refs: tidy up and adjust visibility of the `ref_update` flags

The constants used for `ref_update::flags` were rather disorganized:

* The definitions in `refs.h` were not close to the functions that
  used them.

* Maybe constants were defined in `refs-internal.h`, making them
  visible to the whole refs module, when in fact they only made sense
  for the files backend.

* Their documentation wasn't very consistent and partly still referred
  to sha1s rather than oids.

* The numerical values followed no rational scheme

Fix all of these problems. The main functional improvement is that
some constants' visibility is now limited to `files-backend.c`.

Signed-off-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

2 files changed, 60 insertions, 52 deletions

diff --git a/refs/files-backend.c b/refs/files-backend.c
index a47771e..bbeafe1 100644
--- a/refs/files-backend.c
+++ b/refs/files-backend.c
@@ -10,6 +10,51 @@
 #include "../object.h"
 #include "../dir.h"
 
+/*
+ * This backend uses the following flags in `ref_update::flags` for
+ * internal bookkeeping purposes. Their numerical values must not
+ * conflict with REF_NODEREF, REF_FORCE_CREATE_REFLOG, REF_HAVE_NEW,
+ * REF_HAVE_OLD, or REF_ISPRUNING, which are also stored in
+ * `ref_update::flags`.
+ */
+
+/*
+ * Used as a flag in ref_update::flags when a loose ref is being
+ * pruned. This flag must only be used when REF_NODEREF is set.
+ */
+#define REF_ISPRUNING (1 << 4)
+
+/*
+ * Flag passed to lock_ref_sha1_basic() telling it to tolerate broken
+ * refs (i.e., because the reference is about to be deleted anyway).
+ */
+#define REF_DELETING (1 << 5)
+
+/*
+ * Used as a flag in ref_update::flags when the lockfile needs to be
+ * committed.
+ */
+#define REF_NEEDS_COMMIT (1 << 6)
+
+/*
+ * Used as a flag in ref_update::flags when we want to log a ref
+ * update but not actually perform it.  This is used when a symbolic
+ * ref update is split up.
+ */
+#define REF_LOG_ONLY (1 << 7)
+
+/*
+ * Used as a flag in ref_update::flags when the ref_update was via an
+ * update to HEAD.
+ */
+#define REF_UPDATE_VIA_HEAD (1 << 8)
+
+/*
+ * Used as a flag in ref_update::flags when the loose reference has
+ * been deleted.
+ */
+#define REF_DELETED_LOOSE (1 << 9)
+
 struct ref_lock {
 	char *ref_name;
 	struct lock_file lk;
diff --git a/refs/refs-internal.h b/refs/refs-internal.h
index b0f3e30..2ea02dc 100644
--- a/refs/refs-internal.h
+++ b/refs/refs-internal.h
@@ -8,58 +8,22 @@
  */
 
 /*
- * Flag passed to lock_ref_sha1_basic() telling it to tolerate broken
- * refs (i.e., because the reference is about to be deleted anyway).
+ * The following flags can appear in `ref_update::flags`. Their
+ * numerical values must not conflict with those of REF_NODEREF and
+ * REF_FORCE_CREATE_REFLOG, which are also stored in
+ * `ref_update::flags`.
  */
-#define REF_DELETING	0x02
 
 /*
- * Used as a flag in ref_update::flags when a loose ref is being
- * pruned. This flag must only be used when REF_NODEREF is set.
+ * The reference should be updated to new_sha1.
  */
-#define REF_ISPRUNING	0x04
+#define REF_HAVE_NEW (1 << 2)
 
 /*
- * Used as a flag in ref_update::flags when the reference should be
- * updated to new_sha1.
+ * The current reference's value should be checked to make sure that
+ * it agrees with old_sha1.
  */
-#define REF_HAVE_NEW	0x08
-
-/*
- * Used as a flag in ref_update::flags when old_sha1 should be
- * checked.
- */
-#define REF_HAVE_OLD	0x10
-
-/*
- * Used as a flag in ref_update::flags when the lockfile needs to be
- * committed.
- */
-#define REF_NEEDS_COMMIT 0x20
-
-/*
- * 0x40 is REF_FORCE_CREATE_REFLOG, so skip it if you're adding a
- * value to ref_update::flags
- */
-
-/*
- * Used as a flag in ref_update::flags when we want to log a ref
- * update but not actually perform it.  This is used when a symbolic
- * ref update is split up.
- */
-#define REF_LOG_ONLY 0x80
-
-/*
- * Internal flag, meaning that the containing ref_update was via an
- * update to HEAD.
- */
-#define REF_UPDATE_VIA_HEAD 0x100
-
-/*
- * Used as a flag in ref_update::flags when the loose reference has
- * been deleted.
- */
-#define REF_DELETED_LOOSE 0x200
+#define REF_HAVE_OLD (1 << 3)
 
 /*
  * Return the length of time to retry acquiring a loose reference lock
@@ -141,23 +105,22 @@ int copy_reflog_msg(char *buf, const char *msg);
  * not exist before update.
  */
 struct ref_update {
-
 	/*
-	 * If (flags & REF_HAVE_NEW), set the reference to this value:
+	 * If (flags & REF_HAVE_NEW), set the reference to this value
+	 * (or delete it, if `new_oid` is `null_oid`).
 	 */
 	struct object_id new_oid;
 
 	/*
 	 * If (flags & REF_HAVE_OLD), check that the reference
-	 * previously had this value:
+	 * previously had this value (or didn't previously exist, if
+	 * `old_oid` is `null_oid`).
 	 */
 	struct object_id old_oid;
 
 	/*
-	 * One or more of REF_HAVE_NEW, REF_HAVE_OLD, REF_NODEREF,
-	 * REF_DELETING, REF_ISPRUNING, REF_LOG_ONLY,
-	 * REF_UPDATE_VIA_HEAD, REF_NEEDS_COMMIT, and
-	 * REF_DELETED_LOOSE:
+	 * One or more of REF_NODEREF, REF_FORCE_CREATE_REFLOG,
+	 * REF_HAVE_NEW, REF_HAVE_OLD, or backend-specific flags.
 	 */
 	unsigned int flags;