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

From: Michael Haggerty <redacted>

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 [off-list ref]]

Signed-off-by: Jeff King <redacted>
---
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

@@ -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;