The regex package isn't thread safe (tracking issue)

[makyen]

I ended up taking a look at the code for the regex package. There are some issues in it which make it not thread safe. I've created the issue Thread safety: need lock for both read and write (_cache and other global variables) in that repository. This issue is for tracking it here.

[makyen]

I've forked the regex package into Charcoal-SE/mrabarnett--mrab-regex and added some changes for thread safety. I need to do more review of the code for the package and coordinate with the author of the package wrt. what changes are needed and getting those changes into the base package.

In the meantime, in PR #7074, I've proposed changing requirements.txt to use the Charcoal-SE version of the package.

[makyen]

I've taken a deeper look at the regex package's C code. It appears that a regex that's actually in use is not thread safe (i.e. two threads can't safely perform an operation with the same compiled regex pattern at the same time).

I thought I had accounted for that possibility in the changes I made, which are mentioned above, by always using copy.deepcopy() to copy compiled patterns into and out of the implicit _cache. However, I've now found out that the C code in regex which implements __deepcopy__, the function which is used to perform a copy.deepcopy(), really just returns a reference to the original compiled regex without performing a copy. In other words, making a copy of a regex just gives you a reference to the original regex object. So, the above mentioned changes need some work.

[stale]

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[makyen]

I'm not certain at this point that there's actually a thread safety issue within the C code for the regex package. I have put more work into this and taken a deeper dive through the regex package's C code. I have not found a specific thread safety issue which would be affecting us.

What I have seen is that there are substantial errors when running the CI test using a large number of different processes. Those errors go away if workarounds are in place to be quite careful not to use the regex package in a way that could invoke a thread safety issue. OTOH, I haven't demonstrated that errors are not removed as a result of the relatively minor slowdown which the workarounds introduce.

Basically, I still need to perform more investigation.