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

["SZEDER Gábor" <szeder.dev@gmail.com>]

On Fri, Jun 28, 2019 at 07:02:11AM +0000, Eric Wong wrote:
> Jeff King <peff@peff.net> 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/20190623224244.GB1100@sigill.intra.peff.net/
> 
> Fixes: 36eba0323d3288a8 ("repack: enable bitmaps by default on bare repos")
> Signed-off-by: Eric Wong <e@80x24.org>
> Helped-by: Jeff King <peff@peff.net>
> Reported-by: Janos Farkas <chexum@gmail.com>
> ---
>  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 "$bitmap"
>  '
>  
> +test_expect_success 'no bitmaps created if .keep files present' '
> +	pack=$(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"

This test fails when run with 'GIT_TEST_MULTI_PACK_INDEX=1':

  + ls bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.pack
  + pack=bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.pack
  + test_path_is_file bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.pack
  + test -f bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.pack
  + keep=bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.keep
  +
  + git -C bare.git repack -ad
  + ls bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.bitmap
  + bitmap=bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.bitmap
  + test -z bare.git/objects/pack/pack-51a6a758692f03f9e1f31fd087bba30584afad2f.bitmap
  error: last command exited with $?=1
  not ok 15 - no bitmaps created if .keep files present

When the new has_pack_keep_file() helper function calls
get_packed_git(the_repository) in the loop it returns with NULL
already on the first iteration, so the keep file goes unnoticed.

> +'
> +
>  test_done
> -- 
> EW