$SOURCE_DATE_EPOCH support for build date stamp

If, and _only_ if, $SOURCE_DATE_EPOCH is found in environ during build,
use it to set the timestamp embedded in the binary instead of using
__DATE__ and __TIME__ cpp directives.

This per <https://reproducible-builds.org/specs/source-date-epoch/>
spec.  It's sane and sensible, without removing date stamping which
matters.  The examples encourage packagers to use timestamps which
do change when they backport patches, so that the date remains useful
for distinguishing builds instead of claiming one date forever across
multiple patchlevels.

This change written so that the old behaviour and code is used if the
environment variable is not found, to better continue to support ancient
platforms with other variants of date(1).

Built with and without an override, on macOS.

1 files changed, 34 insertions, 0 deletions

diff --git a/src/scripts/reversion b/src/scripts/reversion
index 9707b9c1c..45755c084 100755
--- a/src/scripts/reversion
+++ b/src/scripts/reversion
@@ -71,10 +71,41 @@ EXIM_COMPILE_NUMBER=$(expr "${EXIM_COMPILE_NUMBER:-0}" + 1)
 
 echo "$EXIM_COMPILE_NUMBER" >cnumber.h
 
+# Reproducible builds, accept a build timestamp override from environ per
+# <https://reproducible-builds.org/specs/source-date-epoch/>.
+# We require a fairly modern date(1) command here, which is not portable
+# to some of the systems Exim is built on.  That's okay, because the scenarios
+# are:
+#  1) Local postmaster building, not using $SOURCE_DATE_EPOCH, doesn't matter
+#  2) Packaging folks who don't care about reproducible builds
+#  3) Packaging folks who care but are using systems where date Just Works
+#  3) Packaging folks who care and can put a modern date(1) in $PATH
+#  4) Packaging folks who care and can supply us with a clean patch to support
+#     their requirements
+#  5) Packaging folks who care but won't do any work to support their strange
+#     old systems and want us to do the work for them.  We don't care either,
+#     they're SOL and have to live without reproducible builds.
+#
+exim_build_date_override=''
+if [ ".${SOURCE_DATE_EPOCH:-}" != "." ]; then
+  fmt='+%d-%b-%Y %H:%M:%S'
+  # Non-reproducible, we use __DATE__ and __TIME__ in C, which respect timezone
+  # (think localtime, not gmtime); for reproduction between systems, UTC makes
+  # more sense and the examples available use UTC without explicitly mandating
+  # it.  I think that we can switch behavior and use UTC for reproducible
+  # builds without it causing any problems: nothing really cares about timezone.
+  # GNU date: "date -d @TS"
+  # BSD date: "date -r TS"
+  exim_build_date_override="$(date -u -d "@${SOURCE_DATE_EPOCH}" "$fmt" 2>/dev/null | date -u -r "${SOURCE_DATE_EPOCH}" "$fmt" 2>/dev/null)"
+fi
+
 ( echo '# automatically generated file - see ../scripts/reversion'
   echo EXIM_RELEASE_VERSION='"'"$EXIM_RELEASE_VERSION"'"'
   echo EXIM_VARIANT_VERSION='"'"$EXIM_VARIANT_VERSION"'"'
   echo EXIM_COMPILE_NUMBER='"'"$EXIM_COMPILE_NUMBER"'"'
+  if [ ".${exim_build_date_override:-}" != "." ]; then
+    echo EXIM_BUILD_DATE_OVERRIDE='"'"${exim_build_date_override}"'"'
+  fi
 ) >version.sh
 
 if [ ! -f version.h ]
@@ -83,6 +114,9 @@ then
   echo '#define EXIM_RELEASE_VERSION "'"$EXIM_RELEASE_VERSION"'"'
   echo '#define EXIM_VARIANT_VERSION "'"$EXIM_VARIANT_VERSION"'"'
   echo '#define EXIM_VERSION_STR EXIM_RELEASE_VERSION EXIM_VARIANT_VERSION'
+  if [ ".${exim_build_date_override:-}" != "." ]; then
+    echo '#define EXIM_BUILD_DATE_OVERRIDE "'"${exim_build_date_override}"'"'
+  fi
 ) >version.h
 fi