Improve diagnostics

src/python/replay_merge.py

@@ -268,11 +268,11 @@ def merge_replay(
                     stderr=subprocess.PIPE,
                 )
                 if len(process.stderr.decode("utf-8")) == 0:
-                    conflict_files = process.stdout.decode("utf-8")
-                    is_conflict = len(conflict_files) > 0
+                    git_conflict_files = process.stdout.decode("utf-8")
+                    is_conflict = len(git_conflict_files) > 0
                     assert (
                         is_conflict == (merge_result == MERGE_STATE.Merge_failed)
-                    ), f"merge_replay: tool{merge_tool} merge_result {merge_result} does not match conflict_files {conflict_files} at path {repo.local_repo_path}"
+                    ), f"merge_replay: tool merge result is inconsistent with `git diff --diff-filter=U`: tool={merge_tool} merge_result={merge_result} git_conflict_files={git_conflict_files} path={repo.local_repo_path}"
 
             result_df.loc[
                 merge_tool.name,
@@ -308,10 +308,11 @@ def merge_replay(
                     store_artifacts(result_df)
                 if delete_workdir:
                     delete_workdirs(result_df)
-                print("=====================================\n")
+                print("fingerprints differ; details follow.")
+                print(f"=================== start of {log_path}:")
                 with open(log_path, "r", encoding="utf-8") as f:
                     print(f.read())
-                print("=====================================\n")
+                print(f"=================== end of {log_path}.")
                 raise Exception(
                     f"fingerprints differ: after merge of {workdir} with {merge_tool}, found"
                     + f" {merge_fingerprint} but expected "

src/python/repo.py

@@ -152,11 +152,20 @@ def stdout_and_stderr(
     source: Union[subprocess.TimeoutExpired, subprocess.CompletedProcess],
 ) -> str:
     """Produces the standard output and standard error of a timedout process."""
-    explanation = "Run Command: " + " ".join(command) + "\nTimed out"
+    explanation = "Here is the output from: " + " ".join(command)
     if source.stdout:
-        explanation += "\nstdout:\n" + source.stdout.decode("utf-8", "replace")
+        explanation += (
+            "\nstdout:\n"
+            + source.stdout.decode("utf-8", "replace")
+            + "\nEnd of stdout."
+        )
     if source.stderr:
-        explanation += "\nstderr:\n" + source.stderr.decode("utf-8", "replace")
+        explanation += (
+            "\nstderr:\n"
+            + source.stderr.decode("utf-8", "replace")
+            + "\nEnd of stderr."
+        )
+    explanation += "\nEnd of output from: " + " ".join(command)
     return explanation