check_ref_format(): tighten refname rules

This changes the rules for refnames to forbid:

 (1) a refname that contains "@{" in it.

     Some people and foreign SCM converter may have named their branches
     as frotz@24 and we still want to keep supporting it.

     However, "git branch frotz@{24}" is a disaster.  It cannot even
     checked out because "git checkout frotz@{24}" will interpret it as
     "detach the HEAD at twenty-fourth reflog entry of the frotz branch".

 (2) a refname that ends with a dot.

     We already reject a path component that begins with a dot, primarily
     to avoid ambiguous range interpretation.  If we allowed ".B" as a
     valid ref, it is unclear if "A...B" means "in dot-B but not in A" or
     "either in A or B but not in both".

     But for this to be complete, we need also to forbid "A." to avoid "in
     B but not in A-dot".  This was not a problem in the original range
     notation, but we should have added this restriction when three-dot
     notation was introduced.

     Unlike "no dot at the beginning of any path component" rule, this
     rule does not have to be "no dot at the end of any path component",
     because you cannot abbreviate the tail end away, similar to you can
     say "dot-B" to mean "refs/heads/dot-B".

For these reasons, it is not likely people created branches with these
names on purpose, but we have allowed such names to be used for quite some
time, and it is possible that people created such branches by mistake or
by accident.

To help people with branches with such unfortunate names to recover,
we still allow "branch -d 'bad.'" to delete such branches, and also allow
"branch -m bad. good" to rename them.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 5 insertions, 1 deletions

diff --git a/Documentation/git-check-ref-format.txt b/Documentation/git-check-ref-format.txt
index 51579f6..d23fd21 100644
--- a/Documentation/git-check-ref-format.txt
+++ b/Documentation/git-check-ref-format.txt
@@ -32,7 +32,9 @@ imposes the following rules on how refs are named:
   caret `{caret}`, colon `:`, question-mark `?`, asterisk `*`,
   or open bracket `[` anywhere;
 
-. It cannot end with a slash `/`.
+. They cannot end with a slash `/` nor a dot `.`.
+
+. They cannot contain a sequence `@{`.
 
 These rules makes it easy for shell script based tools to parse
 refnames, pathname expansion by the shell when a refname is used
@@ -51,6 +53,8 @@ refname expressions (see linkgit:git-rev-parse[1]).  Namely:
   It may also be used to select a specific object such as with
   'git-cat-file': "git cat-file blob v1.3.3:refs.c".
 
+. at-open-brace `@{` is used as a notation to access a reflog entry.
+
 With the `--branch` option, it expands a branch name shorthand and
 prints the name of the branch the shorthand refers to.