Spurious SIGILL (signal 4) when executing `git` from bootstrap test on `x86_64-apple`

[kennytm]

Symptom: on x86_64-apple (macOS), when bootstrap tries to spawn a Command of git, it will crash with signal 4 (SIGILL), which itself will then abort bootstrap.

Sample crash log

[00:53:34] failures:
[00:53:34] 
[00:53:34] ---- builder::__test::dist_with_same_targets_and_hosts stdout ----
[00:53:34]  thread 'builder::__test::dist_with_same_targets_and_hosts' panicked at 'command did not execute successfully: "git" "log" "-1" "--date=short" "--pretty=format:%cd"
[00:53:34] expected success, got: signal: 4', build_helper/lib.rs:122:9
[00:53:34] 
[00:53:34] 
[00:53:34] failures:
[00:53:34]     builder::__test::dist_with_same_targets_and_hosts

...

/Users/travis/Library/Logs/DiagnosticReports/bootstrap-dbf53a170429e672_2018-05-06-124321_Traviss-Mac-1044.crash
Process:               bootstrap-dbf53a170429e672 [31299]
Path:                  /Users/USER/*/bootstrap-dbf53a170429e672
Identifier:            bootstrap-dbf53a170429e672
Version:               0
Code Type:             X86-64 (Native)
Parent Process:        bootstrap-dbf53a170429e672 [31128]
Responsible:           bootstrap-dbf53a170429e672 [31299]
User ID:               501
Date/Time:             2018-05-06 12:43:17.691 +0000
OS Version:            Mac OS X 10.13.3 (17D47)
Report Version:        12
Anonymous UUID:        A91A3A70-3ADA-D7A9-5891-C1A67A5778E7
Time Awake Since Boot: 3300 seconds
System Integrity Protection: enabled
Crashed Thread:        0
Exception Type:        EXC_BAD_INSTRUCTION (SIGILL)
Exception Codes:       0x0000000000000001, 0x0000000000000000
Exception Note:        EXC_CORPSE_NOTIFY
Termination Signal:    Illegal instruction: 4
Termination Reason:    Namespace SIGNAL, Code 0x4
Terminating Process:   exc handler [0]
Application Specific Information:
crashed on child side of fork pre-exec
BUG IN CLIENT OF LIBPLATFORM: os_once_t is corrupt
Thread 0 Crashed:
0   libsystem_platform.dylib       0x00007fff6f9b5ad4 _os_once_gate_corruption_abort + 23
1   libsystem_platform.dylib       0x00007fff6f9b5b6e _os_once_gate_wait_slow + 129
2   libsystem_platform.dylib       0x00007fff6f9b1ed4 _os_alloc_once + 42
3   libsystem_notify.dylib         0x00007fff6f9ac22f _notify_fork_child + 179
4   libSystem.B.dylib              0x00007fff6d4aeb13 libSystem_atfork_child + 49
5   libsystem_c.dylib              0x00007fff6f79083e fork + 47
6   bootstrap-dbf53a170429e672     0x000000010abfd3c0 std::sys::unix::process::process_inner::_$LT$impl$u20$std..sys..unix..process..process_common..Command$GT$::spawn::h6f398f162ba4d9eb + 912 (process_unix.rs:45)
7   bootstrap-dbf53a170429e672     0x000000010ac1bb2c std::process::Command::output::h4459d64179fe8aba + 28 (result.rs:468)
8   bootstrap-dbf53a170429e672     0x000000010abf43e3 build_helper::output::h8dce802bd45620c3 + 131 (lib.rs:114)
9   bootstrap-dbf53a170429e672     0x000000010a821fc7 bootstrap::channel::GitInfo::new::hb7b5bdb1aa6f99de + 887 (channel.rs:57)
10  bootstrap-dbf53a170429e672     0x000000010a73d672 bootstrap::Build::new::h1206ee0cf8c7def9 + 818 (lib.rs:341)

...

First seen on the following PRs:

• Remove unstable macro_reexport #49982 (comment) (2018-04-30T13:42:15Z)
• Issue 49938: Reference tagged unions discr(iminant) as tag #50309 (comment) (2018-05-06T12:44:09Z)

[Mark-Simulacrum]

Since we've moved CI systems and seem to have never seen this after the first few times I'm going to claim this has been "fixed" and close it.