systemd: ensure reproducible builds by clearly exposing the time epoch support
systemd has the ability to check the time on boot and if it's earlier than an
epoch determined at build time, set the time to that epoch. This is useful for
systems where the system time is January 1st 1970 (because the unix timestamp
was 0 at boot) as then at least the time is reset to something approximating the
right year at least.
By default systemd uses the mtime of the NEWS file, which is static for tarballs
and corresponds to the time the release was made, but for git checkouts this is
simply the time do_unpack() was executed. Thus, rebuilding systemd will cause
this embedded timestamp to change.
Remove the PACKAGECONFIG time-epoch which has the logic reversed: enabling
time-epoch will set the epoch to the unix timestamp 0). Replace with
set-time-epoch with the following semantics:
- When disabled, the time epoch is set to 0 (1st January 1970), so there is no
time manipulation on boot.
- When enabled, if reproducible builds are configured by setting
SOURCE_DATE_EPOCH then that timestamp is used for the time epoch. If
reproducible builds are not configured then the timestamp of NEWS (thus the
build time) is used.
The set-time-epoch flag is enabled by default.
[ YOCTO #13473 ]
Signed-off-by: Ross Burton <ross.burton@intel.com>
