odb: introduce odb_for_each_object()

Introduce a new function `odb_for_each_object()` that knows to iterate
through all objects part of a given object database. This function is
essentially a simple wrapper around the object database sources.

Subsequent commits will adapt callers to use this new function.

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

object-file.h

@@ -139,9 +139,10 @@ int for_each_loose_object(struct object_database *odb,
 
 /*
  * Iterate through all loose objects in the given object database source and
- * invoke the callback function for each of them. If given, the object info
- * will be populated with the object's data as if you had called
- * `odb_source_loose_read_object_info()` on the object.
+ * invoke the callback function for each of them. If an object info request is
+ * given, then the object info will be read for every individual object and
+ * passed to the callback as if `odb_source_loose_read_object_info()` was
+ * called for the object.
  */
 int odb_source_loose_for_each_object(struct odb_source *source,
 				     const struct object_info *request,

odb.c

@@ -995,6 +995,35 @@ int odb_freshen_object(struct object_database *odb,
 	return 0;
 }
 
+int odb_for_each_object(struct object_database *odb,
+			const struct object_info *request,
+			odb_for_each_object_cb cb,
+			void *cb_data,
+			unsigned flags)
+{
+	int ret;
+
+	odb_prepare_alternates(odb);
+	for (struct odb_source *source = odb->sources; source; source = source->next) {
+		if (flags & ODB_FOR_EACH_OBJECT_LOCAL_ONLY && !source->local)
+			continue;
+
+		if (!(flags & ODB_FOR_EACH_OBJECT_PROMISOR_ONLY)) {
+			ret = odb_source_loose_for_each_object(source, request,
+							       cb, cb_data, flags);
+			if (ret)
+				return ret;
+		}
+
+		ret = packfile_store_for_each_object(source->packfiles, request,
+						     cb, cb_data, flags);
+		if (ret)
+			return ret;
+	}
+
+	return 0;
+}
+
 void odb_assert_oid_type(struct object_database *odb,
 			 const struct object_id *oid, enum object_type expect)
 {

odb.h

@@ -475,6 +475,26 @@ typedef int (*odb_for_each_object_cb)(const struct object_id *oid,
 				      struct object_info *oi,
 				      void *cb_data);
 
+/*
+ * Iterate through all objects contained in the object database. Note that
+ * objects may be iterated over multiple times in case they are either stored
+ * in different backends or in case they are stored in multiple sources.
+ * If an object info request is given, then the object info will be read and
+ * passed to the callback as if `odb_read_object_info()` was called for the
+ * object.
+ *
+ * Returning a non-zero error code from the callback function will cause
+ * iteration to abort. The error code will be propagated.
+ *
+ * Returns 0 on success, a negative error code in case a failure occurred, or
+ * an arbitrary non-zero error code returned by the callback itself.
+ */
+int odb_for_each_object(struct object_database *odb,
+			const struct object_info *request,
+			odb_for_each_object_cb cb,
+			void *cb_data,
+			unsigned flags);
+
 enum {
 	/*
 	 * By default, `odb_write_object()` does not actually write anything