This fixes a race during import. The existing _PyRuntimeState.imports.pkgcontext is shared between interpreters, and occasionally this would cause a crash when multiple interpreters were importing extensions modules at the same time. To solve this we add a thread-local variable for the value. We also leave the existing state (and infrequent race) in place for platforms that do not support thread-local variables.
The original tool wasn't working right and it was simpler to create a new one, partially re-using some of the old code. At this point the tool runs properly on the master. (Try: ./python Tools/c-analyzer/c-analyzer.py analyze.) It take ~40 seconds on my machine to analyze the full CPython code base.
Note that we'll need to iron out some OS-specific stuff (e.g. preprocessor). We're okay though since this tool isn't used yet in our workflow. We will also need to verify the analysis results in detail before activating the check in CI, though I'm pretty sure it's close.
https://bugs.python.org/issue36876
