Re: [PATCH v2 4/4] pack-refs: add fully-peeled trait

[Michael Haggerty <mhagger@alum.mit.edu>]

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>

and ACK for the whole series, once Junio's points are addressed.

Regarding Junio's readability suggestion: I agree that his versions are
a bit more readable, albeit at the expense of having to evaluate a bit
more logic for each reference rather than just once when the header line
is handled.  So I don't have a preference either way.

Michael

On 03/17/2013 09:28 AM, Jeff King wrote:
> From: Michael Haggerty <mhagger@alum.mit.edu>
> 
> Older versions of pack-refs did not write peel lines for
> refs outside of refs/tags. This meant that on reading the
> pack-refs file, we might set the REF_KNOWS_PEELED flag for
> such a ref, even though we do not know anything about its
> peeled value.
> 
> The previous commit updated the writer to always peel, no
> matter what the ref is. That means that packed-refs files
> written by newer versions of git are fine to be read by both
> old and new versions of git. However, we still have the
> problem of reading packed-refs files written by older
> versions of git, or by other implementations which have not
> yet learned the same trick.
> 
> The simplest fix would be to always unset the
> REF_KNOWS_PEELED flag for refs outside of refs/tags that do
> not have a peel line (if it has a peel line, we know it is
> valid, but we cannot assume a missing peel line means
> anything). But that loses an important optimization, as
> upload-pack should not need to load the object pointed to by
> refs/heads/foo to determine that it is not a tag.
> 
> Instead, we add a "fully-peeled" trait to the packed-refs
> file. If it is set, we know that we can trust a missing peel
> line to mean that a ref cannot be peeled. Otherwise, we fall
> back to assuming nothing.
> 
> [commit message and tests by Jeff King <peff@peff.net>]
> 
> Signed-off-by: Jeff King <peff@peff.net>
> ---
> This uses Michael's approach for managing the flags within
> read_packed_refs, which is more readable. As I picked up his
> code and comments, I realized that there was basically
> nothing of mine left, so I switched the authorship. But do
> note:
> 
>   1. It should have Michael's signoff, which was not present
>      in the commit I lifted the code from.
> 
>   2. I tweaked the big comment above read_packed_refs to
>      reduce some ambiguities. Please double-check that I am
>      not putting inaccurate words in your mouth. :)
> 
>  pack-refs.c         |  2 +-
>  refs.c              | 43 +++++++++++++++++++++++++++++++++++++++++--
>  t/t3211-peel-ref.sh | 22 ++++++++++++++++++++++
>  3 files changed, 64 insertions(+), 3 deletions(-)
> 
> diff --git a/pack-refs.c b/pack-refs.c
> index ebde785..4461f71 100644
> --- a/pack-refs.c
> +++ b/pack-refs.c
> @@ -128,7 +128,7 @@ int pack_refs(unsigned int flags)
>  		die_errno("unable to create ref-pack file structure");
>  
>  	/* perhaps other traits later as well */
> -	fprintf(cbdata.refs_file, "# pack-refs with: peeled \n");
> +	fprintf(cbdata.refs_file, "# pack-refs with: peeled fully-peeled \n");
>  
>  	for_each_ref(handle_one_ref, &cbdata);
>  	if (ferror(cbdata.refs_file))
> diff --git a/refs.c b/refs.c
> index 175b9fc..bdeac28 100644
> --- a/refs.c
> +++ b/refs.c
> @@ -803,11 +803,39 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
>  	return line;
>  }
>  
> +/*
> + * Read f, which is a packed-refs file, into dir.
> + *
> + * A comment line of the form "# pack-refs with: " may contain zero or
> + * more traits. We interpret the traits as follows:
> + *
> + *   No traits:
> + *
> + *	Probably no references are peeled. But if the file contains a
> + *	peeled value for a reference, we will use it.
> + *
> + *   peeled:
> + *
> + *      References under "refs/tags/", if they *can* be peeled, *are*
> + *      peeled in this file. References outside of "refs/tags/" are
> + *      probably not peeled even if they could have been, but if we find
> + *      a peeled value for such a reference we will use it.
> + *
> + *   fully-peeled:
> + *
> + *      All references in the file that can be peeled are peeled.
> + *      Inversely (and this is more important, any references in the
> + *      file for which no peeled value is recorded is not peelable. This
> + *      trait should typically be written alongside "fully-peeled" for
> + *      compatibility with older clients, but we do not require it
> + *      (i.e., "peeled" is a no-op if "fully-peeled" is set).
> + */
>  static void read_packed_refs(FILE *f, struct ref_dir *dir)
>  {
>  	struct ref_entry *last = NULL;
>  	char refline[PATH_MAX];
>  	int flag = REF_ISPACKED;
> +	int refs_tags_peeled = 0;
>  
>  	while (fgets(refline, sizeof(refline), f)) {
>  		unsigned char sha1[20];
> @@ -816,8 +844,10 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
>  
>  		if (!strncmp(refline, header, sizeof(header)-1)) {
>  			const char *traits = refline + sizeof(header) - 1;
> -			if (strstr(traits, " peeled "))
> +			if (strstr(traits, " fully-peeled "))
>  				flag |= REF_KNOWS_PEELED;
> +			else if (strstr(traits, " peeled "))
> +				refs_tags_peeled = 1;
>  			/* perhaps other traits later as well */
>  			continue;
>  		}
> @@ -825,6 +855,8 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
>  		refname = parse_ref_line(refline, sha1);
>  		if (refname) {
>  			last = create_ref_entry(refname, sha1, flag, 1);
> +			if (refs_tags_peeled && !prefixcmp(refname, "refs/tags/"))
> +				last->flag |= REF_KNOWS_PEELED;
>  			add_ref(dir, last);
>  			continue;
>  		}
> @@ -832,8 +864,15 @@ static void read_packed_refs(FILE *f, struct ref_dir *dir)
>  		    refline[0] == '^' &&
>  		    strlen(refline) == 42 &&
>  		    refline[41] == '\n' &&
> -		    !get_sha1_hex(refline + 1, sha1))
> +		    !get_sha1_hex(refline + 1, sha1)) {
>  			hashcpy(last->u.value.peeled, sha1);
> +			/*
> +			 * Regardless of what the file header said,
> +			 * we definitely know the value of *this*
> +			 * reference:
> +			 */
> +			last->flag |= REF_KNOWS_PEELED;
> +		}
>  	}
>  }
>  
> diff --git a/t/t3211-peel-ref.sh b/t/t3211-peel-ref.sh
> index 85f09be..d4d7792 100755
> --- a/t/t3211-peel-ref.sh
> +++ b/t/t3211-peel-ref.sh
> @@ -39,4 +39,26 @@ test_expect_success 'refs are peeled outside of refs/tags (packed)' '
>  	test_cmp expect actual
>  '
>  
> +test_expect_success 'create old-style pack-refs without fully-peeled' '
> +	# Git no longer writes without fully-peeled, so we just write our own
> +	# from scratch; we could also munge the existing file to remove the
> +	# fully-peeled bits, but that seems even more prone to failure,
> +	# especially if the format ever changes again. At least this way we
> +	# know we are emulating exactly what an older git would have written.
> +	{
> +		echo "# pack-refs with: peeled " &&
> +		print_ref "refs/heads/master" &&
> +		print_ref "refs/outside/foo" &&
> +		print_ref "refs/tags/base" &&
> +		print_ref "refs/tags/foo" &&
> +		echo "^$(git rev-parse "refs/tags/foo^{}")"
> +	} >tmp &&
> +	mv tmp .git/packed-refs
> +'
> +
> +test_expect_success 'refs are peeled outside of refs/tags (old packed)' '
> +	git show-ref -d >actual &&
> +	test_cmp expect actual
> +'
> +
>  test_done
> 


-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/