Allow to override build date with SOURCE_DATE_EPOCH

in order to make builds reproducible.
See https://reproducible-builds.org/ for why this is good
and https://reproducible-builds.org/specs/source-date-epoch/
for the definition of this variable.

Without this patch, building python-debtcollector from a tarball
that does not contain a .git dir varied for every build.

Change-Id: Iaf8bc1c559f5b149a48c9dd05beb0961365fda7e
Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>

openstackdocstheme/page_context.py

@@ -11,14 +11,17 @@
 #    under the License.
 
 import datetime
+import os
 import os.path
 import subprocess
+import time
 
 from sphinx.util import logging
 
 LOG = logging.getLogger(__name__)
 
-_default_last_updated = datetime.datetime.now()
+_timeint = int(os.environ.get('SOURCE_DATE_EPOCH', time.time()))
+_default_last_updated = datetime.datetime.utcfromtimestamp(_timeint)
 
 
 def _get_last_updated_file(src_file):

releasenotes/notes/source_date_epoch-1078730ca0b29a4e.yaml

@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Allow to override build date of generated files with SOURCE_DATE_EPOCH
+    in order to make build results of packages fully reproducible
+    especially when building from tarballs without a .git directory.

releasenotes/source/conf.py

@@ -142,6 +142,9 @@ html_static_path = ['_static']
 
 # If not '', a 'Last updated on:' timestamp is inserted at every page bottom,
 # using the given strftime format.
+# If the SOURCE_DATE_EPOCH env var is set, its value will be used as default
+# instead of the build time.
+# See https://reproducible-builds.org/specs/source-date-epoch/
 # html_last_updated_fmt = '%b %d, %Y'
 html_last_updated_fmt = '%Y-%m-%d %H:%M'