MockModuleBuilder: don't accidentally remove log files from other threads

The _purge_useless_log_files() method, if run at the wrong time, would
remove log files that other threads were still creating.

module_build_service/builder/MockModuleBuilder.py

@@ -608,11 +608,15 @@ class MockModuleBuilder(GenericBuilder):
         if os.path.exists(old_log):
             os.rename(old_log, new_log)
 
-    def _purge_useless_logs(self):
+    def _purge_useless_logs(self, artifact_name):
         """
         Remove empty or otherwise useless log files
         """
+        prefix = artifact_name + "-"
+
         for logf in os.listdir(self.resultsdir):
+            if not logf.startswith(prefix):
+                continue
 
             log_path = os.path.join(self.resultsdir, logf)
 
@@ -685,7 +689,7 @@ class MockModuleBuilder(GenericBuilder):
 
         # Depending on the configuration settings, remove/keep useless log files
         if conf.mock_purge_useless_logs:
-            self._purge_useless_logs()
+            self._purge_useless_logs(artifact_name)
 
         # We return BUILDING state here even when we know it is already
         # completed or failed, because otherwise utils.start_build_batch