Importing ProcessPoolExecutor may hang or cause an error when the import
accesses urandom on a low resource platform


https://bugs.python.org/issue29877

Unconditional forcing of ``CHECKED_HASH`` invalidation was introduced in
3.7.0 in bpo-29708.  The change is bad, as it unconditionally overrides
*invalidation_mode*, even if it was passed as an explicit argument to
``py_compile.compile()`` or ``compileall``.  An environment variable
should *never* override an explicit argument to a library function.
That change leads to multiple test failures if the ``SOURCE_DATE_EPOCH``
environment variable is set.

This changes ``py_compile.compile()`` to only look at
``SOURCE_DATE_EPOCH`` if no explicit *invalidation_mode* was specified.
I also made various relevant tests run with explicit control over the
value of ``SOURCE_DATE_EPOCH``.

While looking at this, I noticed that ``zipimport`` does not work
with hash-based .pycs _at all_, though I left the fixes for
subsequent commits.

Python now supports checking bytecode cache up-to-dateness with a hash of the
source contents rather than volatile source metadata. See the PEP for details.

While a fairly straightforward idea, quite a lot of code had to be modified due
to the pervasiveness of pyc implementation details in the codebase. Changes in
this commit include:

- The core changes to importlib to understand how to read, validate, and
  regenerate hash-based pycs.

- Support for generating hash-based pycs in py_compile and compileall.

- Modifications to our siphash implementation to support passing a custom
  key. We then expose it to importlib through _imp.

- Updates to all places in the interpreter, standard library, and tests that
  manually generate or parse pyc files to grok the new format.

- Support in the interpreter command line code for long options like
  --check-hash-based-pycs.

- Tests and documentation for all of the above.

This matches the documentation, and passes the test suite when multithreading
is disabled.

the return values as well as test them.

Thanks to Nicholas Chammas for the bug report and initial patch.

Before the argparse conversion, compileall would (sometimes) accept multiple
paths when -d was specified.  Afterward, it does not.  The corresponding check
in the original code claimed to prevent multiple *directories* from being
specified...but it didn't really work even to do that.  So this patch fixes
the regression by invoking the consenting adults rule: if you specify a
combination of arguments to compileall that produces files with inconsistent
destdirs (which you could do before), it is on you.

Patch by Jake Garver.

Patch by Claudiu Popa.