fsck: unify object-name code

Commit 90cf590f53 (fsck: optionally show more helpful info for broken
links, 2016-07-17) added a system for decorating objects with names. The
code is split across builtin/fsck.c (which gives the initial names) and
fsck.c (which adds to the names as it traverses the object graph). This
leads to some duplication, where both sites have near-identical
describe_object() functions (the difference being that the one in
builtin/fsck.c uses a circular array of buffers to allow multiple calls
in a single printf).

Let's provide a unified object_name API for fsck. That lets us drop the
duplication, as well as making the interface boundaries more clear
(which will let us refactor the implementation more in a future patch).

We'll leave describe_object() in builtin/fsck.c as a thin wrapper around
the new API, as it relies on a static global to make its many callers a
bit shorter.

We'll also convert the bare add_decoration() calls in builtin/fsck.c to
put_object_name(). This fixes two minor bugs:

  1. We leak many small strings. add_decoration() has a last-one-wins
     approach: it updates the decoration to the new string and returns
     the old one. But we ignore the return value, leaking the old
     string. This is quite common to trigger, since we look at reflogs:
     the tip of any ref will be described both by looking at the actual
     ref, as well as the latest reflog entry. So we'd always end up
     leaking one of those strings.

  2. The last-one-wins approach gives us lousy names. For instance, we
     first look at all of the refs, and then all of the reflogs. So
     rather than seeing "refs/heads/master", we're likely to overwrite
     it with "HEAD@{12345678}". We're generally better off using the
     first name we find.

     And indeed, the test in t1450 expects this ugly HEAD@{} name. After
     this patch, we've switched to using fsck_put_object_name()'s
     first-one-wins semantics, and we output the more human-friendly
     "refs/tags/julius" (and the test is updated accordingly).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/fsck.h b/fsck.h
index e479461..6228f0b 100644
--- a/fsck.h
+++ b/fsck.h
@@ -67,4 +67,28 @@ int fsck_object(struct object *obj, void *data, unsigned long size,
  */
 int fsck_finish(struct fsck_options *options);
 
+/*
+ * Subsystem for storing human-readable names for each object.
+ *
+ * If fsck_enable_object_names() has not been called, all other functions are
+ * noops.
+ *
+ * Use fsck_put_object_name() to seed initial names (e.g. from refnames); the
+ * fsck code will extend that while walking trees, etc.
+ *
+ * Use fsck_get_object_name() to get a single name (or NULL if none). Or the
+ * more convenient describe_object(), which always produces an output string
+ * with the oid combined with the name (if any). Note that the return value
+ * points to a rotating array of static buffers, and may be invalidated by a
+ * subsequent call.
+ */
+void fsck_enable_object_names(struct fsck_options *options);
+const char *fsck_get_object_name(struct fsck_options *options,
+				 struct object *obj);
+__attribute__((format (printf,3,4)))
+void fsck_put_object_name(struct fsck_options *options, struct object *obj,
+			  const char *fmt, ...);
+const char *fsck_describe_object(struct fsck_options *options,
+				 struct object *obj);
+
 #endif