[tor-bugs] #21765 [Core Tor/Tor]: PortForwardingHelper stdout/stderr log forwarding seems fragile

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#21765: PortForwardingHelper stdout/stderr log forwarding seems fragile
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     Reporter:  ahf           |      Owner:
         Type:  defect        |     Status:  new
     Priority:  Low           |  Milestone:  Tor: 0.3.1.x-final
    Component:  Core Tor/Tor  |    Version:  Tor: unspecified
     Severity:  Normal        |   Keywords:
Actual Points:                |  Parent ID:
       Points:                |   Reviewer:
      Sponsor:                |
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 While working on a fix for a regression I caused in #21757 I noticed that
 we had code in our utility module that could be affected by the same
 regression as I was trying to fix.

 I've tried to list some of my observations while studying this code:

 - We use non-blocking I/O for stdout and stderr in the spawned `tor-fw-
 helper` child process, but we only call `log_from_pipe()` once immediately
 after spawning the process. From my quick observation the call to
 `log_form_pipe()` almost always result in a `EAGAIN` and we then return
 without processing any lines at all since we never actually try to read
 again.
 - We have duplicated code in `handle_fw_helper_line()` and
 `log_from_pipe()` to handle `SPAWN_ERROR_MESSAGE` error messages during
 child creation with a comment related to the first observation.

 I tried to test this with a simple dummy `PortForwardingHelper` script
 which outputs a set of lines on stdout and stderr, sleeps for a while,
 then continues to write some more content before it exits and only a
 subset of these lines are actually forwarded to tor's logging subsystem.

 The application seems to work correctly and contact my router, so this
 issue is purely related to the forwarding of stdout and stderr from the
 helper process.

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Ticket URL: <https://trac.torproject.org/projects/tor/ticket/21765>
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