`Unexpected status 0x1057f (PTRACE_EVENT_FORK)` during diversion

Unfortunately, I don't know exactly what I did to get into this state, and it doesn't easily reproduce:

(rr) p jl_dump_llvm_value(0x7b2000007788)
; Function Attrs: allocsize(1)
declare noalias nonnull {} addrspace(10)* @ijl_gc_pool_alloc[FATAL /home/keno/rr/src/AutoRemoteSyscalls.cc:344:syscall_base()] 
 (task 1666792 (rec:1665936) at time 0)
 -> Assertion `false' failed to hold. Unexpected status 0x1057f (PTRACE_EVENT_FORK)
=== Start rr backtrace:
/home/keno/rr-build/bin/rr(_ZN2rr13dump_rr_stackEv+0x5d)[0x562107483876]
/home/keno/rr-build/bin/rr(_ZN2rr9GdbServer15emergency_debugEPNS_4TaskE+0x1a6)[0x5621072a7e4a]
/home/keno/rr-build/bin/rr(+0x39d2f6)[0x5621072db2f6]
/home/keno/rr-build/bin/rr(_ZN2rr21EmergencyDebugOstreamD1Ev+0x66)[0x5621072db560]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls12syscall_baseEiRNS_9RegistersE+0x96b)[0x5621072328bd]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi7EEEliRNS_9RegistersE+0x2c)[0x56210723adb6]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi6EmJEEEliRNS_9RegistersET0_DpT1_+0x43)[0x56210723ad2d]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi5EmJmEEEliRNS_9RegistersET0_DpT1_+0x49)[0x562107247807]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi4EmJmmEEEliRNS_9RegistersET0_DpT1_+0x54)[0x562107247730]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi3EmJmmmEEEliRNS_9RegistersET0_DpT1_+0x58)[0x562107247412]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi2EmJmmmmEEEliRNS_9RegistersET0_DpT1_+0x5f)[0x5621072470f7]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls14syscall_helperILi1EmJmmmmmEEEliRNS_9RegistersET0_DpT1_+0x5e)[0x562107246cce]
/home/keno/rr-build/bin/rr(_ZN2rr18AutoRemoteSyscalls7syscallIJmmmmmmEEEliDpT_+0xb8)[0x5621072464c4]
/home/keno/rr-build/bin/rr(+0x30593f)[0x56210724393f]
/home/keno/rr-build/bin/rr(+0x306d2b)[0x562107244d2b]
/home/keno/rr-build/bin/rr(+0x305a42)[0x562107243a42]
/home/keno/rr-build/bin/rr(_ZN2rr16DiversionSession14diversion_stepEPNS_4TaskENS_10RunCommandEi+0x616)[0x5621072440b2]
/home/keno/rr-build/bin/rr(_ZN2rr9GdbServer6divertERNS_13ReplaySessionE+0x31f)[0x5621072a3655]
/home/keno/rr-build/bin/rr(_ZN2rr9GdbServer25process_debugger_requestsENS0_11ReportStateE+0x1af)[0x5621072a3a6b]
/home/keno/rr-build/bin/rr(_ZN2rr9GdbServer14debug_one_stepERNS_10GdbRequestE+0x183)[0x5621072a484d]
/home/keno/rr-build/bin/rr(_ZN2rr9GdbServer12serve_replayERKNS0_15ConnectionFlagsE+0x54f)[0x5621072a6f15]
/home/keno/rr-build/bin/rr(+0x485456)[0x5621073c3456]
/home/keno/rr-build/bin/rr(_ZN2rr13ReplayCommand3runERSt6vectorINSt7__cxx1112basic_stringIcSt11char_traitsIcESaIcEEESaIS7_EE+0x404)[0x5621073c3ee0]
/home/keno/rr-build/bin/rr(main+0x21f)[0x56210749f77b]
/lib/x86_64-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0x7fd62445d0b3]
/home/keno/rr-build/bin/rr(_start+0x2e)[0x5621071ff63e]
=== End rr backtrace

I tried some obvious things like p fork(), but that seems to not cause the same issue.

Start at AutoRemoteSyscalls.cc:344 and trace the call from DiversionSession::diversion_step shown in the backtrace. Use the reported replay scenario if it can be reproduced, then investigate why PTRACE_EVENT_FORK reaches the assertion during diversion; done means the failure is reproduced and either prevented or covered by a regression test.