update-ref: -d flag and ref creation safety.

This adds -d flag to update-ref to allow safe deletion of ref.
Before deleting it, the command checks if the given <oldvalue>
still matches the value the caller thought the ref contained.

Similarly, it also accepts 0{40} or an empty string as <oldvalue>
to allow safe creation of a new ref.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-update-ref.txt

@@ -7,7 +7,7 @@ git-update-ref - update the object name stored in a ref safely
 
 SYNOPSIS
 --------
-'git-update-ref' [-m <reason>] <ref> <newvalue> [<oldvalue>]
+'git-update-ref' [-m <reason>] (-d <ref> <oldvalue> | <ref> <newvalue> [<oldvalue>])
 
 DESCRIPTION
 -----------
@@ -20,7 +20,9 @@ possibly dereferencing the symbolic refs, after verifying that
 the current value of the <ref> matches <oldvalue>.
 E.g. `git-update-ref refs/heads/master <newvalue> <oldvalue>`
 updates the master branch head to <newvalue> only if its current
-value is <oldvalue>.
+value is <oldvalue>.  You can specify 40 "0" or an empty string
+as <oldvalue> to make sure that the ref you are creating does
+not exist.
 
 It also allows a "ref" file to be a symbolic pointer to another
 ref file by starting with the four-byte header sequence of
@@ -49,6 +51,10 @@ for reading but not for writing (so we'll never write through a
 ref symlink to some other tree, if you have copied a whole
 archive by creating a symlink tree).
 
+With `-d` flag, it deletes the named <ref> after verifying it
+still contains <oldvalue>.
+
+
 Logging Updates
 ---------------
 If config parameter "core.logAllRefUpdates" is true or the file

builtin-update-ref.c

@@ -3,15 +3,16 @@
 #include "builtin.h"
 
 static const char git_update_ref_usage[] =
-"git-update-ref <refname> <value> [<oldval>] [-m <reason>]";
+"git-update-ref [-m <reason>] (-d <refname> <value> | <refname> <value> [<oldval>])";
 
 int cmd_update_ref(int argc, const char **argv, const char *prefix)
 {
 	const char *refname=NULL, *value=NULL, *oldval=NULL, *msg=NULL;
 	struct ref_lock *lock;
 	unsigned char sha1[20], oldsha1[20];
-	int i;
+	int i, delete;
 
+	delete = 0;
 	setup_ident();
 	git_config(git_default_config);
 
@@ -26,6 +27,10 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 				die("Refusing to perform update with \\n in message.");
 			continue;
 		}
+		if (!strcmp("-d", argv[i])) {
+			delete = 1;
+			continue;
+		}
 		if (!refname) {
 			refname = argv[i];
 			continue;
@@ -44,8 +49,15 @@ int cmd_update_ref(int argc, const char **argv, const char *prefix)
 
 	if (get_sha1(value, sha1))
 		die("%s: not a valid SHA1", value);
+
+	if (delete) {
+		if (oldval)
+			usage(git_update_ref_usage);
+		return delete_ref(refname, sha1);
+	}
+
 	hashclr(oldsha1);
-	if (oldval && get_sha1(oldval, oldsha1))
+	if (oldval && *oldval && get_sha1(oldval, oldsha1))
 		die("%s: not a valid old SHA1", oldval);
 
 	lock = lock_any_ref_for_update(refname, oldval ? oldsha1 : NULL);

cache.h

@@ -179,6 +179,7 @@ struct lock_file {
 extern int hold_lock_file_for_update(struct lock_file *, const char *path, int);
 extern int commit_lock_file(struct lock_file *);
 extern void rollback_lock_file(struct lock_file *);
+extern int delete_ref(const char *, unsigned char *sha1);
 
 /* Environment bits from configuration mechanism */
 extern int use_legacy_headers;

refs.c

@@ -378,6 +378,32 @@ int get_ref_sha1(const char *ref, unsigned char *sha1)
 	return read_ref(mkpath("refs/%s", ref), sha1);
 }
 
+int delete_ref(const char *refname, unsigned char *sha1)
+{
+	struct ref_lock *lock;
+	int err, i, ret = 0;
+
+	lock = lock_any_ref_for_update(refname, sha1);
+	if (!lock)
+		return 1;
+	i = strlen(lock->lk->filename) - 5; /* .lock */
+	lock->lk->filename[i] = 0;
+	err = unlink(lock->lk->filename);
+	if (err) {
+		ret = 1;
+		error("unlink(%s) failed: %s",
+		      lock->lk->filename, strerror(errno));
+	}
+	lock->lk->filename[i] = '.';
+
+	err = unlink(lock->log_file);
+	if (err && errno != ENOENT)
+		fprintf(stderr, "warning: unlink(%s) failed: %s",
+			lock->log_file, strerror(errno));
+
+	return ret;
+}
+
 /*
  * Make sure "ref" is something reasonable to have under ".git/refs/";
  * We do not like it if: