rr fails to record mach mochitest-devtools

I built git master rr on x86-64 Fedora 20. I commented out the 32-bit tests (didn't want to install the 32-bit toolchain) but the 64-bit tests all pass.

I am building firefox fx-team. I've tried various configurations but all with this in .mozconfig:

ac_add_options --enable-devtools-perf

I tried to record a test run using:

./mach mochitest-devtools --slowscript --debugger rr  browser/devtools/performance/test/browser_perf-compatibility-04.js

This fails like:

[FATAL /home/tromey/firefox-git/rr/src/task.cc:562:fstat() errno: 13 'Permission denied']
 (task 26191 (rec:26191) at time 3637)
 ->  Assertion `backing_fd.is_open()' failed to hold.
[FATAL /home/tromey/firefox-git/rr/src/log.cc:16:emergency_debug() errno: 0 'Success']
 -> (session doesn't look interactive, aborting emergency debugging)

Re-running with rr in gdb, the backtrace is:

(gdb) bt
#0  0x00007ffff6ab101f in accept4 (fd=4, addr=..., addr_len=0x7fffffffae9c, 
    flags=-1) at ../sysdeps/unix/sysv/linux/accept4.c:37
#1  0x0000000000548ef7 in GdbConnection::await_debugger (this=0x6ab670, 
    listen_fd=...) at /home/tromey/firefox-git/rr/src/GdbConnection.cc:109
#2  0x0000000000549119 in GdbConnection::await_client_connection (
    desired_port=25814, probe=GdbConnection::PROBE_PORT, tgid=25814, 
    exe_image="", client_params_fd=0x0)
    at /home/tromey/firefox-git/rr/src/GdbConnection.cc:142
#3  0x0000000000554eed in GdbServer::emergency_debug (t=0x690c40)
    at /home/tromey/firefox-git/rr/src/GdbServer.cc:896
#4  0x000000000056cfd8 in emergency_debug (t=0x690c40)
    at /home/tromey/firefox-git/rr/src/log.cc:20
#5  0x000000000056d09a in EmergencyDebugOstream::~EmergencyDebugOstream (
    this=0x7fffffffd220, __in_chrg=<optimized out>)
    at /home/tromey/firefox-git/rr/src/log.cc:27
#6  0x00000000005e954f in Task::fstat (this=0x690c40, fd=9)
    at /home/tromey/firefox-git/rr/src/task.cc:562
#7  0x000000000058a827 in process_mmap (t=0x690c40, length=217032, prot=1, 
    flags=1, fd=9, offset_pages=0)
    at /home/tromey/firefox-git/rr/src/record_syscall.cc:2424
#8  0x000000000059470b in rec_process_syscall_arch<rr::X64Arch> (t=0x690c40, 
    syscall_state=...)
    at /home/tromey/firefox-git/rr/src/record_syscall.cc:2645
#9  0x000000000058b0a4 in rec_process_syscall_internal (t=0x690c40, 
    syscall_state=...)
    at /home/tromey/firefox-git/rr/src/record_syscall.cc:2805
#10 0x000000000058b0da in rec_process_syscall (t=0x690c40)
    at /home/tromey/firefox-git/rr/src/record_syscall.cc:2810
#11 0x0000000000580131 in RecordSession::syscall_state_changed (this=0x68f450, 
    t=0x690c40, step_state=0x7fffffffdeb0)
    at /home/tromey/firefox-git/rr/src/RecordSession.cc:531
#12 0x0000000000582ad1 in RecordSession::record_step (this=0x68f450)
    at /home/tromey/firefox-git/rr/src/RecordSession.cc:1065
#13 0x000000000057c12f in record (
    args=std::vector of length 5, capacity 8 = {...}, flags=...)
    at /home/tromey/firefox-git/rr/src/RecordCommand.cc:187
#14 0x000000000057c374 in RecordCommand::run (
    this=0x68ae40 <RecordCommand::singleton>, 
    args=std::vector of length 5, capacity 8 = {...})
    at /home/tromey/firefox-git/rr/src/RecordCommand.cc:233
#15 0x000000000056ea1e in main (argc=6, argv=0x7fffffffe128)
    at /home/tromey/firefox-git/rr/src/main.cc:241

Reproduce the failure with the shown mach mochitest-devtools command and inspect rr/src/task.cc at Task::fstat, then follow the mmap path in rr/src/record_syscall.cc. Use the provided backtrace and the GdbConnection emergency-debug path to determine why recording receives EACCES. Done means the test run records successfully without the assertion.