BLD: GH30873 Workaround parallel build failures
Fix parallel build race conditions due to the same source files being
used in multiple targets.  To workaround the distutils bug, create
unique .c files for each target that just #include the original file.

- [x] closes #30873 
- [ ] tests added / passed
- [ ] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [ ] whatsnew entry

I know that this solution somewhat sucks but I don't know of a better one.

Before the patch, the breakage can be proven by grepping the build log:
```
$ grep -o -- '-o [^ ]*' build.log  | sort | uniq -c | grep -v 1
      3 -o build/temp.linux-x86_64-3.9/pandas/_libs/src/parser/tokenizer.o
      4 -o build/temp.linux-x86_64-3.9/pandas/_libs/tslibs/src/datetime/np_datetime.o
      2 -o build/temp.linux-x86_64-3.9/pandas/_libs/tslibs/src/datetime/np_datetime_strings.o
```

Since all commands are run in parallel, we're dealing with 2-4 commands writing to the same file simultaneously, and racing with linking commands that use these files. We've had mysterious bug for a long time now, and we've finally figured out the root cause. For example, in dask:

```pytb
_____________ ERROR collecting dask/array/tests/test_array_core.py _____________
ImportError while importing test module '/var/tmp/portage/dev-python/dask-2021.3.0/work/dask-2021.3.0/dask/array/tests/test_array_core.py'.
Hint: make sure your test modules/packages have valid Python names.
Traceback:
/usr/lib/python3.8/site-packages/pandas/__init__.py:29: in <module>
    from pandas._libs import hashtable as _hashtable, lib as _lib, tslib as _tslib
/usr/lib/python3.8/site-packages/pandas/_libs/__init__.py:13: in <module>
    from pandas._libs.interval import Interval
pandas/_libs/interval.pyx:1: in init pandas._libs.interval
    ???
pandas/_libs/hashtable.pyx:1: in init pandas._libs.hashtable
    ???
pandas/_libs/missing.pyx:1: in init pandas._libs.missing
    ???
/usr/lib/python3.8/site-packages/pandas/_libs/tslibs/__init__.py:30: in <module>
    from .conversion import OutOfBoundsTimedelta, localize_pydatetime
E   ImportError: /usr/lib/python3.8/site-packages/pandas/_libs/tslibs/conversion.cpython-38-x86_64-linux-gnu.so: undefined symbol: pandas_datetime_to_datetimestruct
```

We're building via `python setup.py build -j 36`. can u point to a bug in disutils itself or a bug report? No, I don't have any bug link handy. And I don't think there's any point in wasting my time filing one given that Python upstream has abandoned distutils. > No, I don't have any bug link handy. And I don't think there's any point in wasting my time filing one given that Python upstream has abandoned distutils.

and then how do you expect us to merge this?

this is a very hacky solution  - if you said the dependencies are not specified correctly then that’s something - but this appears to be a bandaid that may or may not work and is adding a lot of complexity I'm doing the best with the very little time I have. If you don't believe me, just run `setup.py build -j100` or alike and see for yourself that multiple compiler instances with the same output file are started simultaneously and don't order correctly. The likeliness that Python upstream fixes this is close to zero and even if they do, all the old Python versions that do not receive bugfixes will still be affected. why is j more than like 4 actually useful? in any event we need a solution that follows c standards
duplicating files is not it It's pretty normal for people to set `-j` to the number of their processors/cores/threads globally. > why is j more than like 4 actually useful? in any event we need a solution that follows c standards
> duplicating files is not it

Have you ever compiled stuff on a 16-core machine? `-j16` is essential to scaling build systems to many-core machines Here's the numpy issue: https://github.com/numpy/numpy/issues/15957 > why is j more than like 4 actually useful? in any event we need a solution that follows c standards
> duplicating files is not it

@jreback I initially filed this issue in #30873, because I use `-j 8` and always get these failures.  And it does speed up the build compared to `-j 4` especially when you are merging with `master` and everything has to get rebuilt.
 @Dr-Irv I am not averse to solving this. why is this: https://stackoverflow.com/questions/11013851/speeding-up-build-process-with-distutils not the patch.

 > @Dr-Irv I am not averse to solving this. why is this: https://stackoverflow.com/questions/11013851/speeding-up-build-process-with-distutils not the patch.

@jreback I don't think that solution will work here.  What it does is spawn off separate processes to do the compilation.  I think if we did that we'd still have the same issue.  What is happening with pandas is that we compile the same file multiple times, and the compilations interfere with each other when done in parallel since they are writing to the same file.   So the workaround proposed here in the PR is to ensure that we don't have those collisions.
 I am not an expert but I have never seen anyone ever suggesting including a C file directly in another to be a good thing. If anything, I only see dire warnings against doing something like this:

https://stackoverflow.com/questions/10448047/include-c-file-in-another

I am generally -1 on adding complexity to pandas for this, especially if we haven't checked upstream where this should really be solved #30873 seems worth fixing, but this doesn't seem like an approach we want