Bug 246385

Created attachment 214394 [details]
reduced test case, see description for how to compile

fish-shell is observing a hang under FreeBSD CURRENT which I believe to be a regression in FreeBSD signal handling, relative to 12.1.

Originally reported as https://github.com/fish-shell/fish-shell/issues/6919

High level: if a process forks and the child exits before fork is complete in the parent, then SIGCHLD will not be delivered.

Details: fork() will block signals using sigfastblock. If the child dies before signals are unblocked, the SIGCHLD will be marked as pending via the sigfastblock word. In this case fork() will issue a syscall to `sigfastblock(SIGFASTBLOCK_UNBLOCK)`; however SIGCHLD will NOT be delivered.

Reduced test case attached as `demo.c`. This test case uses a loop which creates a child and waits for SIGCHLD to be delivered via the self-pipe trick. It counts every 256 iterations.

To reproduce:

    clang demo.c ; ./a.out # this does not hang
    
    clang -lpthread demo.c ; ./a.out # this hangs on FreeBSD CURRENT only


ktrace output:

Good iteration (SIGCHLD generated while signals NOT blocked):
 27181 a.out    CALL  fork
 27181 a.out    RET   fork 27581/0x6bbd
 27181 a.out    CALL  read(0x3,0x7fffffffe8e0,0x200)
 27181 a.out    RET   read RESTART
 27181 a.out    PSIG  SIGCHLD caught handler=0x800258dd0 mask=0x0 code=CLD_EXITED
 27181 a.out    CALL  sigprocmask(SIG_SETMASK,0x7fffffffe24c,0)
 27181 a.out    RET   sigprocmask 0
 27181 a.out    CALL  write(0x4,0x7fffffffde6b,0x1)
 27181 a.out    GIO   fd 4 wrote 1 byte
 27181 a.out    RET   write 1
 27181 a.out    CALL  sigreturn(0x7fffffffde80)
 27181 a.out    RET   sigreturn JUSTRETURN
 27181 a.out    CALL  read(0x3,0x7fffffffe8e0,0x200)
 27181 a.out    GIO   fd 3 read 1 byte


Hanging iteration (SIGCHLD generated while signals ARE blocked):

 27181 a.out    RET   read 1
 27181 a.out    CALL  wait4(0x6bbd,0x7fffffffe8d4,0x6<WUNTRACED|WCONTINUED>,0)
 27181 a.out    RET   wait4 27581/0x6bbd
 27181 a.out    CALL  fork
 27181 a.out    RET   fork 27582/0x6bbe
 27181 a.out    CALL  sigfastblock(0x2,0)
 27181 a.out    RET   sigfastblock 0
 27181 a.out    CALL  read(0x3,0x7fffffffe8e0,0x200)

Note the call to sigfastblock(0x2) (pending signal) but PSIG  SIGCHLD is not generated.