Re: [screen-devel] Current screen HEAD can't reattach to a detached screen 4.0.3

[Axel Beckert]

Hi Fernando,

On Thu, Nov 17, 2011 at 04:12:13PM +0100, Fernando Vezzosi wrote:
> > Is this issue known? Not circumventable? An unexpected bug? Anyone has
> > an idea where this comes from? Or does it not happen at all with vanilla
> > screen versions?
> 
> I noticed the message while upgrading screen version sid → experimental,
> thanks for making it loud.

I'm glad that this note in the Debian packaged works and gets read. :-)

> > I tried to strace "screen -r" to find any differences, but
> > interestingly it behaves differently when being traced (bails out with
> > claiming /var/run/screen has wrong permissions or so) than when not.
> 
> This is because screen is setgid utmp:

Ah, forgot that.

> The difference I see at the end is:
> 
>   4.0.3 → write(4, "\0gsm\2\0\0\0/dev/pts/46\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
> 12336) = 12336
>   4.1.0 → write(4, "\2gsm\2\0\0\0/dev/pts/46\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
> 12376) = 12376
> 
> The first byte was \0, now it is \2.
> 
> Is tracing the background process perhaps more interesting?
> 
>   # aptitude install screen=4.0.3-14
>   # strace -t -o /tmp/server_log_4.0.3 -p 26322
>   [... works, I detach, then ^C this strace ...]
>   # aptitude install screen=4.1.0~20110819git450e8f3-1
>   # strace -t -o /tmp/server_log_4.1.0 -p 26322 
>   [... hangs, I kill client, then ^C this strace ...]
> 
> Differences here:
> 
> 4.0.3:
>   0.000083 read(3, "\0gsm\2\0\0\0/dev/pts/46\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
> 12336) = 12336
>   0.000125 close(3)                  = 0
>   0.000075 geteuid()                 = 0
>   0.000067 getegid()                 = 43
>   0.000067 getgid()                  = 0
>   0.000068 setregid(43, 0)           = 0
>   0.000077 open("/var/run/screen/S-root/26322.test", O_RDONLY|O_NONBLOCK) = 3
>   0.000106 geteuid()                 = 0
>   0.000068 getegid()                 = 0
>   0.000067 getgid()                  = 43
>   0.000069 setregid(0, 43)           = 0
>   0.000074 kill(26948, SIG_0)        = 0
>   0.000074 geteuid()                 = 0
>   0.000067 getegid()                 = 43
>   0.000067 getgid()                  = 0
>   0.000068 setregid(43, 0)           = 0
>   0.000072 open("/dev/pts/46", O_RDWR|O_NONBLOCK) = 5
>   [... goes on and on ...]
> 
> 4.1.0:
>   0.000087 read(3, "\2gsm\2\0\0\0/dev/pts/46\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 
> 12336) = 12336
>   0.000123 close(3)                  = 0
>   0.000079 geteuid()                 = 0
>   0.000073 getegid()                 = 43
>   0.000072 getgid()                  = 0
>   0.000074 setregid(43, 0)           = 0
>   0.000081 open("/var/run/screen/S-root/26322.test", O_RDONLY|O_NONBLOCK) = 3
>   0.000112 geteuid()                 = 0
>   0.000073 getegid()                 = 0
>   0.000072 getgid()                  = 43
>   0.000074 setregid(0, 43)           = 0
>   0.000089 select(1024, [3 4], [], NULL, NULL) = ? ERESTARTNOHAND (To be 
> restarted)
>   5.869203 --- SIGINT (Interrupt) @ 0 (0) ---
> 
> So yeah, the background screen is waiting for more traffic from the new
> version, which it doesn't get.

This sounds fixable, good.

> The screen.git repo has no tags.  The experimental package has a commit SHA1
> in its version string, which helps.  How do I find out exactly which commit
> corresponds to "Screen version 4.00.03jw4 (FAU) 2-May-06" ?OB

Not sure. But as the source tar ball of screen 4.0.3 got uploaded in
March 2007 [1], I'd start with a commit before that date.

  [1] http://packages.qa.debian.org/s/screen/news/20070305T083205Z.html

> I'd like to bisect the issue, and knowing where to start would be a
> good start :)

Thanks for that! This information would help a lot, but I must admit,
I haven't found the time to even inspect the commits of which I think
that they could cause such an issue.

                Kind regards, Axel
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