ash: [JOBS] Fix dowait signal race

ash: [JOBS] Fix dowait signal race

Upstream commit:

    Date: Sun, 22 Feb 2009 18:10:01 +0800
    [JOBS] Fix dowait signal race

    This test program by Alexey Gladkov can cause dash to enter an
    infinite loop in waitcmd.

    #!/bin/dash
    trap "echo TRAP" USR1
    stub() {
        echo ">>> STUB $1" >&2
        sleep $1
        echo "<<< STUB $1" >&2
        kill -USR1 $$
    }
    stub 3 &
    stub 2 &
    until { echo "###"; wait; } do
    echo "*** $?"
    done

    The problem is that if we get a signal after the wait3 system
    call has returned but before we get to INTON in dowait, then
    we can jump back up to the top and lose the exit status.  So
    if we then wait for the job that has just exited, then it'll
    stay there forever.

    I made the original change that caused this bug to fix pretty
    much the same bug but in the opposite direction.  That is, if
    we get a signal after we enter wait3 but before we hit the kernel
    then it too can cause the wait to go on forever (assuming the
    child doesn't exit).

    In fact this is pretty much exactly the scenario that you'll
    find in glibc's documentation on pause().  The solution is given
    there too, in the form of sigsuspend, which is the only way to
    do the check and wait atomically.

    So this patch fixes Alexey's race without reintroducing the old
    bug by converting the blocking wait3 to a sigsuspend.

    In order to do this we need to set a signal handler for SIGCHLD,
    so the code has been modified to always do that.

    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
I failed to reproduce the bug (it requires precise timing), but it seems real.

function                                             old     new   delta
dowait                                               284     463    +179
setsignal                                            301     326     +25
signal_handler                                        59      76     +17
ash_main                                            1481    1487      +6
localcmd                                             350     348      -2
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 4/1 up/down: 227/-2)            Total: 225 bytes

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>