Non-reproducible failure in RestartEventLogging test in `threaded1` way.

Summary

Running validate by hand the test typically succeeds, but on one run I got:

...
--- tests/rts/RestartEventLogging.run/RestartEventLogging.stdout.normalised 2021-04-20 21:29:06.924934000 -0400
+++ tests/rts/RestartEventLogging.run/RestartEventLogging.run.stdout.normalised   2021-04-20 21:29:06.925062000 -0400
@@ -8,6 +8,11 @@
 stop
 init
 Event log started with EVENT_HEADER_BEGIN
+ERROR: event does not start with EVENT_HEADER_BEGIN
+0x0 != 0x68
+0x12 != 0x64
+0x0 != 0x72
+0x0 != 0x62
 stop
 init
 Event log started with EVENT_HEADER_BEGIN
...
tests/rts/RestartEventLogging.run  RestartEventLogging [bad stdout] (threaded1)

The expected value is:

includes/rts/EventLogFormat.h:#define EVENT_HEADER_BEGIN    0x68647262 /* 'h' 'd' 'r' 'b' */

but we got '\0\x12\0\0'.

Steps to reproduce

Not easily reproducible, could even be a hardware glitch, though that seems unlikely...

Expected behavior

The test should ideally pass consistently

Environment

• GHC version used: commit 4cfb6b89 (HEAD -> freebsd-tlsgd, v/freebsd-tlsgd) of MR !5561 (closed). The merge-base with head is: commit 0619fb0f (origin/master, origin/HEAD, master)

Optional:

• Operating System: FreeBSD 12.2
• System Architecture: X86_64