adding a ref checker util

repo2docker/app.py

@@ -34,7 +34,7 @@ from .buildpacks import (
 )
 from .utils import (
     execute_cmd, ByteSpecification, maybe_cleanup, is_valid_docker_image_name,
-    validate_and_generate_port_mapping
+    validate_and_generate_port_mapping, check_ref
 )
 
 
@@ -175,11 +175,7 @@ class Repo2Docker(Application):
             sys.exit(1)
 
         if ref:
-            if self.repo_type == "remote" and not ref.startswith('origin/'):
+            ref = check_ref(ref)
-                # Ensure that the ref has `origin` in front
-                self.log.info("Ref did not have a remote specified, "
-                              "adding 'origin/' to ref.")
-                ref = '/'.join(["origin", ref])
             try:
                 for line in execute_cmd(['git', 'reset', '--hard', ref],
                                         cwd=checkout_path,

repo2docker/utils.py

@@ -270,3 +270,28 @@ class ByteSpecification(Integer):
             )
         else:
             return int(float(num) * self.UNIT_SUFFIXES[suffix])
+
+
+def check_ref(ref):
+    """Prepare a ref and ensure it works with git reset --hard."""
+    ref_checks = {'tag': "refs/tags/{}",
+                  'branch': "refs/heads/{}",
+                  'remote': "refs/remotes/origin/{}",
+                  'commit': "{}^{{commit}}"}
+    chosen_kind = None
+    for kind, check in ref_checks:
+        check = check.split('/')[-1]
+        parts = ["git", "show-ref", "--verify", check.format(ref)]
+        success = subprocess.call(parts)
+        if success:
+            break
+
+    if chosen_kind is None:
+        raise ValueError("Could not resolve the ref properly, ensure that "
+                         "it is a commit hash / tag / branch name.")
+    if chosen_kind == "remote" and not check.startswith('origin/'):
+        # Ensure that the ref has `origin` in front
+        self.log.info("Ref did not have a remote specified, "
+                      "adding 'origin/' to ref.\n")
+        check = '/'.join(["origin", check])
+    return check