fetch: only opportunistically update references based on command line

The old code processed (tags == TAGS_SET) before adding the entries
used to opportunistically update references mentioned on the command
line.  The result was that all tags were also considered candidates
for opportunistic updating.

This is harmless for two reasons: (a) because it would only add
entries if there is a configured refspec that covers tags *and* both
--tags and another refspec appear on the command-line; (b) because any
extra entries would be deleted later by the call to
ref_remove_duplicates() anyway.

But, to avoid extra work and extra memory usage, and to make the
implementation better match the intention, change the algorithm
slightly: compute the opportunistic refspecs based only on the
command-line arguments, storing the results into a separate temporary
list.  Then add the tags (which have to come earlier in the list so
that they are not de-duped in favor of an opportunistic entry).  Then
concatenate the temporary list onto the main list.

This change will also make later changes easier.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/fetch.c

@@ -272,32 +272,50 @@ static struct ref *get_ref_map(struct transport *transport,
 	const struct ref *remote_refs = transport_get_remote_refs(transport);
 
 	if (refspec_count || tags == TAGS_SET) {
-		struct ref **old_tail;
+		/* opportunistically-updated references: */
+		struct ref *orefs = NULL, **oref_tail = &orefs;
 
 		for (i = 0; i < refspec_count; i++) {
 			get_fetch_map(remote_refs, &refspecs[i], &tail, 0);
 			if (refspecs[i].dst && refspecs[i].dst[0])
 				*autotags = 1;
 		}
-		/* Merge everything on the command line, but not --tags */
+		/* Merge everything on the command line (but not --tags) */
 		for (rm = ref_map; rm; rm = rm->next)
 			rm->fetch_head_status = FETCH_HEAD_MERGE;
-		if (tags == TAGS_SET)
-			get_fetch_map(remote_refs, tag_refspec, &tail, 0);
 
 		/*
-		 * For any refs that we happen to be fetching via command-line
-		 * arguments, take the opportunity to update their configured
-		 * counterparts. However, we do not want to mention these
-		 * entries in FETCH_HEAD at all, as they would simply be
-		 * duplicates of existing entries.
+		 * For any refs that we happen to be fetching via
+		 * command-line arguments, the destination ref might
+		 * have been missing or have been different than the
+		 * remote-tracking ref that would be derived from the
+		 * configured refspec.  In these cases, we want to
+		 * take the opportunity to update their configured
+		 * remote-tracking reference.  However, we do not want
+		 * to mention these entries in FETCH_HEAD at all, as
+		 * they would simply be duplicates of existing
+		 * entries, so we set them FETCH_HEAD_IGNORE below.
+		 *
+		 * We compute these entries now, based only on the
+		 * refspecs specified on the command line.  But we add
+		 * them to the list following the refspecs resulting
+		 * from the tags option so that one of the latter,
+		 * which has FETCH_HEAD_NOT_FOR_MERGE, is not removed
+		 * by ref_remove_duplicates() in favor of one of these
+		 * opportunistic entries with FETCH_HEAD_IGNORE.
 		 */
-		old_tail = tail;
 		for (i = 0; i < transport->remote->fetch_refspec_nr; i++)
 			get_fetch_map(ref_map, &transport->remote->fetch[i],
-				      &tail, 1);
-		for (rm = *old_tail; rm; rm = rm->next)
+				      &oref_tail, 1);
+
+		if (tags == TAGS_SET)
+			get_fetch_map(remote_refs, tag_refspec, &tail, 0);
+
+		*tail = orefs;
+		for (rm = orefs; rm; rm = rm->next) {
 			rm->fetch_head_status = FETCH_HEAD_IGNORE;
+			tail = &rm->next;
+		}
 	} else {
 		/* Use the defaults */
 		struct remote *remote = transport->remote;
@@ -334,8 +352,10 @@ static struct ref *get_ref_map(struct transport *transport,
 			tail = &ref_map->next;
 		}
 	}
+
 	if (tags == TAGS_DEFAULT && *autotags)
 		find_non_local_tags(transport, &ref_map, &tail);
+
 	ref_remove_duplicates(ref_map);
 
 	return ref_map;