The dangers of upgrading

One of the guys in the office was having trouble since the upgrade to python3.6 – a bunch of test code was breaking when run under the debugger.

The issue seemed to be related to the use of pexpect 4.6 in the new environment as opposed to pexpect 3.3 in the old environment.

This is made very difficult to debug as the ptyprocess code closes all the file descriptors of the child process before attempting to exec. As a result random exceptions were being swallowed, the entire thing was crashing out, and the code was locking up in an error read loop.

I hemmed and hawed about downgrading to the 3.3 version of pexpect, but decided to investigate further, rather than leave the problem as is.

Addressing the debugging problem involved replacing all the code that closed all the file descriptors with code that marked all the file descriptors as close on exec, so that when I saw the exception, I was able to deal with it. This was done in the ptyprocess module. The solution is linux only, but TBH at this time it’s all I’m concerned with.

Addressing the pexpect problem involved just removing the code that re-encoded the arguments when the encoding argument was passed, and just leave them as-is.

The confusion is because encoding was for the I/O, not for the arguments on the command line, and when the change was made it relied on this argument, rather than adding an extra argument to deal with it.

Fixes the problem in my case, but it was a complete pain to debug.

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