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

[Fernando Vezzosi]

On Sat, Oct 08, 2011 at 02:45:27PM +0200, Axel Beckert wrote:
> Hi,
> 
> I'm currently preparing an upload of the current screen HEAD to Debian
> (either Unstable or Experimental, not yet decided) and I noticed that,
> if I have a screen 4.0.3 (as currently in Debian Stable/Unstable) is
> running, I can't reattach to that with the new screen 4.1.0 snapshot,
> it just hangs until I kill it with "kill -TERM" as Ctrl-C does not
> help.
> 
> As for me the common way to dist-upgrade Debian or Ubuntu boxes
> (especially remote servers) is to run the whole process inside a
> screen, this is a quite critical issue. So I wonder:
> 
> 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 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:

  % ls -l =screen
  -rwxr-sr-x 1 root utmp 414432 Oct  9 05:02 /usr/bin/screen*

strace() will defuse setuid/setgid so it won't work as expected, but we can
always try as root:

  # aptitude install screen=4.0.3-14
  # screen -dmS test
  # strace -o /tmp/log_4.0.3 screen -r test
  [... works, I detach ...]
  # aptitude install screen=4.1.0~20110819git450e8f3-1
  # strace -o /tmp/log_4.1.0 screen -r test
  [... hangs, gets killed ...]

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.

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

-- 
  Fernando Vezzosi    perl -E'$_="Pop Corn",s/\b\S(?{$0=$&^_^L})/$0/g,say'
         qw(MDAx MTAw MDEw MDEx MDAw _5 MTEw _6 _1 _5 _5 _4 _2 _2 _6
         MTEx _1 _5 _5 _4 _5 _2 _6 _1 _1 _2 _2 _3 _5 _5 _6 _1 _1 _2
         _5 _4 _3 _5 _2 _5 _1 _2 _3 _4 _5 MA==)