DWIM "git checkout frotz" to "git checkout -b frotz origin/frotz"

When 'frotz' is not a valid object name and not a tracked filename,
we used to complain and failed this command.  When there is only
one remote that has 'frotz' as one of its tracking branches, we can
DWIM it as a request to create a local branch 'frotz' forking from
the matching remote tracking branch.

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

diff --git a/builtin-checkout.c b/builtin-checkout.c
index d050c3789fcdaf1b50c12a20c28cbeaf774048eb..fb7e68ac5f2f0f7d224da93ae57867b24247c7ca 100644 (file)
--- a/builtin-checkout.c
+++ b/builtin-checkout.c
@@ -572,6 +572,40 @@ static int interactive_checkout(const char *revision, const char **pathspec,
        return run_add_interactive(revision, "--patch=checkout", pathspec);
 }
 
+struct tracking_name_data {
+       const char *name;
+       char *remote;
+       int unique;
+};
+
+static int check_tracking_name(const char *refname, const unsigned char *sha1,
+                              int flags, void *cb_data)
+{
+       struct tracking_name_data *cb = cb_data;
+       const char *slash;
+
+       if (prefixcmp(refname, "refs/remotes/"))
+               return 0;
+       slash = strchr(refname + 13, '/');
+       if (!slash || strcmp(slash + 1, cb->name))
+               return 0;
+       if (cb->remote) {
+               cb->unique = 0;
+               return 0;
+       }
+       cb->remote = xstrdup(refname);
+       return 0;
+}
+
+static const char *unique_tracking_name(const char *name)
+{
+       struct tracking_name_data cb_data = { name, NULL, 1 };
+       for_each_ref(check_tracking_name, &cb_data);
+       if (cb_data.unique)
+               return cb_data.remote;
+       free(cb_data.remote);
+       return NULL;
+}
 
 int cmd_checkout(int argc, const char **argv, const char *prefix)
 {
@@ -630,8 +664,6 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
                opts.new_branch = argv0 + 1;
        }
 
-       if (opts.track == BRANCH_TRACK_UNSPECIFIED)
-               opts.track = git_branch_track;
        if (conflict_style) {
                opts.merge = 1; /* implied */
                git_xmerge_config("merge.conflictstyle", conflict_style, NULL);
@@ -655,6 +687,11 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
         *   With no paths, if <something> is a commit, that is to
         *   switch to the branch or detach HEAD at it.
         *
+        *   With no paths, if <something> is _not_ a commit, no -t nor -b
+        *   was given, and there is a tracking branch whose name is
+        *   <something> in one and only one remote, then this is a short-hand
+        *   to fork local <something> from that remote tracking branch.
+        *
         *   Otherwise <something> shall not be ambiguous.
         *   - If it's *only* a reference, treat it like case (1).
         *   - If it's only a path, treat it like case (2).
@@ -677,7 +714,20 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
                if (get_sha1(arg, rev)) {
                        if (has_dash_dash)          /* case (1) */
                                die("invalid reference: %s", arg);
-                       goto no_reference;          /* case (3 -> 2) */
+                       if (!patch_mode &&
+                           opts.track == BRANCH_TRACK_UNSPECIFIED &&
+                           !opts.new_branch &&
+                           !check_filename(NULL, arg) &&
+                           argc == 1) {
+                               const char *remote = unique_tracking_name(arg);
+                               if (!remote || get_sha1(remote, rev))
+                                       goto no_reference;
+                               opts.new_branch = arg;
+                               arg = remote;
+                               /* DWIMmed to create local branch */
+                       }
+                       else
+                               goto no_reference;
                }
 
                /* we can't end up being in (2) anymore, eat the argument */
@@ -715,6 +765,10 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
        }
 
 no_reference:
+
+       if (opts.track == BRANCH_TRACK_UNSPECIFIED)
+               opts.track = git_branch_track;
+
        if (argc) {
                const char **pathspec = get_pathspec(prefix, argv);