We no longer rely on now_offset stored in the shm-stats-file. Instead
haproxy automatically computes the now_offset relative to the monotonic
clock and the shared global clock.
Indeed, the previous model based on static now_offset when monotonic
clock is available proved to be insufficient when used in
combination with shm-stats-file (that is when monotonic clock is shared
between multiple co-processes). In ideal situation co-processes would
correctly apply the offset to their local monotonic clock and end up
with consistent now_ns. But when restarting from an existing
shm-stats-file from a previous session (ie: prior to reboot), then the
local monotonic clock would no longer be consistent with the one used
to update the file previously, so applying a static offset would fail
to restore clock consistency.
For this specific issue, a workaround was brought by 09bf116
("BUG/MEDIUM: stats-file: detect and fix inconsistent shared clock when resuming from shm-stats-file")
but the solution implemented there was deemed too fragile, because there
is a 60sec window where the fix would fail to detect inconsistent clock
and would leave haproxy with a broken clock ranging from 0 to 60 seconds,
which can be huge..
By simply recomputing the now_offset each time we learn from another
process (through the shared map by reading global_now_ns), we simply
recompute our local offset (difference between OUR monotonic clock
and the SHARED one). Also, in clock_update_global_date(), we make
sure we always recompute the now_offset as now_ms may have been
updated from shared clock if shared clock was ahead of us.
Thanks to that new logic, interrupted processes, resumed processes,
processed started with shm-stats-file from previous session now
correctly recover from those various situations and multiple
co-processes with diverting clocks on startup end up converging to
the same values.
Since it is no longer relevant to save now_offset in the map, it was
removed but to prevent shm-stats-file incompatibility with previous
versions, 8-byte hole was forced, and we didn't bump the shm-stats-file
version on purpose.
This patch may be backported in 3.3 after a solid period of observation
to ensure we didn't break things.
4319c20363