show proper error message when head commit not found (#878)

fixes #848

src/popper/scm.py

@@ -167,10 +167,19 @@ def get_sha(repo, short=None):
     if not repo:
         return None
 
-    if short:
-        return repo.git.rev_parse(repo.head.object.hexsha, short=short)
+    try:
+        if short:
+            sha = repo.git.rev_parse(repo.head.object.hexsha, short=short)
+        else:
+            sha = repo.git.rev_parse(repo.head.object.hexsha)
+
+    except ValueError as e:
+        sha = None
+        log.warning(
+            f"Could not obtain commit ID (SHA1) due to the Git repository at {repo.git_dir} being empty."
+        )
 
-    return repo.git.rev_parse(repo.head.object.hexsha)
+    return sha
 
 
 def get_branch(repo):

src/test/test_scm.py

@@ -53,6 +53,16 @@ def test_get_branch_in_detached_head_state(self):
         self.assertIsNone(scm.get_sha(None, short=8))
         self.assertIsNone(scm.get_branch(None))
 
+        # drop head commit
+        with self.assertLogs("popper", level="WARNING") as cm:
+            repo.git.update_ref("-d", "HEAD")
+            self.assertEqual(scm.get_sha(repo), None)
+            self.assertEqual(len(cm.output), 1)
+            self.assertTrue(
+                f"WARNING:popper:Could not obtain commit ID (SHA1) due to the Git repository at {repo.git_dir} being empty."
+                in cm.output[0]
+            )
+
     def test_clone(self):
         tempdir = tempfile.mkdtemp()
         tdir = os.path.join(tempdir, "test_clone")