branch -d: avoid repeated symref resolution

If a repository gets in a broken state with too much symref nesting,
it cannot be repaired with "git branch -d":

 $ git symbolic-ref refs/heads/nonsense refs/heads/nonsense
 $ git branch -d nonsense
 error: branch 'nonsense' not found.

Worse, "git update-ref --no-deref -d" doesn't work for such repairs
either:

 $ git update-ref -d refs/heads/nonsense
 error: unable to resolve reference refs/heads/nonsense: Too many levels of symbolic links

Fix both by teaching resolve_ref_unsafe a new RESOLVE_REF_NO_RECURSE
flag and passing it when appropriate.

Callers can still read the value of a symref (for example to print a
message about it) with that flag set --- resolve_ref_unsafe will
resolve one level of symrefs and stop there.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Reviewed-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -236,7 +236,8 @@ static int delete_branches(int argc, const char **argv, int force, int kinds,
 		free(name);
 
 		name = mkpathdup(fmt, bname.buf);
-		target = resolve_ref_unsafe(name, 0, sha1, &flags);
+		target = resolve_ref_unsafe(name, RESOLVE_REF_NO_RECURSE,
+					    sha1, &flags);
 		if (!target ||
 		    (!(flags & REF_ISSYMREF) && is_null_sha1(sha1))) {
 			error(remote_branch

cache.h

@@ -973,6 +973,11 @@ extern int read_ref(const char *refname, unsigned char *sha1);
  *   "writing" to the ref, the return value is the name of the ref
  *   that will actually be created or changed.
  *
+ * If the RESOLVE_REF_NO_RECURSE flag is passed, only resolves one
+ * level of symbolic reference.  The value stored in sha1 for a symbolic
+ * reference will always be null_sha1 in this case, and the return
+ * value is the reference that the symref refers to directly.
+ *
  * If flags is non-NULL, set the value that it points to the
  * combination of REF_ISPACKED (if the reference was found among the
  * packed references), REF_ISSYMREF (if the initial reference was a
@@ -985,6 +990,7 @@ extern int read_ref(const char *refname, unsigned char *sha1);
  * errno is set to something meaningful on error.
  */
 #define RESOLVE_REF_READING 0x01
+#define RESOLVE_REF_NO_RECURSE 0x02
 extern const char *resolve_ref_unsafe(const char *ref, int resolve_flags, unsigned char *sha1, int *flags);
 extern char *resolve_refdup(const char *ref, int resolve_flags, unsigned char *sha1, int *flags);
 

refs.c

@@ -1467,6 +1467,10 @@ const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned
 				refname = refname_buffer;
 				if (flags)
 					*flags |= REF_ISSYMREF;
+				if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
+					hashclr(sha1);
+					return refname;
+				}
 				continue;
 			}
 		}
@@ -1523,13 +1527,17 @@ const char *resolve_ref_unsafe(const char *refname, int resolve_flags, unsigned
 		buf = buffer + 4;
 		while (isspace(*buf))
 			buf++;
+		refname = strcpy(refname_buffer, buf);
+		if (resolve_flags & RESOLVE_REF_NO_RECURSE) {
+			hashclr(sha1);
+			return refname;
+		}
 		if (check_refname_format(buf, REFNAME_ALLOW_ONELEVEL)) {
 			if (flags)
 				*flags |= REF_ISBROKEN;
 			errno = EINVAL;
 			return NULL;
 		}
-		refname = strcpy(refname_buffer, buf);
 	}
 }
 
@@ -2170,6 +2178,8 @@ static struct ref_lock *lock_ref_sha1_basic(const char *refname,
 
 	if (mustexist)
 		resolve_flags |= RESOLVE_REF_READING;
+	if (flags & REF_NODEREF && flags & REF_DELETING)
+		resolve_flags |= RESOLVE_REF_NO_RECURSE;
 
 	refname = resolve_ref_unsafe(refname, resolve_flags,
 				     lock->old_sha1, &type);
@@ -3664,13 +3674,16 @@ int ref_transaction_commit(struct ref_transaction *transaction,
 	/* Acquire all locks while verifying old values */
 	for (i = 0; i < n; i++) {
 		struct ref_update *update = updates[i];
+		int flags = update->flags;
 
+		if (is_null_sha1(update->new_sha1))
+			flags |= REF_DELETING;
 		update->lock = lock_ref_sha1_basic(update->refname,
 						   (update->have_old ?
 						    update->old_sha1 :
 						    NULL),
 						   NULL,
-						   update->flags,
+						   flags,
 						   &update->type);
 		if (!update->lock) {
 			ret = (errno == ENOTDIR)

refs.h

@@ -177,10 +177,12 @@ extern int peel_ref(const char *refname, unsigned char *sha1);
  * ref_transaction_create(), etc.
  * REF_NODEREF: act on the ref directly, instead of dereferencing
  *              symbolic references.
+ * REF_DELETING: tolerate broken refs
  *
  * Flags >= 0x100 are reserved for internal use.
  */
 #define REF_NODEREF	0x01
+#define REF_DELETING	0x02
 /*
  * This function sets errno to something meaningful on failure.
  */

t/t1400-update-ref.sh

@@ -110,6 +110,40 @@ test_expect_success "delete symref without dereference when the referred ref is
 cp -f .git/HEAD.orig .git/HEAD
 git update-ref -d $m
 
+test_expect_success 'update-ref -d is not confused by self-reference' '
+	git symbolic-ref refs/heads/self refs/heads/self &&
+	test_when_finished "rm -f .git/refs/heads/self" &&
+	test_path_is_file .git/refs/heads/self &&
+	test_must_fail git update-ref -d refs/heads/self &&
+	test_path_is_file .git/refs/heads/self
+'
+
+test_expect_success 'update-ref --no-deref -d can delete self-reference' '
+	git symbolic-ref refs/heads/self refs/heads/self &&
+	test_when_finished "rm -f .git/refs/heads/self" &&
+	test_path_is_file .git/refs/heads/self &&
+	git update-ref --no-deref -d refs/heads/self &&
+	test_path_is_missing .git/refs/heads/self
+'
+
+test_expect_success 'update-ref --no-deref -d can delete reference to bad ref' '
+	>.git/refs/heads/bad &&
+	test_when_finished "rm -f .git/refs/heads/bad" &&
+	git symbolic-ref refs/heads/ref-to-bad refs/heads/bad &&
+	test_when_finished "rm -f .git/refs/heads/ref-to-bad" &&
+	test_path_is_file .git/refs/heads/ref-to-bad &&
+	git update-ref --no-deref -d refs/heads/ref-to-bad &&
+	test_path_is_missing .git/refs/heads/ref-to-bad
+'
+
+test_expect_success 'update-ref --no-deref -d can delete reference to broken name' '
+	git symbolic-ref refs/heads/badname refs/heads/broken...ref &&
+	test_when_finished "rm -f .git/refs/heads/badname" &&
+	test_path_is_file .git/refs/heads/badname &&
+	git update-ref --no-deref -d refs/heads/badname &&
+	test_path_is_missing .git/refs/heads/badname
+'
+
 test_expect_success '(not) create HEAD with old sha1' "
 	test_must_fail git update-ref HEAD $A $B
 "

t/t3200-branch.sh

@@ -285,6 +285,15 @@ test_expect_success 'deleting a dangling symref' '
 	test_i18ncmp expect actual
 '
 
+test_expect_success 'deleting a self-referential symref' '
+	git symbolic-ref refs/heads/self-reference refs/heads/self-reference &&
+	test_path_is_file .git/refs/heads/self-reference &&
+	echo "Deleted branch self-reference (was refs/heads/self-reference)." >expect &&
+	git branch -d self-reference >actual &&
+	test_path_is_missing .git/refs/heads/self-reference &&
+	test_i18ncmp expect actual
+'
+
 test_expect_success 'renaming a symref is not allowed' '
 	git symbolic-ref refs/heads/master2 refs/heads/master &&
 	test_must_fail git branch -m master2 master3 &&