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Processes started under "stock" tmux and screen can not access the
Mac OS X pasteboard.
Apple publishes their modified source for screen.
Someone adapted Apple's 10.6 screen changes to tmux.
Basically, it involves replacing the call to daemon() with a
call to the undocumented function _vprocmgr_detach_from_console().
Note: 'upstream' screen vs. Apple's 10.6.6 screen
The call to _vprocmgr_detach_from_console was a pure
addition, not a replacement for daemon(3) like the above
patch does in tmux.
When I patched recent tmux HEAD (via
git:// master), the resulting
under-tmux shells could access the pasteboard (e.g. pbpaste).
However, it resulted in many warning messages from libevent.
[warn] event_del: event has no event_base set.
I see eleven of these messages just doing
./tmux new 'echo something'
Additionally, if I start a shell (instead of just an "echo"), I see
the message after every key press, until I press ESC. Then the
message per key press disappears and I only see ten of the messages
after tmux exiting the shell (and thus tmux).
This appears to be a bad interaction between not forking (via
daemon(3)) and the way libevent is used (maybe just libevent 2.0).
What I want to do here is to make a minimal test rig for
daemon()/_vprocmgr_detach_from_console() and libevent to see what
can be done to ameliorate the problem.
Findings from testing on 10.6.6:
Calling _vprocmgr_detach_from_console() by itself does not
impede a child from successfully using pbpaste. The result from
getpid() is the same before and after the call, so whatever it
does, it does not fork internally.
Calling daemon() does prevent a child from successfully using pbpaste.
A hand-rolled daemon() (fork, setsid, open(/dev/null), dup2 for
0-2) does not impede a child from successfully using pbpaste.
I was able to dig up daemon() and the vproc functions. Here are the
sources for 10.6.6:
The annoying thing that the system daemon(3) is doing is that it
moves to the root "bootstrap namespace". The bootstrap service is
what programs use to get access to other services. Being in the root
namespace prevents the process from accessing the pasteboard service
(which is only in a per-user namespace in 10.5 and beyond).
After some more subtle testing, I found that (when using
a hand-rolled daemon()) the call to _vprocmgr_detach_from_console()
lets the children of tmux (the whole per-session bootstrap
namespace?) maintain access to the pasteboard even after the session
initiator has exited. For example: during a GUI login, SSH back to
localhost, start a new tmux server there, detach from it. Attach
from elsewhere and pbpaste still works. Exit from the SSH connection
(the one that started the tmux process). If we used
_vprocmgr_detach_from_console, then the children of tmux can still
access the pasteboard. If we did not call it, then they lose access
to the pasteboard (the namespace was revoked?).
Possible Resolutions
Use daemon() from compat/daemon.c and _vprocmgr_detach_from_console()
check for 'broken' daemon would require Darwin-specific stuff
Not even sure how best to do that. Could check whether
(after a call to daemon()) the output from `launchctl
bslist` contains "" (or do the
equivalent in straight C, if it does not use even more
private APIs).
assume Darwin daemon(3) is broken for our purposes
use compat/daemon
would it be deterministically used instead of libSystem daemon()?
could #define daemon compat_daemon for compat/daemon.c and server.c
still need call to _vprocmgr_detach_from_console()
make osdep_daemon() and call it after (compat) daemon()
Use system daemon(3) and find a way to re-attach to the/a user
Per TN2083, launchd creates non-GUI per-session namespaces for
accepted SSH connections based on the SSH launchd plist having
a SessionCreate key. Looking at the launchd code, it seems that
it does this via CreateSession(0,0) from the Security Framework.
launchd_SessionCreate in
Or maybe _vprocmgr_move_subset_to_user(), like the 10.5(?)
screen code used?
Current Thinking
Adding calls to undocumented (private?) Apple APIs in tmux seems
like a fragile solution. Each release of Mac OS X might break
pre-built tmux binaries. The risks of "hard" breaks (tmux errors
out on startup) could be eliminated by using dlsym(3) to access
the required function(s). Even with that precaution, Apple may
break the functions in incompatible ways (they added a parameter
to _vprocmgr_move_subset_to_user() in 10.6; the parameter seems to
be unused, so we may be able to "get away" with calling it without
the extra parameter, but that depends entirely on the calling
Since tmux's call to daemon(3) effectively isolates it from the
per-user bootstrap namespace, there is no way to "reconnect" tmux
itself without patching it. But, we can arrange for (some of)
tmux's children to be reconnected. Any child can make its own call
to _vprocmgr_move_subset_to_user() to reconnect itself (and its
future(?) children).
I have written a wrapper program that calls the private function
and then execs into the command specified in its arguments. I have
configured tmux's default-command to be "wrapper -l shell" (-l
makes the shell's argv[0] be "-shell"). This has successfully let
me run pbpaste(1) (and other commands that access the pasteboard)
from inside tmux sessions. Not all children of tmux are covered by
this workaround, but it seems that it is a "90% solution".
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