refs: introduce wrapper struct for `each_ref_fn`

The `each_ref_fn` callback function type is used across our code base
for several different functions that iterate through reference. There's
a bunch of callbacks implementing this type, which makes any changes to
the callback signature extremely noisy. An example of the required churn
is e8207717f1 (refs: add referent to each_ref_fn, 2024-08-09): adding a
single argument required us to change 48 files.

It was already proposed back then [1] that we might want to introduce a
wrapper structure to alleviate the pain going forward. While this of
course requires the same kind of global refactoring as just introducing
a new parameter, it at least allows us to more change the callback type
afterwards by just extending the wrapper structure.

One counterargument to this refactoring is that it makes the structure
more opaque. While it is obvious which callsites need to be fixed up
when we change the function type, it's not obvious anymore once we use
a structure. That being said, we only have a handful of sites that
actually need to populate this wrapper structure: our ref backends,
"refs/iterator.c" as well as very few sites that invoke the iterator
callback functions directly.

Introduce this wrapper structure so that we can adapt the iterator
interfaces more readily.

[1]: <ZmarVcF5JjsZx0dl@tanuki>

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

1 files changed, 22 insertions, 4 deletions

diff --git a/refs.h b/refs.h
index 4e6bd63aa8..68d235438c 100644
--- a/refs.h
+++ b/refs.h
@@ -355,14 +355,32 @@ struct ref_transaction;
  */
 #define REF_BAD_NAME 0x08
 
+/* A reference passed to `for_each_ref()`-style callbacks. */
+struct reference {
+	/* The fully-qualified name of the reference. */
+	const char *name;
+
+	/* The target of a symbolic ref. `NULL` for direct references. */
+	const char *target;
+
+	/*
+	 * The object ID of a reference. Either the direct object ID or the
+	 * resolved object ID in the case of a symbolic ref. May be the zero
+	 * object ID in case the symbolic ref cannot be resolved.
+	 */
+	const struct object_id *oid;
+
+	/* A bitfield of `REF_` flags. */
+	int flags;
+};
+
 /*
  * The signature for the callback function for the for_each_*()
- * functions below.  The memory pointed to by the refname and oid
- * arguments is only guaranteed to be valid for the duration of a
+ * functions below.  The memory pointed to by the `struct reference`
+ * argument is only guaranteed to be valid for the duration of a
  * single callback invocation.
  */
-typedef int each_ref_fn(const char *refname, const char *referent,
-			const struct object_id *oid, int flags, void *cb_data);
+typedef int each_ref_fn(const struct reference *ref, void *cb_data);
 
 /*
  * The following functions invoke the specified callback function for