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bpo-29708: support SOURCE_DATE_EPOCH env var in py_compile (allow for reproducible builds of python packages) #296

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bpo-29708: support SOURCE_DATE_EPOCH env var in py_compile (allow for reproducible builds of python packages) #296

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bpo-29708: support SOURCE_DATE_EPOCH env var in py_compile
bmwiedemann Feb 25, 2017
3182f49
Add markup and change notice to the docs
brettcannon Jan 11, 2018
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5 changes: 5 additions & 0 deletions Doc/library/py_compile.rst
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Expand Up @@ -57,6 +57,10 @@ byte-code cache files in the directory containing the source code.
enum and controls how the generated ``.pyc`` files are invalidated at
runtime.

If the :envvar:`SOURCE_DATE_EPOCH` environment variable is set, the timestamp
entry in the ``.pyc`` file header will be set to this value.
See https://reproducible-builds.org/specs/source-date-epoch/ for more info.

.. versionchanged:: 3.2
Changed default value of *cfile* to be :PEP:`3147`-compliant. Previous
default was *file* + ``'c'`` (``'o'`` if optimization was enabled).
Expand All @@ -71,6 +75,7 @@ byte-code cache files in the directory containing the source code.

.. versionchanged:: 3.7
The *invalidation_mode* parameter was added as specified in :pep:`552`.
Support for :envvar:`SOURCE_DATE_EPOCH` was also added.


.. class:: PycInvalidationMode
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8 changes: 8 additions & 0 deletions Lib/py_compile.py
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Expand Up @@ -147,6 +147,14 @@ def compile(file, cfile=None, dfile=None, doraise=False, optimize=-1,
pass
if invalidation_mode == PycInvalidationMode.TIMESTAMP:
source_stats = loader.path_stats(file)
source_date_epoch = os.environ.get('SOURCE_DATE_EPOCH')
if source_date_epoch:
try:
source_date_epoch = int(source_date_epoch)
except ValueError:
raise ValueError("SOURCE_DATE_EPOCH is not a valid integer")
if source_stats['mtime'] > source_date_epoch:
source_stats['mtime'] = source_date_epoch
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Why are we making judgment calls about which one is greater? SOURCE_DATE_EPOCH means I decided I know better than you what the time is, that is the entire point. For example, Arch Linux stores all files in the package archive using the modification time of SOURCE_DATE_EPOCH.

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OTOH rpm and tar's --clamp-mtime option implement mtime clamping, so that old files keep their timestamps and just timestamps of files created during a build are modified to be SOURCE_DATE_EPOCH
can be tricky to get this right for both cases...

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I think in that case, the mtime clamp will set the pyc modification time to SOURCE_DATE_EPOCH anyway.

It would result in general files being clamped to a "latest" mtime of SOURCE_DATE_EPOCH and the pyc header timestamps being exactly equal to SOURCE_DATE_EPOCH, so everyone wins.

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@bmwiedemann bmwiedemann Jan 10, 2018 •
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But python does an exact match when checking if it can use pyc files, so old .py files having .pyc files with 'too new' SOURCE_DATE_EPOCH in header would also result in .pyc files not to be used.

I think we might need something like

if source_stats['mtime'] > source_date_epoch or os.environ.get('ALWAYS_SET_MTIME'):
    source_stats['mtime'] = source_date_epoch

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I think worrying about whether .pyc files will be regenerated based on the static date set in them is getting beyond the scope of reproducible builds. If this is going to have a chance to land in Python 3.7 then let's keep this PR just to the production of .pyc files and not their consumption.

bytecode = importlib._bootstrap_external._code_to_timestamp_pyc(
code, source_stats['mtime'], source_stats['size'])
else:
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12 changes: 12 additions & 0 deletions Lib/test/test_py_compile.py
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Expand Up @@ -98,6 +98,18 @@ def test_bad_coding(self):
self.assertFalse(os.path.exists(
importlib.util.cache_from_source(bad_coding)))

def test_source_date_epoch(self):
testtime = 123456789
with support.EnvironmentVarGuard() as env:
env["SOURCE_DATE_EPOCH"] = str(testtime)
py_compile.compile(self.source_path, self.pyc_path)
self.assertTrue(os.path.exists(self.pyc_path))
self.assertFalse(os.path.exists(self.cache_path))
with open(self.pyc_path, "rb") as f:
f.read(4) # Skip the magic number.
timebytes = f.read(4) # Read timestamp.
self.assertEqual(testtime, int.from_bytes(timebytes, 'little'))

@unittest.skipIf(sys.flags.optimize > 0, 'test does not work with -O')
def test_double_dot_no_clobber(self):
# http://bugs.python.org/issue22966
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2 changes: 2 additions & 0 deletions Misc/NEWS.d/next/Build/2017-09-07-04-20-49.bpo-29708.YCaHEx.rst
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@@ -0,0 +1,2 @@
If the SOURCE_DATE_EPOCH environment variable is set,
py_compile will use it to override the timestamps it puts into .pyc files.