bpo-34022: Stop forcing of hash-based invalidation with SOURCE_DATE_EPOCH (GH-9607)

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.

1 files changed, 3 insertions, 0 deletions

diff --git a/Misc/NEWS.d/next/Library/2018-09-27-13-14-15.bpo-34022.E2cl0r.rst b/Misc/NEWS.d/next/Library/2018-09-27-13-14-15.bpo-34022.E2cl0r.rst
new file mode 100644
index 0000000000..efebb84304
--- /dev/null
+++ b/Misc/NEWS.d/next/Library/2018-09-27-13-14-15.bpo-34022.E2cl0r.rst
@@ -0,0 +1,3 @@
+The :envvar:`SOURCE_DATE_EPOCH` environment variable no longer overrides the
+value of the *invalidation_mode* argument to :func:`py_compile.compile`, and
+determines its default value instead.