Corrputed stacktrace?

[Nikolaus Rath]

Hello,

I'm trying to debug a problem. As far as I can tell, one of my methods
is called at a point where it really should not be called. When setting
a breakpoint in the function, I'm getting this:

> /home/nikratio/in-progress/s3ql/src/s3ql/backends/s3c.py(693)close()
-> if not self.md5_checked:
(Pdb) bt
  /usr/lib/python3.3/threading.py(878)_bootstrap()
-> self._bootstrap_inner()
  /usr/lib/python3.3/threading.py(901)_bootstrap_inner()
-> self.run()
  /usr/lib/python3.3/threading.py(858)run()
-> self._target(*self._args, **self._kwargs)
  /usr/lib/python3.3/socketserver.py(610)process_request_thread()
-> self.finish_request(request, client_address)
  /usr/lib/python3.3/socketserver.py(345)finish_request()
-> self.RequestHandlerClass(request, client_address, self)
  /usr/lib/python3.3/socketserver.py(666)__init__()
-> self.handle()
  /home/nikratio/in-progress/s3ql/tests/mock_server.py(77)handle()
-> return super().handle()
  /usr/lib/python3.3/http/server.py(402)handle()
-> self.handle_one_request()
  /usr/lib/python3.3/http/server.py(388)handle_one_request()
-> method()
  /home/nikratio/in-progress/s3ql/tests/mock_server.py(169)do_GET()
-> q = parse_url(self.path)
  /home/nikratio/in-progress/s3ql/tests/mock_server.py(52)parse_url()
-> p.params = urllib.parse.parse_qs(q.query)
  /usr/lib/python3.3/urllib/parse.py(553)parse_qs()
-> encoding=encoding, errors=errors)
  /usr/lib/python3.3/urllib/parse.py(585)parse_qsl()
-> pairs = [s2 for s1 in qs.split('&') for s2 in s1.split(';')]
  /usr/lib/python3.3/urllib/parse.py(585)<listcomp>()
-> pairs = [s2 for s1 in qs.split('&') for s2 in s1.split(';')]
  /home/nikratio/in-progress/s3ql/src/s3ql/backends/common.py(853)close()
-> self.fh.close()
> /home/nikratio/in-progress/s3ql/src/s3ql/backends/s3c.py(693)close()
-> if not self.md5_checked:


To me this does not make any sense.

Firstly, the thread that is (apparently) calling close should never ever
reach code in common.py. This thread is executing a socketserver handler
that is entirely contained in mock_server.py and only communicates with
the rest of the program via tcp.

Secondly, the backtrace does not make sense. How can evaluation of 

 pairs = [s2 for s1 in qs.split('&') for s2 in s1.split(';')]

in urllib/parse.py() result in a method call in backends/common.py?
There is no trickery going on, qs is a regular string:

(Pdb) up
(Pdb) up
(Pdb) up
(Pdb) l
580  	        into Unicode characters, as accepted by the bytes.decode() method.
581  	
582  	    Returns a list, as G-d intended.
583  	    """
584  	    qs, _coerce_result = _coerce_args(qs)
585  ->	    pairs = [s2 for s1 in qs.split('&') for s2 in s1.split(';')]
586  	    r = []
587  	    for name_value in pairs:
588  	        if not name_value and not strict_parsing:
589  	            continue
590  	        nv = name_value.split('=', 1)
(Pdb) whatis qs
<class 'str'>
(Pdb) p qs
''
(Pdb)

I have also tried to get a backtrace with the faulthandler module, but
it gives the same result:

Thread 0x00007f7dafdb4700:
  File "/usr/lib/python3.3/cmd.py", line 126 in cmdloop
  File "/usr/lib/python3.3/pdb.py", line 318 in _cmdloop
  File "/usr/lib/python3.3/pdb.py", line 345 in interaction
  File "/usr/lib/python3.3/pdb.py", line 266 in user_line
  File "/usr/lib/python3.3/bdb.py", line 65 in dispatch_line
  File "/usr/lib/python3.3/bdb.py", line 47 in trace_dispatch
  File "/home/nikratio/in-progress/s3ql/src/s3ql/backends/s3c.py", line 693 in clos
  File "/home/nikratio/in-progress/s3ql/src/s3ql/backends/common.py", line 853 in c
  File "/usr/lib/python3.3/urllib/parse.py", line 585 in <listcomp>
  File "/usr/lib/python3.3/urllib/parse.py", line 585 in parse_qsl
  File "/usr/lib/python3.3/urllib/parse.py", line 553 in parse_qs
  File "/home/nikratio/in-progress/s3ql/tests/mock_server.py", line 52 in parse_url
  File "/home/nikratio/in-progress/s3ql/tests/mock_server.py", line 169 in do_GET
  File "/usr/lib/python3.3/http/server.py", line 388 in handle_one_request
  File "/usr/lib/python3.3/http/server.py", line 402 in handle
  File "/home/nikratio/in-progress/s3ql/tests/mock_server.py", line 77 in handle
  File "/usr/lib/python3.3/socketserver.py", line 666 in __init__
  File "/usr/lib/python3.3/socketserver.py", line 345 in finish_request
  File "/usr/lib/python3.3/socketserver.py", line 610 in process_request_thread
  File "/usr/lib/python3.3/threading.py", line 858 in run
  File "/usr/lib/python3.3/threading.py", line 901 in _bootstrap_inner
  File "/usr/lib/python3.3/threading.py", line 878 in _bootstrap


Is it possible that the stack got somehow corrupted?

Does anyone have a suggestion how I could go about debugging this?

I am using Python 3.3.

Best,
-Nikolaus

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