Assertion `!is_in_untraced_syscall() || 0 == (SYSCALLBUF_LOCKED_TRACEE & read_mem(REMOTE_PTR_FIELD(syscallbuf_child, locked)))' failed to hold.  #2276

I've just run into this assertion while debugging a Firefox marionette test in chaos mode:

[FATAL /var/tmp/portage/dev-util/rr-5.2.0/work/rr-5.2.0/src/Task.cc:2017:reset_syscallbuf()]
(task 22180 (rec:22180) at time 2076543)
-> Assertion `!is_in_untraced_syscall() || 0 == (SYSCALLBUF_LOCKED_TRACEE & read_mem(REMOTE_PTR_FIELD(syscallbuf_child, locked)))' failed to hold.
=== Start rr backtrace:
rr(_ZN2rr13dump_rr_stackEv+0x41)[0x564ef24159b1]
rr(_ZN2rr9GdbServer15emergency_debugEPNS_4TaskE+0x5c5)[0x564ef233ff45]
rr(_ZN2rr21EmergencyDebugOstreamD1Ev+0x11e)[0x564ef234df6e]
rr(_ZN2rr4Task16reset_syscallbufEv+0xb3)[0x564ef23fd673]
rr(_ZN2rr10RecordTask22maybe_reset_syscallbufEv+0xb0)[0x564ef23af070]
rr(_ZN2rr10RecordTask12record_eventERKNS_5EventENS0_15FlushSyscallbufEPKNS_9RegistersE+0x164)[0x564ef23ae744]
rr(+0xbaa0b)[0x564ef236da0b]
rr(_ZN2rr13RecordSession11record_stepEv+0x2b9)[0x564ef2371f19]
rr(_ZN2rr13RecordCommand3runERSt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS7_EE+0x95f)[0x564ef2367cbf]
rr(main+0x1da)[0x564ef22f20fa]
/lib64/libc.so.6(__libc_start_main+0xf1)[0x7f327500b011]
rr(_start+0x2a)[0x564ef22f22ca]
=== End rr backtrace

I hit this twice while running the tests with the following command:

./mach marionette-test -z testing/marionette/harness/marionette_harness/tests/unit/test_select.py testing/marionette/harness/marionette_harness/tests/unit/test_crash.py

This was on mozilla-central revision 90853a59691e with a local build. I could reproduce the issue twice in four runs so it's not hard to reproduce. I can provide all the build details if needed (mozconfig contents, etc...). BTW this was on Gentoo Linux, kernel 4.14.65 and glibc 2.26.