[PATCH/RFD 3/6] rev-list: list all heads with --all

[Michael J Gruber <git@drmicha.warpmail.net>]

HEAD is a worktree specific sysmref, so that a repository with multiple
worktrees can have multiple HEADs, or HEADs in a worktree different from
the current worktree.

So far, "rev-parse --all" adds only the HEAD from the current worktree
to the list of refs (besides branches etc.). So, a detached HEAD from a
different checkout would be missed unless a shared ref (or current HEAD)
points to it (or descents from it). As a consequence, "git prune" can
prune detached HEADs from worktrees and leave the repo in an
inconsistent state.

Make "rev-parse --all" add the HEADs from all worktrees. This results in
a non-worktree-specific ref list and solves the pruning problem.

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
---

Notes:
    This patch solves the pruning problem with worktrees, but I feel quite
    uneasy about substituting the ref solving at the very heart of refs.c. So
    please consider this a mere poc and a request for discussion/input
    about how to do this the right way.
    
    In essence, I feel the worktree interface still has to evolve a bit: I'd
    rather for_each_worktree than loop myself, and if many call sites need to
    be aware of multiple heads or worktrees than get_worktrees() should be
    part of our init stuff, not here. [I may be out of sync of newer progress.]

 refs/files-backend.c | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/refs/files-backend.c b/refs/files-backend.c
index 81f68f8..5bdb568 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -4,6 +4,7 @@
 #include "../lockfile.h"
 #include "../object.h"
 #include "../dir.h"
+#include "../worktree.h"
 
 struct ref_lock {
 	char *ref_name;
@@ -1748,7 +1749,8 @@ static int do_for_each_ref(struct ref_cache *refs, const char *base,
 static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
 {
 	struct object_id oid;
-	int flag;
+	struct worktree **worktrees;
+	int i, retval;
 
 	if (submodule) {
 		if (resolve_gitlink_ref(submodule, "HEAD", oid.hash) == 0)
@@ -1757,10 +1759,15 @@ static int do_head_ref(const char *submodule, each_ref_fn fn, void *cb_data)
 		return 0;
 	}
 
-	if (!read_ref_full("HEAD", RESOLVE_REF_READING, oid.hash, &flag))
-		return fn("HEAD", &oid, flag, cb_data);
+	worktrees = get_worktrees();
+	retval = 0;
+	for (i=0; worktrees[i]; i++) {
+		hashcpy(oid.hash, worktrees[i]->head_sha1);
+		retval = retval || fn("HEAD", &oid, worktrees[i]->is_detached ? 0 : REF_ISSYMREF, cb_data);
+	}
 
-	return 0;
+	free_worktrees(worktrees);
+	return retval;
 }
 
 int head_ref(each_ref_fn fn, void *cb_data)
-- 
2.7.2.618.g7a61b68