Split pack_refs() into multiple passes:

* Iterate over loose refs.  For each one that can be turned into a
  packed ref, create a corresponding entry in the packed refs cache.

* Write the packed refs to the packed-refs file.

This change isolates the mutation of the packed-refs file to a single
place.

Signed-off-by: Michael Haggerty <mhag...@alum.mit.edu>
---
 refs.c | 48 ++++++++++++++++++++++++++++++++++--------------
 1 file changed, 34 insertions(+), 14 deletions(-)

diff --git a/refs.c b/refs.c
index dccfe14..bb3640f 100644
--- a/refs.c
+++ b/refs.c
@@ -2019,35 +2019,50 @@ struct ref_to_prune {
 
 struct pack_refs_cb_data {
        unsigned int flags;
+       struct ref_dir *packed_refs;
        struct ref_to_prune *ref_to_prune;
-       int fd;
 };
 
-static int pack_one_ref(struct ref_entry *entry, void *cb_data)
+/*
+ * An each_ref_entry_fn that is run over loose references only.  If
+ * the loose reference can be packed, add an entry in the packed ref
+ * cache.  If the reference should be pruned, also add it to
+ * ref_to_prune in the pack_refs_cb_data.
+ */
+static int pack_if_possible_fn(struct ref_entry *entry, void *cb_data)
 {
        struct pack_refs_cb_data *cb = cb_data;
        enum peel_status peel_status;
+       struct ref_entry *packed_entry;
        int is_tag_ref = !prefixcmp(entry->name, "refs/tags/");
 
-       /* ALWAYS pack refs that were already packed or are tags */
-       if (!(cb->flags & PACK_REFS_ALL) && !is_tag_ref &&
-           !(entry->flag & REF_ISPACKED))
+       /* ALWAYS pack tags */
+       if (!(cb->flags & PACK_REFS_ALL) && !is_tag_ref)
                return 0;
 
        /* Do not pack symbolic or broken refs: */
        if ((entry->flag & REF_ISSYMREF) || !ref_resolves_to_object(entry))
                return 0;
 
+       /* Add a packed ref cache entry equivalent to the loose entry. */
        peel_status = peel_entry(entry, 1);
        if (peel_status != PEEL_PEELED && peel_status != PEEL_NON_TAG)
                die("internal error peeling reference %s (%s)",
                    entry->name, sha1_to_hex(entry->u.value.sha1));
-       write_packed_entry(cb->fd, entry->name, entry->u.value.sha1,
-                          peel_status == PEEL_PEELED ?
-                          entry->u.value.peeled : NULL);
+       packed_entry = find_ref(cb->packed_refs, entry->name);
+       if (packed_entry) {
+               /* Overwrite existing packed entry with info from loose entry */
+               packed_entry->flag = REF_ISPACKED | REF_KNOWS_PEELED;
+               hashcpy(packed_entry->u.value.sha1, entry->u.value.sha1);
+       } else {
+               packed_entry = create_ref_entry(entry->name, 
entry->u.value.sha1,
+                                               REF_ISPACKED | 
REF_KNOWS_PEELED, 0);
+               add_ref(cb->packed_refs, packed_entry);
+       }
+       hashcpy(packed_entry->u.value.peeled, entry->u.value.peeled);
 
-       /* If the ref was already packed, there is no need to prune it. */
-       if ((cb->flags & PACK_REFS_PRUNE) && !(entry->flag & REF_ISPACKED)) {
+       /* Schedule the loose reference for pruning if requested. */
+       if ((cb->flags & PACK_REFS_PRUNE)) {
                int namelen = strlen(entry->name) + 1;
                struct ref_to_prune *n = xcalloc(1, sizeof(*n) + namelen);
                hashcpy(n->sha1, entry->u.value.sha1);
@@ -2114,16 +2129,21 @@ static struct lock_file packlock;
 int pack_refs(unsigned int flags)
 {
        struct pack_refs_cb_data cbdata;
+       int fd;
 
        memset(&cbdata, 0, sizeof(cbdata));
        cbdata.flags = flags;
 
-       cbdata.fd = hold_lock_file_for_update(&packlock, 
git_path("packed-refs"),
-                                             LOCK_DIE_ON_ERROR);
+       fd = hold_lock_file_for_update(&packlock, git_path("packed-refs"),
+                                      LOCK_DIE_ON_ERROR);
+       cbdata.packed_refs = get_packed_refs(&ref_cache);
 
-       write_or_die(cbdata.fd, PACKED_REFS_HEADER, strlen(PACKED_REFS_HEADER));
+       do_for_each_entry_in_dir(get_loose_refs(&ref_cache), 0,
+                                pack_if_possible_fn, &cbdata);
+
+       write_or_die(fd, PACKED_REFS_HEADER, strlen(PACKED_REFS_HEADER));
+       do_for_each_entry_in_dir(cbdata.packed_refs, 0, write_packed_entry_fn, 
&fd);
 
-       do_for_each_entry(&ref_cache, "", pack_one_ref, &cbdata);
        if (commit_lock_file(&packlock) < 0)
                die_errno("unable to overwrite old ref-pack file");
        prune_refs(cbdata.ref_to_prune);
-- 
1.8.3

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