Improve event log reader code

gaphor/storage/recovery.py

@@ -228,25 +228,34 @@ def write(self, event):
 
     def read(self):
         self.close()
-        checksum_failed = False
+        filename = self._filename or self._template
+        if not filename:
+            return
+
         try:
             with self._log_name.open(mode="r", encoding="utf-8") as f:
+                preamble_line = f.readline()
+                preamble = ast.literal_eval(preamble_line)
+                if not isinstance(preamble, dict):
+                    raise ChecksumFailed()
+
+                filename = (
+                    self._template if preamble.get("template") else self._filename
+                )
+                if not filename or sha256sum(filename) != preamble.get("sha256"):
+                    raise ChecksumFailed()
+
                 for line in f:
                     events = ast.literal_eval(line.rstrip("\r\n"))
-                    if (
-                        isinstance(events, dict)
-                        and self._filename
-                        and sha256sum(self._filename) != events.get("sha256")
-                    ):
-                        checksum_failed = True
-                        break
                     yield events
-            if checksum_failed:
-                log.info("Recovery file hash does not match.")
-                self.move_aside()
+
         except FileNotFoundError:
             # Log does not exist, no problem
             pass
+        except ChecksumFailed:
+            # Move file after it's closed.
+            log.info("Recovery file hash does not match.")
+            self.move_aside()
 
     def close(self):
         if self._file:
@@ -258,6 +267,10 @@ def move_aside(self):
         _move_aside(self._log_name)
 
 
+class ChecksumFailed(Exception):
+    pass
+
+
 def _move_aside(path: Path):
     backup = path.with_suffix(".recovery.bak")
     path.rename(backup)

tests/test_session_recovery.py

@@ -177,7 +177,8 @@ def test_broken_recovery_log(
     assert "Could not recover model changes" in caplog.text
 
 
-def test_recover_from_session_files(application: Application, test_models):
+@pytest.mark.parametrize("template", [True, False])
+def test_recover_from_session_files(application: Application, test_models, template):
     session_id = "1234"
     class_id = "9876"
 
@@ -186,7 +187,7 @@ def test_recover_from_session_files(application: Application, test_models):
         {
             "path": str(test_models / "all-elements.gaphor"),
             "sha256": sha256sum(test_models / "all-elements.gaphor"),
-            "template": True,
+            "template": template,
         },
         [("c", "Class", class_id, None)],
     )