[#316] Extend parallel exception message output

Signed-off-by: a.berezin <a.berezin@yadro.com>

src/frostfs_testlib/testing/parallel.py

@@ -1,4 +1,5 @@
 import itertools
+import traceback
 from concurrent.futures import Future, ThreadPoolExecutor
 from contextlib import contextmanager
 from typing import Callable, Collection, Optional, Union
@@ -55,7 +56,42 @@ def parallel(
     # Check for exceptions
     exceptions = [future.exception() for future in futures if future.exception()]
     if exceptions:
-        message = "\n".join([str(e) for e in exceptions])
+        # Prettify exception in parallel with all underlying stack traces
+        # For example, we had 3 RuntimeError exceptions during parallel. This format will give us something like
+        #
+        # RuntimeError: The following exceptions occured during parallel run:
+        # 1) Exception one text
+        # 2) Exception two text
+        # 3) Exception three text
+        # TRACES:
+        # ==== 1 ====
+        # Traceback (most recent call last):
+        #   File "/usr/lib/python3.10/concurrent/futures/thread.py", line 58, in run
+        #     result = self.fn(*self.args, **self.kwargs)
+        #   File "frostfs_testcases/pytest_tests/testsuites/object/test_object_tombstone.py", line 17, in check_service
+        #     raise RuntimeError(f"Exception one text")
+        # RuntimeError: Exception one text
+        #
+        # ==== 2 ====
+        # Traceback (most recent call last):
+        #   File "/usr/lib/python3.10/concurrent/futures/thread.py", line 58, in run
+        #     result = self.fn(*self.args, **self.kwargs)
+        #   File "frostfs_testcases/pytest_tests/testsuites/object/test_object_tombstone.py", line 17, in check_service
+        #     raise RuntimeError(f"Exception two text")
+        # RuntimeError: Exception two text
+        #
+        # ==== 3 ====
+        # Traceback (most recent call last):
+        #   File "/usr/lib/python3.10/concurrent/futures/thread.py", line 58, in run
+        #     result = self.fn(*self.args, **self.kwargs)
+        #   File "frostfs_testcases/pytest_tests/testsuites/object/test_object_tombstone.py", line 17, in check_service
+        #     raise RuntimeError(f"Exception three text")
+        # RuntimeError: Exception three text
+        short_summary = "\n".join([f"{i}) {str(e)}" for i, e in enumerate(exceptions, 1)])
+        stack_traces = "\n".join(
+            [f"==== {i} ====\n{''.join(traceback.TracebackException.from_exception(e).format())}" for i, e in enumerate(exceptions, 1)]
+        )
+        message = f"{short_summary}\nTRACES:\n{stack_traces}"
         raise RuntimeError(f"The following exceptions occured during parallel run:\n{message}")
     return futures