git check-ref-format --print

Tolerating empty path components in ref names means each ref does
not have a unique name.  This creates difficulty for porcelains
that want to see if two branches are equal.  Add a helper associating
to each ref a canonical name.

If a user asks a porcelain to create a ref "refs/heads//master",
the porcelain can run "git check-ref-format --print refs/heads//master"
and only deal with "refs/heads/master" from then on.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-check-ref-format.txt

@@ -9,6 +9,7 @@ SYNOPSIS
 --------
 [verse]
 'git check-ref-format' <refname>
+'git check-ref-format' --print <refname>
 'git check-ref-format' [--branch] <branchname-shorthand>
 
 DESCRIPTION
@@ -63,16 +64,28 @@ reference name expressions (see linkgit:git-rev-parse[1]):
 
 . at-open-brace `@{` is used as a notation to access a reflog entry.
 
+With the `--print` option, if 'refname' is acceptable, it prints the
+canonicalized name of a hypothetical reference with that name.  That is,
+it prints 'refname' with any extra `/` characters removed.
+
 With the `--branch` option, it expands a branch name shorthand and
 prints the name of the branch the shorthand refers to.
 
-EXAMPLE
--------
-
-git check-ref-format --branch @{-1}::
-
-Print the name of the previous branch.
+EXAMPLES
+--------
 
+* Print the name of the previous branch:
++
+------------
+$ git check-ref-format --branch @{-1}
+------------
+
+* Determine the reference name to use for a new branch:
++
+------------
+$ ref=$(git check-ref-format --print "refs/heads/$newbranch") ||
+die "we do not like '$newbranch' as a branch name."
+------------
 
 GIT
 ---

builtin-check-ref-format.c

@@ -17,6 +17,16 @@ int cmd_check_ref_format(int argc, const char **argv, const char *prefix)
 		printf("%s\n", sb.buf + 11);
 		exit(0);
 	}
+	if (argc == 3 && !strcmp(argv[1], "--print")) {
+		char *refname = xmalloc(strlen(argv[2]) + 1);
+
+		if (check_ref_format(argv[2]))
+			exit(1);
+		if (normalize_path_copy(refname, argv[2]))
+			die("Could not normalize ref name '%s'", argv[2]);
+		printf("%s\n", refname);
+		exit(0);
+	}
 	if (argc != 2)
 		usage("git check-ref-format refname");
 	return !!check_ref_format(argv[1]);

t/t1402-check-ref-format.sh

@@ -41,4 +41,21 @@ test_expect_success "check-ref-format --branch @{-1}" '
 	refname2=$(git check-ref-format --branch @{-2}) &&
 	test "$refname2" = master'
 
+valid_ref_normalized() {
+	test_expect_success "ref name '$1' simplifies to '$2'" "
+		refname=\$(git check-ref-format --print '$1') &&
+		test \"\$refname\" = '$2'"
+}
+invalid_ref_normalized() {
+	test_expect_success "check-ref-format --print rejects '$1'" "
+		test_must_fail git check-ref-format --print '$1'"
+}
+
+valid_ref_normalized 'heads/foo' 'heads/foo'
+valid_ref_normalized 'refs///heads/foo' 'refs/heads/foo'
+invalid_ref_normalized 'foo'
+invalid_ref_normalized 'heads/foo/../bar'
+invalid_ref_normalized 'heads/./foo'
+invalid_ref_normalized 'heads\foo'
+
 test_done