remerge-diff: lazily prepare temporary objdir on demand

It is error prone for each caller that sets revs.remerge_diff bit
to be responsible for preparing a temporary object directory and
rotate it into the list of alternate object stores, making it the
primary object store.

Instead, remove the code to set up and arrange the temporary object
directory from the current callers and implement it in the code that
runs remerge-diff logic.  The code to undo the futzing of the list
of alternate object store is still spread across the callers, but we
will deal with it in future steps.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 12 insertions, 4 deletions

diff --git a/log-tree.c b/log-tree.c
index 337b9334cd..83e648b7cd 100644
--- a/log-tree.c
+++ b/log-tree.c
@@ -1019,6 +1019,17 @@ static int do_remerge_diff(struct rev_info *opt,
 	struct strbuf parent1_desc = STRBUF_INIT;
 	struct strbuf parent2_desc = STRBUF_INIT;
 
+	/*
+	 * Lazily prepare a temporary object directory and rotate it
+	 * into the alternative object store list as the primary.
+	 */
+	if (opt->remerge_diff && !opt->remerge_objdir) {
+		opt->remerge_objdir = tmp_objdir_create("remerge-diff");
+		if (!opt->remerge_objdir)
+			return error(_("unable to create temporary object directory"));
+		tmp_objdir_replace_primary_odb(opt->remerge_objdir, 1);
+	}
+
 	/* Setup merge options */
 	init_merge_options(&o, the_repository);
 	o.show_rename_progress = 0;
@@ -1053,10 +1064,7 @@ static int do_remerge_diff(struct rev_info *opt,
 	merge_finalize(&o, &res);
 
 	/* Clean up the contents of the temporary object directory */
-	if (opt->remerge_objdir)
-		tmp_objdir_discard_objects(opt->remerge_objdir);
-	else
-		BUG("did a remerge diff without remerge_objdir?!?");
+	tmp_objdir_discard_objects(opt->remerge_objdir);
 
 	return !opt->loginfo;
 }