refs: add ability for backends to special-case reading of symbolic refs

Reading of symbolic and non-symbolic references is currently treated the
same in reference backends: we always call `refs_read_raw_ref()` and
then decide based on the returned flags what type it is. This has one
downside though: symbolic references may be treated different from
normal references in a backend from normal references. The packed-refs
backend for example doesn't even know about symbolic references, and as
a result it is pointless to even ask it for one.

There are cases where we really only care about whether a reference is
symbolic or not, but don't care about whether it exists at all or may be
a non-symbolic reference. But it is not possible to optimize for this
case right now, and as a consequence we will always first check for a
loose reference to exist, and if it doesn't, we'll query the packed-refs
backend for a known-to-not-be-symbolic reference. This is inefficient
and requires us to search all packed references even though we know to
not care for the result at all.

Introduce a new function `refs_read_symbolic_ref()` which allows us to
fix this case. This function will only ever return symbolic references
and can thus optimize for the scenario layed out above. By default, if
the backend doesn't provide an implementation for it, we just use the
old code path and fall back to `read_raw_ref()`. But in case the backend
provides its own, more efficient implementation, we will use that one
instead.

Note that this function is explicitly designed to not distinguish
between missing references and non-symbolic references. If it did, we'd
be forced to always search the packed-refs backend to see whether the
symbolic reference the user asked for really doesn't exist, or if it
exists as a non-symbolic reference.

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

refs.c

@@ -1672,6 +1672,23 @@ int refs_read_raw_ref(struct ref_store *ref_store, const char *refname,
 					   type, failure_errno);
 }
 
+int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+			   struct strbuf *referent)
+{
+	struct object_id oid;
+	int ret, failure_errno = 0;
+	unsigned int type = 0;
+
+	if (ref_store->be->read_symbolic_ref)
+		return ref_store->be->read_symbolic_ref(ref_store, refname, referent);
+
+	ret = refs_read_raw_ref(ref_store, refname, &oid, referent, &type, &failure_errno);
+	if (ret || !(type & REF_ISSYMREF))
+		return -1;
+
+	return 0;
+}
+
 const char *refs_resolve_ref_unsafe(struct ref_store *refs,
 				    const char *refname,
 				    int resolve_flags,

refs.h

@@ -82,6 +82,9 @@ int read_ref_full(const char *refname, int resolve_flags,
 		  struct object_id *oid, int *flags);
 int read_ref(const char *refname, struct object_id *oid);
 
+int refs_read_symbolic_ref(struct ref_store *ref_store, const char *refname,
+			   struct strbuf *referent);
+
 /*
  * Return 0 if a reference named refname could be created without
  * conflicting with the name of an existing reference. Otherwise,

refs/debug.c

@@ -435,6 +435,7 @@ struct ref_storage_be refs_be_debug = {
 
 	debug_ref_iterator_begin,
 	debug_read_raw_ref,
+	NULL,
 
 	debug_reflog_iterator_begin,
 	debug_for_each_reflog_ent,

refs/files-backend.c

@@ -3286,6 +3286,7 @@ struct ref_storage_be refs_be_files = {
 
 	files_ref_iterator_begin,
 	files_read_raw_ref,
+	NULL,
 
 	files_reflog_iterator_begin,
 	files_for_each_reflog_ent,

refs/packed-backend.c

@@ -1684,6 +1684,7 @@ struct ref_storage_be refs_be_packed = {
 
 	packed_ref_iterator_begin,
 	packed_read_raw_ref,
+	NULL,
 
 	packed_reflog_iterator_begin,
 	packed_for_each_reflog_ent,

refs/refs-internal.h

@@ -649,6 +649,21 @@ typedef int read_raw_ref_fn(struct ref_store *ref_store, const char *refname,
 			    struct object_id *oid, struct strbuf *referent,
 			    unsigned int *type, int *failure_errno);
 
+/*
+ * Read a symbolic reference from the specified reference store. This function
+ * is optional: if not implemented by a backend, then `read_raw_ref_fn` is used
+ * to read the symbolcic reference instead. It is intended to be implemented
+ * only in case the backend can optimize the reading of symbolic references.
+ *
+ * Return 0 on success, or -1 on failure. `referent` will be set to the target
+ * of the symbolic reference on success. This function explicitly does not
+ * distinguish between error cases and the reference not being a symbolic
+ * reference to allow backends to optimize this operation in case symbolic and
+ * non-symbolic references are treated differently.
+ */
+typedef int read_symbolic_ref_fn(struct ref_store *ref_store, const char *refname,
+				 struct strbuf *referent);
+
 struct ref_storage_be {
 	struct ref_storage_be *next;
 	const char *name;
@@ -668,6 +683,7 @@ struct ref_storage_be {
 
 	ref_iterator_begin_fn *iterator_begin;
 	read_raw_ref_fn *read_raw_ref;
+	read_symbolic_ref_fn *read_symbolic_ref;
 
 	reflog_iterator_begin_fn *reflog_iterator_begin;
 	for_each_reflog_ent_fn *for_each_reflog_ent;