masked]>
> ---
>  builtin/repack.c  | 18 ++++++++++++++++--
>  t/t7700-repack.sh | 10 ++++++++++
>  2 files changed, 26 insertions(+), 2 deletions(-)
>
> diff --git a/builtin/repack.c b/builtin/repack.c
> index caca113927..a9529d1afc 100644
> --- a/builtin/repack.c
> +++ b/builtin/repack.c
> @@ -89,6 +89,17 @@ static void remove_pack_on_signal(int signo)
>  	raise(signo);
>  }
>
> +static int has_pack_keep_file(void)
> +{
> +	struct packed_git *p;
> +
> +	for (p = get_packed_git(the_repository); p; p = p->next) {
> +		if (p->pack_keep)
> +			return 1;
> +	}
> +	return 0;
> +}
> +
>  /*
>   * Adds all packs hex strings to the fname list, which do not
>   * have a corresponding .keep file. These packs are not to
> @@ -343,9 +354,12 @@ int cmd_repack(int argc, const char **argv, const char *prefix)
>  	    (unpack_unreachable || (pack_everything & LOOSEN_UNREACHABLE)))
>  		die(_("--keep-unreachable and -A are incompatible"));
>
> -	if (write_bitmaps < 0)
> +	if (write_bitmaps < 0) {
>  		write_bitmaps = (pack_everything & ALL_INTO_ONE) &&
> -				 is_bare_repository();
> +				 is_bare_repository() &&
> +				 keep_pack_list.nr == 0 &&
> +				 !has_pack_keep_file();
> +	}
>  	if (pack_kept_objects < 0)
>  		pack_kept_objects = write_bitmaps;
>
> diff --git a/t/t7700-repack.sh b/t/t7700-repack.sh
> index 86d05160a3..0acde3b1f8 100755
> --- a/t/t7700-repack.sh
> +++ b/t/t7700-repack.sh
> @@ -239,4 +239,14 @@ test_expect_success 'bitmaps can be disabled on bare repos' '
>  	test -z "$bitmap"
>  '

I have the feedback I posted before this patch in
https://public-inbox.org/git/[email-masked]/

In particular "b" there since "a" is clearly more work. I.e. shouldn't
we at least in interactive mode on a "gc" print something about skipping
what we'd otherwise do.

Maybe that's tricky with the gc.log functionality, but I think we should
at least document this before the next guy shows up with "sometimes my
.bitmap files aren't generated...".

> +test_expect_success 'no bitmaps created if .keep files present' '
> +	pack=$(ls bare.git/objects/pack/*.pack) &&
> +	test_path_is_file "$pack" &&
> +	keep=${pack%.pack}.keep &&
> +	>"$keep" &&
> +	git -C bare.git repack -ad &&
> +	bitmap=$(ls bare.git/objects/pack/*.bitmap 2>/dev/null || :) &&
> +	test -z "$bitmap"

Maybe more readable as:

	find .git/objects/pack/ -type f -name '*.bitmap' >actual &&
	test_must_be_empty actual

I.e. it avoids the "|| :" & subshell.

> +'
> +
>  test_done