refspec: add support for negative refspecs

Both fetch and push support pattern refspecs which allow fetching or
pushing references that match a specific pattern. Because these patterns
are globs, they have somewhat limited ability to express more complex
situations.

For example, suppose you wish to fetch all branches from a remote except
for a specific one. To allow this, you must setup a set of refspecs
which match only the branches you want. Because refspecs are either
explicit name matches, or simple globs, many patterns cannot be
expressed.

Add support for a new type of refspec, referred to as "negative"
refspecs. These are prefixed with a '^' and mean "exclude any ref
matching this refspec". They can only have one "side" which always
refers to the source. During a fetch, this refers to the name of the ref
on the remote. During a push, this refers to the name of the ref on the
local side.

With negative refspecs, users can express more complex patterns. For
example:

 git fetch origin refs/heads/*:refs/remotes/origin/* ^refs/heads/dontwant

will fetch all branches on origin into remotes/origin, but will exclude
fetching the branch named dontwant.

Refspecs today are commutative, meaning that order doesn't expressly
matter. Rather than forcing an implied order, negative refspecs will
always be applied last. That is, in order to match, a ref must match at
least one positive refspec, and match none of the negative refspecs.
This is similar to how negative pathspecs work.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 8 insertions, 1 deletions

diff --git a/remote.h b/remote.h
index 5e3ea5a26d..104e75e0f7 100644
--- a/remote.h
+++ b/remote.h
@@ -193,6 +193,12 @@ int resolve_remote_symref(struct ref *ref, struct ref *list);
  */
 struct ref *ref_remove_duplicates(struct ref *ref_map);
 
+/*
+ * Remove all entries in the input list which match any negative refspec in
+ * the refspec list.
+ */
+struct ref *apply_negative_refspecs(struct ref *ref_map, struct refspec *rs);
+
 int query_refspecs(struct refspec *rs, struct refspec_item *query);
 char *apply_refspecs(struct refspec *rs, const char *name);
 
@@ -205,7 +211,8 @@ void set_ref_status_for_push(struct ref *remote_refs, int send_mirror,
 /*
  * Given a list of the remote refs and the specification of things to
  * fetch, makes a (separate) list of the refs to fetch and the local
- * refs to store into.
+ * refs to store into. Note that negative refspecs are ignored here, and
+ * should be handled separately.
  *
  * *tail is the pointer to the tail pointer of the list of results
  * beforehand, and will be set to the tail pointer of the list of