Repository: pallets/werkzeug (python)
Issue: Empty response with failure from ProcessPoolExecutor
When a flask app uses a ProcessPoolExecutor and one of its tasks fails, werkzeug can't handle the stack trace.

```
from flask import Flask
import concurrent.futures

app = Flask(__name__)

def fail():
    raise ValueError('Failed')

@app.route("/")
def test():
    executor = concurrent.futures.ProcessPoolExecutor(max_workers=1)
    future = executor.submit(fail)
    return future.result()

if __name__ == '__main__':
    app.run(debug=True)
```

```
$ curl http://localhost:5000
curl: (52) Empty reply from server
```

```
Exception occurred during processing of request from ('127.0.0.1', 52594)
concurrent.futures.process._RemoteTraceback: 
"""
Traceback (most recent call last):
  File "/home/kjurka/tmp/mclatest/lib/python3.9/concurrent/futures/process.py", line 243, in _process_worker
    r = call_item.fn(*call_item.args, **call_item.kwargs)
  File "/home/kjurka/tmp/mclatest/flask_processpool.py", line 7, in fail
    raise ValueError('Failed')
ValueError: Failed
"""

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/__init__.py", line 309, in debug_application
    app_iter = self.app(environ, start_response)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/flask/app.py", line 2088, in __call__
    return self.wsgi_app(environ, start_response)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/flask/app.py", line 2073, in wsgi_app
    response = self.handle_exception(e)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/flask/app.py", line 2070, in wsgi_app
    response = self.full_dispatch_request()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/flask/app.py", line 1515, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/flask/app.py", line 1513, in full_dispatch_request
    rv = self.dispatch_request()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/flask/app.py", line 1499, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "/home/kjurka/tmp/mclatest/flask_processpool.py", line 13, in test
    return future.result()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/concurrent/futures/_base.py", line 445, in result
    return self.__get_result()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/concurrent/futures/_base.py", line 390, in __get_result
    raise self._exception
ValueError: Failed

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/serving.py", line 319, in run_wsgi
    execute(self.server.app)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/serving.py", line 310, in execute
    for data in application_iter:
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/__init__.py", line 316, in debug_application
    traceback = get_current_traceback(
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/tbtools.py", line 182, in get_current_traceback
    tb.filter_hidden_frames()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/tbtools.py", line 248, in filter_hidden_frames
    group.filter_hidden_frames()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/tbtools.py", line 388, in filter_hidden_frames
    elif self.frames[-1] in new_frames:
IndexError: list index out of range

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/kjurka/tmp/mclatest/lib/python3.9/socketserver.py", line 683, in process_request_thread
    self.finish_request(request, client_address)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/socketserver.py", line 360, in finish_request
    self.RequestHandlerClass(request, client_address, self)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/socketserver.py", line 747, in __init__
    self.handle()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/serving.py", line 342, in handle
    BaseHTTPRequestHandler.handle(self)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/http/server.py", line 427, in handle
    self.handle_one_request()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/serving.py", line 374, in handle_one_request
    self.run_wsgi()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/serving.py", line 327, in run_wsgi
    traceback = get_current_traceback(ignore_system_exceptions=True)
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/tbtools.py", line 182, in get_current_traceback
    tb.filter_hidden_frames()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/tbtools.py", line 248, in filter_hidden_frames
    group.filter_hidden_frames()
  File "/home/kjurka/tmp/mclatest/lib/python3.9/site-packages/werkzeug/debug/tbtools.py", line 388, in filter_hidden_frames
    elif self.frames[-1] in new_frames:
IndexError: list index out of range
----------------------------------------
```


Environment:

- Python version: 3.9.5
- Werkzeug version: 2.0.1
- Flask version: 2.0.1

Pull request description: Handle an Exception without a traceback.
When an exception does not have a traceback there are no frames
to filter, but there's an explicit check for the last frame which
will fail if there are no frames.

An example exception with this problem can be seen when using
concurrent.futures.ProcessPoolExecutor.  When it reconstitutes a
remote exception it has serialized the traceback to a string it
includes as the error message instead of an object that can be
inspected.

- fixes #2217 


Patch:
diff --git a/CHANGES.rst b/CHANGES.rst
index a63e0e18..915ca7dd 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -39,6 +39,9 @@ Unreleased
 -   Fix setting CSP header options on the response. :pr:`2237`
 -   Fix an issue with with the interactive debugger where lines would
     not expand on click for very long tracebacks. :pr:`2239`
+-   The interactive debugger handles displaying an exception that does
+    not have a traceback, such as from ``ProcessPoolExecutor``.
+    :issue:`2217`
 
 
 Version 2.0.1
diff --git a/src/werkzeug/debug/tbtools.py b/src/werkzeug/debug/tbtools.py
index a8e2c96e..5ffd6542 100644
--- a/src/werkzeug/debug/tbtools.py
+++ b/src/werkzeug/debug/tbtools.py
@@ -357,6 +357,11 @@ class Group:
             tb = tb.tb_next  # type: ignore
 
     def filter_hidden_frames(self) -> None:
+        # An exception may not have a traceback to filter frames, such
+        # as one re-raised from ProcessPoolExecutor.
+        if not self.frames:
+            return
+
         new_frames: t.List[Frame] = []
         hidden = False
 


Test patch:
diff --git a/tests/test_debug.py b/tests/test_debug.py
index 19318c72..a5c6a7ad 100644
--- a/tests/test_debug.py
+++ b/tests/test_debug.py
@@ -6,6 +6,7 @@ import pytest
 
 from werkzeug.debug import console
 from werkzeug.debug import DebuggedApplication
+from werkzeug.debug import get_current_traceback
 from werkzeug.debug import get_machine_id
 from werkzeug.debug.console import HTMLStringO
 from werkzeug.debug.repr import debug_repr
@@ -350,3 +351,12 @@ def test_non_hashable_exception():
     except MutableException:
         # previously crashed: `TypeError: unhashable type 'MutableException'`
         Traceback(*sys.exc_info())
+
+
+def test_exception_without_traceback():
+    try:
+        raise Exception("msg1")
+    except Exception as e:
+        # filter_hidden_frames should skip this since it has no traceback
+        e.__context__ = Exception("msg2")
+        get_current_traceback()