odb/source: introduce source type for robustness

When a caller holds a `struct odb_source`, they have no way of telling
what type the source is. This doesn't really cause any problems in the
current status quo as we only have a single type anyway, "files". But
going forward we expect to add more types, and if so it will become
necessary to tell the sources apart.

Introduce a new enum to cover this use case and assert that the given
source actually matches the target source when performing the downcast.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: pks-t

odb/source-files.c

@@ -36,7 +36,7 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 	struct odb_source_files *files;
 
 	CALLOC_ARRAY(files, 1);
-	odb_source_init(&files->base, odb, path, local);
+	odb_source_init(&files->base, odb, ODB_SOURCE_FILES, path, local);
 	files->loose = odb_source_loose_new(&files->base);
 	files->packed = packfile_store_new(&files->base);
 

odb/source-files.h

@@ -25,10 +25,13 @@ struct odb_source_files *odb_source_files_new(struct object_database *odb,
 void odb_source_files_free(struct odb_source_files *files);
 
 /*
- * Cast the given object database source to the files backend.
+ * Cast the given object database source to the files backend. This will cause
+ * a BUG in case the source doesn't use this backend.
  */
 static inline struct odb_source_files *odb_source_files_downcast(struct odb_source *source)
 {
+	if (source->type != ODB_SOURCE_FILES)
+		BUG("trying to downcast source of type '%d' to files", source->type);
 	return container_of(source, struct odb_source_files, base);
 }
 

odb/source.c

@@ -13,10 +13,12 @@ struct odb_source *odb_source_new(struct object_database *odb,
 
 void odb_source_init(struct odb_source *source,
 		     struct object_database *odb,
+		     enum odb_source_type type,
 		     const char *path,
 		     bool local)
 {
 	source->odb = odb;
+	source->type = type;
 	source->local = local;
 	source->path = xstrdup(path);
 }

odb/source.h

@@ -1,6 +1,17 @@
 #ifndef ODB_SOURCE_H
 #define ODB_SOURCE_H
 
+enum odb_source_type {
+	/*
+	 * The "unknown" type, which should never be in use. This type mostly
+	 * exists to catch cases where the type field remains zeroed out.
+	 */
+	ODB_SOURCE_UNKNOWN,
+
+	/* The "files" backend that uses loose objects and packfiles. */
+	ODB_SOURCE_FILES,
+};
+
 /*
  * The source is the part of the object database that stores the actual
  * objects. It thus encapsulates the logic to read and write the specific
@@ -19,6 +30,9 @@ struct odb_source {
 	/* Object database that owns this object source. */
 	struct object_database *odb;
 
+	/* The type used by this source. */
+	enum odb_source_type type;
+
 	/*
 	 * Figure out whether this is the local source of the owning
 	 * repository, which would typically be its ".git/objects" directory.
@@ -58,6 +72,7 @@ struct odb_source *odb_source_new(struct object_database *odb,
  */
 void odb_source_init(struct odb_source *source,
 		     struct object_database *odb,
+		     enum odb_source_type type,
 		     const char *path,
 		     bool local);