如何等待孙进程（由于 SIG CHLD，`bash` retval 在 Perl 中变为 -1）

Question

我有一个在 cron 中运行的 Perl 脚本（下面的片段）来执行系统检查。我 fork 一个 child 作为超时，并使用 SIG{CHLD} 获取它。 Perl 对 Bash 脚本进行多次系统调用并检查它们的退出状态。一个 bash 脚本在大约 5% 的时间内没有错误地失败。 Bash 脚本以 0 存在并且 Perl 看到 $?作为 -1 和 $!作为 "No child processes"。

此 bash 脚本测试编译器许可证，并且在 Bash 脚本完成后留下英特尔 icc（下面的 ps 输出）。我认为 icc 僵尸完成，迫使 Perl 进入 SIG{CHLD} 处理程序，这会吹走 $?在我能够阅读它之前的状态。

Compile status -1; No child processes

#!/usr/bin/perl
use strict;
use POSIX ':sys_wait_h';

my $GLOBAL_TIMEOUT = 1200;

### Timer to notify if this program hangs
my $timer_pid;
$SIG{CHLD} = sub {
    local ($!, $?);
    while((my $pid = waitpid(-1, WNOHANG)) > 0)
    {
        if($pid == $timer_pid)
        {
            die "Timeout\n";
        }
    }
};

die "Unable to fork\n" unless(defined($timer_pid = fork));
if($timer_pid == 0)  # child
{
    sleep($GLOBAL_TIMEOUT);
    exit;
}
### End Timer

### Compile test
my @compile = `./compile_test.sh 2>&1`;
my $status = $?;
print "Compile status $status; $!\n";
if($status != 0)
{
    print "@compile\n";
}

END  # Timer cleanup
{
    if($timer_pid != 0)
    {
        $SIG{CHLD} = 'IGNORE';
        kill(15, $timer_pid);
    }
}

exit(0);

#!/bin/sh

cc compile_test.c
if [ $? -ne 0 ]; then
    echo "Cray compiler failure"
    exit 1
fi

module swap PrgEnv-cray PrgEnv-intel
cc compile_test.c
if [ $? -ne 0 ]; then
    echo "Intel compiler failure"
    exit 1
fi

wait
ps
exit 0

等待并没有真正等待，因为 cc 调用 icc，它创建了一个等待（或等待 PID）不会阻塞的僵尸 grandchild 进程。（等待 `pidof icc`，在本例中为 31589，给出 "not a child of this shell"）

user 31589     1  0 12:47 pts/15   00:00:00 icc

我只是不知道如何在 Bash 或 Perl 中解决这个问题。

谢谢，克里斯

Answer 1

这不是 alarm 的用例吗？扔掉你的 SIGCHLD 处理程序并说

local $? = -1;
eval {
    local $SIG{ALRM} = sub { die "Timeout\n" };
    alarm($GLOBAL_TIMEOUT);
    @compile = `./compile_test.sh 2>&1`;
    alarm(0);
};

my $status = $?;

相反。

Answer 2

我认为最快的解决方案是在 bash 脚本底部添加一两秒的休眠以等待僵尸 icc 完成。但这没有用。

如果我还没有 SIG ALRM（在实际程序中），我同意最好的选择是将整个事情包装在一个 eval 中。甚至认为这对于一个 500 行的程序来说会很丑陋。

没有本地 ($?)，每个 `system` 调用都会得到 $? = -1。 $？在这种情况下，我需要在 waitpid 之后，不幸的是在 sig 处理程序退出后设置为 -1。所以我觉得这行得通。 新行显示为###

my $timer_pid;
my $chld_status;    ###
$SIG{CHLD} = sub {
    local($!, $?);
    while((my $pid = waitpid(-1, WNOHANG)) > 0)
    {
        $chld_status = $?;    ###
        if($pid == $timer_pid)
        {
            die "Timeout\n";
        }
    }
};

...
my @compile = `./compile_test.sh 2>&1`;
my $status = ($? == -1) ? $chld_status : $?;    ###
...

Answer 3

我们有一个类似的问题，这是我们的解决方案：将 write-side 文件描述符泄漏到孙子中并从中读取（），这将阻塞直到它退出。

另请参阅：wait for children and grand-children

use Fcntl;

# OCF scripts invoked by Pacemaker will be killed by Pacemaker with
# a SIGKILL if the script exceeds the configured resource timeout. In
# addition to killing the script, Pacemaker also kills all of the children
# invoked by that script. Because it is a kill, the scripts cannot trap
# the signal and clean up; because all of the children are killed as well,
# we cannot simply fork and have the parent wait on the child. In order
# to work around that, we need the child not to have a parent proccess
# of the OCF script---and the only way to do that is to grandchild the
# process. However, we still want the parent to wait for the grandchild
# process to exit so that the OCF script exits when the grandchild is
# done and not before. This is done by leaking the write file descriptor
# from pipe() into the grandchild and then the parent reads the read file
# descriptor, thus blocking until it gets IO or the grandchild exits. Since
# the file descriptor is never written to by the grandchild, the parent
# blocks until the child exits.
sub grandchild_wait_exit
{
    # We use "our" instead of "my" for the write side of the pipe. If
    # we did not, then when the sub exits and $w goes out of scope,
    # the file descriptor will close and the parent will exit.
    pipe(my $r, our $w);

    # Enable leaking the file descriptor into the children
    my $flags = fcntl($w, F_GETFD, 0) or warn $!;
    fcntl($w, F_SETFD, $flags & (~FD_CLOEXEC)) or die "Can't set flags: $!\n";

    # Fork the child
    my $child = fork();
    if ($child) {
        # We are the parent, waitpid for the child and
        # then read to wait for the grandchild.
        close($w);
        waitpid($child, 0);
        <$r>;
        exit;
    }

    # Otherwise we are the child, so close the read side of the pipe.
    close($r);

    # Fork a grandchild, exit the child.
    if (fork()) {
        exit;
    }

    # Turn off leaking of the file descriptor in the grandchild so
    # that no other process can write to the open file descriptor
    # that would prematurely exit the parent.
    $flags = fcntl($w, F_GETFD, 0) or warn $!;
    fcntl($w, F_SETFD, $flags | FD_CLOEXEC) or die "Can't set flags: $!\n";
}

grandchild_wait_exit();

sleep 1;
print getppid() . "\n";
print "$$: gc\n";
sleep 30;
exit;

如何等待孙进程（由于 SIG CHLD，`bash` retval 在 Perl 中变为 -1）

How to wait for grandchild process (`bash` retval becomes -1 in Perl due to SIG CHLD)

bash

perl

background-process

wait

child-process