linux perl - interrupted system calls not restarted?

flipper has asked for the wisdom of the Perl Monks concerning the following question:

Hi monks,

I've spent some time tracking down a strange problem in a daemon script - ultimately I've no-one to blame but myself, as I wasn't checking my return codes and/or $!, but I've found some surprising behaviour... Much reduced code:

#!/usr/bin/perl -w
use strict;
use IO::Socket::INET;
my $server =IO::Socket::INET->new(LocalPort => 4848, Listen=> 1,ReuseA
+ddr => 1) or die "listen: $!";
my $client;
USER: while($client= $server->accept()){
        if (my $pid = fork()){
                #parent
                close $client;
                $SIG{CHLD} = sub {};
        }else{
                print $client "hello, world!\n";
                select(undef,undef,undef,2);
                exit;
        }
}
warn "fell out of loop - $!";
[download]

The sleep in the child process causes SIGCHLD to be delivered to the parent when it is in blocking in accept(), and sure enough I get fell out of loop - Interrupted system call at /tmp/z.pl line 17.

I understand from chapter 16 of the Camel Book that this could happen with old, horrible Unices, but I'm surprised it happens with This is perl, v5.10.0 built for i486-linux-gnu-thread-multi on Debian 5.0.3

Can anyone shed some light on this?? An strace looks like it's going to work, but doesn't...

bind(3, {sa_family=AF_INET, sin_port=htons(4848), sin_addr=inet_addr("
+0.0.0.0")}, 16) = 0
listen(3, 1)                            = 0
accept(3, {sa_family=AF_INET, sin_port=htons(23694), sin_addr=inet_add
+r("127.0.0.1")}, [16]) = 4
ioctl(4, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbffa26c8) = -1 EINVAL (Inval
+id argument)
_llseek(4, 0, 0xbffa2710, SEEK_CUR)     = -1 ESPIPE (Illegal seek)
ioctl(4, SNDCTL_TMR_TIMEBASE or TCGETS, 0xbffa26c8) = -1 EINVAL (Inval
+id argument)
_llseek(4, 0, 0xbffa2710, SEEK_CUR)     = -1 ESPIPE (Illegal seek)
fcntl64(4, F_SETFD, FD_CLOEXEC)         = 0
clone(child_stack=0, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIG
+CHLD, child_tidptr=0xb7d9f908) = 5206
close(4)                                = 0
rt_sigprocmask(SIG_BLOCK, [CHLD], [], 8) = 0
rt_sigaction(SIGCHLD, {0x809a270, [], 0}, {SIG_DFL}, 8) = 0
rt_sigprocmask(SIG_SETMASK, [], NULL, 8) = 0
accept(3, 0xbffa28c8, [4096])           = ? ERESTARTSYS (To be restart
+ed)
--- SIGCHLD (Child exited) @ 0 (0) ---
sigreturn()                             = ? (mask now [])
rt_sigprocmask(SIG_BLOCK, [CHLD], NULL, 8) = 0
rt_sigaction(SIGCHLD, NULL, {0x809a270, [], 0}, 8) = 0
rt_sigprocmask(SIG_UNBLOCK, [CHLD], NULL, 8) = 0
open("/usr/share/locale/locale.alias", O_RDONLY) = 4
fstat64(4, {st_mode=S_IFREG|0644, st_size=2586, ...}) = 0
mmap2(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1,
+ 0) = 0xb7f7a000
read(4, "# Locale name alias data base.\n# "..., 4096) = 2586
read(4, ""..., 4096)                    = 0
close(4)                                = 0
munmap(0xb7f7a000, 4096)                = 0
open("/usr/share/locale/en_GB.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = 
+-1 ENOENT (No such file or directory)
open("/usr/share/locale/en_GB.utf8/LC_MESSAGES/libc.mo", O_RDONLY) = -
+1 ENOENT (No such file or directory)
open("/usr/share/locale/en_GB/LC_MESSAGES/libc.mo", O_RDONLY) = 4
fstat64(4, {st_mode=S_IFREG|0644, st_size=1474, ...}) = 0
mmap2(NULL, 1474, PROT_READ, MAP_PRIVATE, 4, 0) = 0xb7f7a000
close(4)                                = 0
open("/usr/share/locale/en.UTF-8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 
+ENOENT (No such file or directory)
open("/usr/share/locale/en.utf8/LC_MESSAGES/libc.mo", O_RDONLY) = -1 E
+NOENT (No such file or directory)
open("/usr/share/locale/en/LC_MESSAGES/libc.mo", O_RDONLY) = -1 ENOENT
+ (No such file or directory)
write(2, "fell out of loop - Interrupted sy"..., 65) = 65
close(3)                                = 0
exit_group(0)                           = ?
[download]

Thanks!

Comment on linux perl - interrupted system calls not restarted? Select or Download Code

Replies are listed 'Best First'.
Re: linux perl - interrupted system calls not restarted? by jakobi (Pilgrim) on Oct 22, 2009 at 13:29 UTC
consider $SIG{CHLD} = 'IGNORE', which indeed avoids both ending the loop and zombies. Search for reaper and IGNORE. As you said you need IO::Socket below: Possibly IO::Socket gets irritated by some %SIG handlers (couldn't find anything on the quick in the source or docs though). 2 possible workarounds I currently see: perlipc: search for sigaction and SA_RESTART then read the section on EINTR directly below IGNORE as handler and place the 'rc' into a file in the child, then in the mainloop regularly waitpid over the children and check for possible rc files from children died with rc!=0. Missing 'rc' and missing child is indicating a more severe error. Seems a known issue: SOLVED: Re: TCP Client-Server: Server exits though it shouldn't loops forever, and retries accept in case of EINTR. But the loop in the reaper looks like it can stop early; and other syscalls may mess up the detection of EINTR, which seems quite far away from the interrupted syscall. Maybe just retry accept() upto n times in a row if $client is false. Note that's there's a small race of the parent running w/o SIG handler, ~~and later children possibly running with it (if it's inheritable by fork; CHECKED: doesn't seem to be inherited)~~ Please also post your updated code when done, thanx, Peter	[reply]
Re^2: linux perl - interrupted system calls not restarted? by flipper (Beadle) on Oct 22, 2009 at 14:27 UTC
Indeed - `perldoc perlipc` suggests that the info in my camel book is out of date, guess I should update! Thanks for your help!	[reply] [d/l]
Re^2: linux perl - interrupted system calls not restarted? by flipper (Beadle) on Oct 22, 2009 at 13:39 UTC
I need to use SIG{CHLD} in this case - the parent accepts connections, if there is no child running, it starts a child to service the new connection. If there is an existing child, it tells the new client it can't connect as it is in use from ip:port. The child can exit nonzero - when this happens, the parent needs to exit immediately (not at the next `accept()`. So the parent needs to detect new connections, and a child exiting. The issue I'm concerned with is more general - If I'm using signals anywhere (eg `sleep()`), do I need to wrap every system call in a loop to retry it??	[reply] [d/l] [select]
Re^3: linux perl - interrupted system calls not restarted? by ikegami (Patriarch) on Oct 22, 2009 at 14:37 UTC
I need to use SIG{CHLD} in this case Then you need to check for EINTR `use Errno qw( EINTR ); for (;;) { my $client = $server->accept(); if (!$client) { next if $! == EINTR; die("Can't accept: $!\n"); } ... }` [download] If I'm using signals anywhere (eg sleep()), do I need to wrap every system call in a loop to retry it?? Well, those that are interruptable, yes. That includes `sysread`. But `sysread` should already be in a loop since it's not guaranteed to return as many bytes as you requested.	[reply] [d/l] [select]


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