tree 3b64e281d4c3fa1a0265564c519d11746ced4eac
parent 6c3a681bd61a3fd54de8fe644db17b9676939396
author Willy Tarreau <w@1wt.eu> 1583517895 +0100
committer Willy Tarreau <w@1wt.eu> 1583557875 +0100
encoding latin1

BUG/MEDIUM: random: implement per-thread and per-process random sequences

As mentioned in previous patch, the random number generator was never
made thread-safe, which used not to be a problem for health checks
spreading, until the uuid sample fetch function appeared. Currently
it is possible for two threads or processes to produce exactly the
same UUID. In fact it's extremely likely that this will happen for
processes, as can be seen with this config:

    global
        nbproc 8

    frontend f
        bind :4445
        mode http
        log stdout daemon format raw
        log-format "%[uuid] %pid"
        redirect location /

It typically produces this log:

  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30645
  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30641
  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30644
  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30639
  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30646
  07764439-c24d-4e6f-a5a6-0138be59e7a8 30645
  07764439-c24d-4e6f-a5a6-0138be59e7a8 30639
  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30643
  07764439-c24d-4e6f-a5a6-0138be59e7a8 30646
  b6773fdd-678f-4d04-96f2-4fb11ad15d6b 30646
  551ce567-0bfb-4bbd-9b58-cdc7e9365325 30642
  07764439-c24d-4e6f-a5a6-0138be59e7a8 30642

What this patch does is to use a distinct per-thread and per-process
seed to make sure the same sequences will not appear, and will then
extend these seeds by "burning" a number of randoms that depends on
the global random seed, the thread ID and the process ID. This adds
roughly 20 extra bits of randomness, resulting in 52 bits total per
thread and per process.

It only takes a few milliseconds to burn these randoms and given
that threads start with a different seed, we know they will not
catch each other. So these random extra bits are essentially added
to ensure randomness between boots and cluster instances.

This replaces all uses of random() with ha_random() which uses the
thread-local state.

This must be backported as far as 2.0 or any version having the
UUID sample-fetch function since it's the main victim here.

It's important to note that this patch, in addition to depending on
the previous one "BUG/MEDIUM: init: initialize the random pool a bit
better", also depends on the preceeding build fixes to address a
circular dependency issue in the include files that prevented it
from building. Part or all of these patches may need to be backported
or adapted as well.