ducktape: Copy segment to debug if parse failed

When parse fails in compute_storage we emit a special JSON which has the
segment path. This segment is then copied to test results for analysis.

tests/rptest/services/redpanda.py

@@ -4396,7 +4396,18 @@ def node_storage(self,
         output = node.account.ssh_output(shlex.join(cmd),
                                          combine_stderr=False,
                                          timeout_sec=10)
-        namespaces = json.loads(output)
+        json_output = json.loads(output)
+        if "parse_failed" in json_output:
+            self.logger.error(f"segment parsing failed: {json_output}")
+            segment_src = json_output["segment"]
+            segment_destination = os.path.join(
+                TestContext.results_dir(self._context,
+                                        self._context.test_index), segment_src)
+            self.logger.info(
+                f"copying segment from {segment_src} to {segment_destination}")
+            node.account.copy_from(segment_src, segment_destination)
+            raise AssertionError
+        namespaces = json_output
         for ns, topics in namespaces.items():
             ns_path = os.path.join(store.data_dir, ns)
             ns = store.add_namespace(ns, ns_path)