[tor-bugs] #11820 [Obfsproxy]: circuit `NoneType` in obfs3 handshake callbacks

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#11820: circuit `NoneType` in obfs3 handshake callbacks
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     Reporter:  asn        |      Owner:  asn
         Type:  defect     |     Status:  new
     Priority:  normal     |  Milestone:
    Component:  Obfsproxy  |    Version:
   Resolution:             |   Keywords:
Actual Points:             |  Parent ID:
       Points:             |
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Comment (by asn):

 I'm not sure if it makes sense checking whether `self.circuit` exists, if
 we remove the code that sets it to `None`.

 As a fix to this bug, I'm thinking of using the fix to #11769 (that adds
 `self.closed` guards) and also removing the part of the code that sets
 `self.transport.circuit` to `None`.

 If this thing bites us again, we can do more elaborate fixes like the
 `@property` idea.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/11820#comment:3>
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