Signal safety

[tornaria]

Follow up to #162.

The first commit gets rid of all usage of stdio in signal handlers (mostly, but not only, inside ENABLE_DEBUG_CYSIGNALS).

The second commit replaces use of gettimeofday() by clock_gettime(), the latter being safe according to signal-safety(7). It's also monotonic, unlike gettimeofday().

Other stuff that happens inside the signal handler:

• cysigs_interrupt_handler() calls PyErr_SetInterrupt() (probably ok?).

• when debug_level >= 2 the function do_raise_exception() calls PyGILState_Ensure / PyErr_Occurred / PyGILState_Release, I wonder if this could deadlock for some reason; in any case I don't understand why the GIL is necessary if the return value of PyErr_Occurred() is not used, just printed.

• function sigdie() uses getenv() which is not safe, maybe the env variables should be read by init_cysignals() instead.

• function print_enhanced_backtrace() uses execvp(), this is probably ok although the "correct" way would be to read PATH on init and here use execv() with the full path to cysignals-CSI already precomputed.

• the function sig_raise_exception() is in cython and calls lots of python stuff, I wonder if this should be called after the longjmp() instead (it probably makes no difference since I don't think longjmp() will fix internal state of e.g. stdio).

[mkoeppe]

Rebased, to run the current tests

[mkoeppe]

@malb @dimpase should this be merged? It seems to pass all tests.
@tornaria Are you using these fixes in production already?

[tornaria]

@malb @dimpase should this be merged? It seems to pass all tests. @tornaria Are you using these fixes in production already?

I'm only using #162 (see https://github.com/void-linux/void-packages/tree/master/srcpkgs/python3-cysignals/patches). This is the only issue that I know how to reproduce. This PR "fixes" potential issues of the same kind that I don't know how to reproduce.

Note that #162 is merged but never released. It would be nice to have a new release which fixes this issue and fully supports cython 3. The easy way to support cython 3 is to use legacy_implicit_noexcept but a proper fix is to actually add all the noexcept clauses so the code builds fine with cython 0.29 or cython 3.

[dimpase]

rebased over the latest main, to see how it goes with CI