odb: return temporary ODB source when set

When invoked, `odb_set_temporary_primary_source()` installs a temporary
object directory as the new primary ODB source. A caller that wants to
operate on the ODB source of the open transaction must assume that it is
the first entry in the ODB source list which is a bit awkward and
fragile.

Instead, return the newly installed source directly and report the
previous primary source via a new `prev_source` out parameter. Propagate
the installed source through `tmp_objdir_replace_primary_odb()` and
start storing it in the "files" ODB transaction so a subsequent commit
can easily access it without relying on the ODB source list ordering.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object-file.c

diff --git a/object-file.c b/object-file.c
index 4d03c16..db63587 100644
--- a/object-file.c
+++ b/object-file.c
@@ -485,6 +485,7 @@ struct odb_transaction_files {
 	struct odb_transaction base;
 
 	struct tmp_objdir *objdir;
+	struct odb_source *quarantine;
 	struct transaction_packfile packfile;
 	const char *prefix;
 };
@@ -507,7 +508,7 @@ int odb_transaction_files_prepare(struct odb_transaction *base)
 	if (!transaction->objdir)
 		return error(_("unable to create temporary object directory"));
 
-	tmp_objdir_replace_primary_odb(transaction->objdir, 0);
+	transaction->quarantine = tmp_objdir_replace_primary_odb(transaction->objdir, 0);
 
 	return 0;
 }

odb.c

diff --git a/odb.c b/odb.c
index caf1d0f..8afcb6b 100644
--- a/odb.c
+++ b/odb.c
@@ -226,7 +226,8 @@ struct odb_source *odb_add_to_alternates_memory(struct object_database *odb,
 }
 
 struct odb_source *odb_set_temporary_primary_source(struct object_database *odb,
-						    const char *dir, int will_destroy)
+						    const char *dir, int will_destroy,
+						    struct odb_source **prev_source)
 {
 	struct odb_source *source;
 
@@ -250,7 +251,11 @@ struct odb_source *odb_set_temporary_primary_source(struct object_database *odb,
 	source->will_destroy = will_destroy;
 	source->next = odb->sources;
 	odb->sources = source;
-	return source->next;
+
+	if (prev_source)
+		*prev_source = source->next;
+
+	return source;
 }
 
 void odb_restore_primary_source(struct object_database *odb,

odb.h

diff --git a/odb.h b/odb.h
index fca67e8..bdfcb95 100644
--- a/odb.h
+++ b/odb.h
@@ -199,10 +199,12 @@ struct odb_source *odb_find_source_or_die(struct object_database *odb, const cha
 
 /*
  * Replace the current writable object directory with the specified temporary
- * object directory; returns the former primary source.
+ * object directory and return the newly installed primary source. The former
+ * primary source is reported via `prev_source` when non-NULL.
  */
 struct odb_source *odb_set_temporary_primary_source(struct object_database *odb,
-						    const char *dir, int will_destroy);
+						    const char *dir, int will_destroy,
+						    struct odb_source **prev_source);
 
 /*
  * Restore the primary source that was previously replaced by

tmp-objdir.c

diff --git a/tmp-objdir.c b/tmp-objdir.c
index d199d39..e633d97 100644
--- a/tmp-objdir.c
+++ b/tmp-objdir.c
@@ -327,11 +327,13 @@ void tmp_objdir_add_as_alternate(const struct tmp_objdir *t)
 	odb_add_to_alternates_memory(t->repo->objects, t->path.buf);
 }
 
-void tmp_objdir_replace_primary_odb(struct tmp_objdir *t, int will_destroy)
+struct odb_source *tmp_objdir_replace_primary_odb(struct tmp_objdir *t,
+						  int will_destroy)
 {
 	if (t->prev_source)
 		BUG("the primary object database is already replaced");
-	t->prev_source = odb_set_temporary_primary_source(t->repo->objects,
-							  t->path.buf, will_destroy);
 	t->will_destroy = will_destroy;
+
+	return odb_set_temporary_primary_source(t->repo->objects, t->path.buf,
+						will_destroy, &t->prev_source);
 }

tmp-objdir.h

diff --git a/tmp-objdir.h b/tmp-objdir.h
index ccf800f..81eb927 100644
--- a/tmp-objdir.h
+++ b/tmp-objdir.h
@@ -64,8 +64,10 @@ void tmp_objdir_add_as_alternate(const struct tmp_objdir *);
 /*
  * Replaces the writable object store in the current process with the temporary
  * object directory and makes the former main object store an alternate.
- * If will_destroy is nonzero, the object directory may not be migrated.
+ * If will_destroy is nonzero, the object directory may not be migrated. Returns
+ * the newly installed primary source.
  */
-void tmp_objdir_replace_primary_odb(struct tmp_objdir *, int will_destroy);
+struct odb_source *tmp_objdir_replace_primary_odb(struct tmp_objdir *,
+						  int will_destroy);
 
 #endif /* TMP_OBJDIR_H */