[PATCH v4 0/3] repack -ad: fix after fetch --prune in a shallow repository

Under certain circumstances, commits that were reachable can be made unreachable, e.g. via git fetch --prune. These commits might have been packed already, in which case git repack -adlf will just drop them without giving them the usual grace period before an eventual git prune (via git gc) prunes them.

This is a problem when the shallow file still lists them, which is the reason why git prune edits that file. And with the proposed changes, git repack -ad will now do the same.

Reported by Alejandro Pauly.

Changes since v3:

 * Made the regression test less confusing with regards to tags that might
   need to be dereferenced.
 * Introduced new global constants PRUNE_SHOW_ONLY and PRUNE_QUICK instead
   of extending the signature of prune_shallow() with Boolean parameters.
 * While at it, renamed the file-local constant from QUICK_PRUNE to QUICK.
 * Replaced the lookup_commit() && parse_commit() cadence (that wants to
   test for the existence of a commit) to has_object_file(), for ease of
   readability, and also to make it more obvious how to add a call to 
   is_promisor_object() if we ever figure out that that would be necessary.

Changes since v2:

 * Fixed a typo in the last commit message.
 * Added an explanation to the last commit message why we do not simply skip
   non-existing shallow commits at fetch time.
 * Introduced a new, "quick prune" mode where prune_shallow() does not try
   to drop unreachable commits, but only non-existing ones.
 * Rebased to current master because there were too many merge conflicts for
   my liking otherwise.

Changes since v1:

 * Also trigger prune_shallow() when --unpack-unreachable=<approxidate> was
   passed to git repack.
 * No need to trigger prune_shallow() when git repack was called with -k.

Johannes Schindelin (3):
  repack: point out a bug handling stale shallow info
  shallow: offer to prune only non-existing entries
  repack -ad: prune the list of shallow commits

 builtin/prune.c          |  2 +-
 builtin/repack.c         |  6 ++++++
 commit.h                 |  4 +++-
 shallow.c                | 23 +++++++++++++++++------
 t/t5537-fetch-shallow.sh | 27 +++++++++++++++++++++++++++
 5 files changed, 54 insertions(+), 8 deletions(-)

base-commit: c4df23f7927d8d00e666a3c8d1b3375f1dc8a3c1
Published-As: https://github.com/gitgitgadget/git/releases/tags/pr-9%2Fdscho%2Fshallow-and-fetch-prune-v4
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-9/dscho/shallow-and-fetch-prune-v4
Pull-Request: https://github.com/gitgitgadget/git/pull/9

Range-diff vs v3:

 1:  ed8559b91 ! 1:  d9720bd25 repack: point out a bug handling stale shallow info
     @@ -29,7 +29,7 @@
      +	test_commit -C shallow-server C &&
      +	test_commit -C shallow-server E &&
      +	test_commit -C shallow-server D &&
     -+	d="$(git -C shallow-server rev-parse --verify D)" &&
     ++	d="$(git -C shallow-server rev-parse --verify D^0)" &&
      +	git -C shallow-server checkout master &&
      +
      +	git clone --depth=1 --no-tags --no-single-branch \
     @@ -43,7 +43,7 @@
      +	test_must_fail git -C shallow-client rev-parse --verify $d^0 &&
      +	! grep $d shallow-client/.git/shallow &&
      +
     -+	git -C shallow-server branch branch-orig D^0 &&
     ++	git -C shallow-server branch branch-orig $d &&
      +	git -C shallow-client fetch --prune --depth=2 \
      +		origin "+refs/heads/*:refs/remotes/origin/*"
      +'
 2:  f085eb4f7 ! 2:  18308c13e shallow: offer to prune only non-existing entries
     @@ -12,6 +12,10 @@
      
          Let's support that use case.
      
     +    Rather than extending the signature of `prune_shallow()` to accept
     +    another Boolean, let's turn it into a bit field and declare constants,
     +    for readability.
     +
          Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
      
      diff --git a/builtin/prune.c b/builtin/prune.c
     @@ -22,7 +26,7 @@
       
       	if (is_repository_shallow(the_repository))
      -		prune_shallow(show_only);
     -+		prune_shallow(show_only, 0);
     ++		prune_shallow(show_only ? PRUNE_SHOW_ONLY : 0);
       
       	return 0;
       }
     @@ -35,7 +39,9 @@
       					   int *ref_status);
       extern int delayed_reachability_test(struct shallow_info *si, int c);
      -extern void prune_shallow(int show_only);
     -+extern void prune_shallow(int show_only, int quick_prune);
     ++#define PRUNE_SHOW_ONLY 1
     ++#define PRUNE_QUICK 2
     ++extern void prune_shallow(unsigned options);
       extern struct trace_key trace_shallow;
       
       extern int interactive_add(int argc, const char **argv, const char *prefix, int patch);
     @@ -47,7 +53,7 @@
       
       #define SEEN_ONLY 1
       #define VERBOSE   2
     -+#define QUICK_PRUNE 4
     ++#define QUICK 4
       
       struct write_shallow_data {
       	struct strbuf *out;
     @@ -56,9 +62,8 @@
       	if (graft->nr_parent != -1)
       		return 0;
      -	if (data->flags & SEEN_ONLY) {
     -+	if (data->flags & QUICK_PRUNE) {
     -+		struct commit *c = lookup_commit(the_repository, &graft->oid);
     -+		if (!c || parse_commit(c))
     ++	if (data->flags & QUICK) {
     ++		if (!has_object_file(&graft->oid))
      +			return 0;
      +	} else if (data->flags & SEEN_ONLY) {
       		struct commit *c = lookup_commit(the_repository, &graft->oid);
     @@ -74,18 +79,19 @@
      + * commits that do not exist (any longer).
        */
      -void prune_shallow(int show_only)
     -+void prune_shallow(int show_only, int quick_prune)
     ++void prune_shallow(unsigned options)
       {
       	struct lock_file shallow_lock = LOCK_INIT;
       	struct strbuf sb = STRBUF_INIT;
      +	unsigned flags = SEEN_ONLY;
       	int fd;
       
     -+	if (quick_prune)
     -+		flags |= QUICK_PRUNE;
     -+
     - 	if (show_only) {
     +-	if (show_only) {
      -		write_shallow_commits_1(&sb, 0, NULL, SEEN_ONLY | VERBOSE);
     ++	if (options & PRUNE_QUICK)
     ++		flags |= QUICK;
     ++
     ++	if (options & PRUNE_SHOW_ONLY) {
      +		flags |= VERBOSE;
      +		write_shallow_commits_1(&sb, 0, NULL, flags);
       		strbuf_release(&sb);
 3:  1f9ff57d5 ! 3:  2858bc886 repack -ad: prune the list of shallow commits
     @@ -52,7 +52,7 @@
      +		    (!(pack_everything & LOOSEN_UNREACHABLE) ||
      +		     unpack_unreachable) &&
      +		    is_repository_shallow(the_repository))
     -+			prune_shallow(0, 1);
     ++			prune_shallow(PRUNE_QUICK);
       	}
       
       	if (!no_update_server_info)