remote: Fix a stuck remote call pipeline causing testing to hang

remote: Fix a stuck remote call pipeline causing testing to hang

Fix a stuck remote call pipeline comprised of multiple processes causing
testing to hang and requiring a manual intervention to either terminate
or proceed, like below (here with the GCC `c' testsuite invoked with
`execute.exp=postmod-1.c' for 8 compilation and 8 execution tests on a
remote QEMU target run in the system emulation mode):

PASS: gcc.c-torture/execute/postmod-1.c   -O0  (test for excess errors)
Executing on remote-localhost: .../gcc/testsuite/gcc/postmod-1.exe    (timeout = 15)
spawn [open ...]
WARNING: program timed out
got a INT signal, interrupted by user

		=== gcc Summary ===

# of expected passes		1

by not killing the pending force-kills in `close_wait_program' and also
by setting the channel associated with the pipeline to the nonblocking
mode when it is about to be closed afterwards.

The situation here is as follows.  A connection to the remote target
board is requested by `rsh_exec' with input redirection requested from
`/dev/null'.  The request is handled by `local_exec' and the redirection
causes a Tcl command pipeline channel to be opened.  The list of PIDs of
the processes comprising the pipeline is determined and then the channel
is assigned an Expect spawn ID.  The spawn ID is then waited for output
produced by the remote target (here accessed with SSH) and, ultimately,
completion marked by the end-of-file condition.

As SSH gets stuck and does not complete the timeout eventually fires and
a kill sequence is initiated, by calling `close_wait_program' with the
list of PIDs previously obtained to kill given as one of the procedure's
arguments.  Seeing the list of PIDs rather than -1 `close_wait_program'
issues SIGINT to all the requested processes right away and schedules a
delayed sequence called "force-kills" to them, which sends SIGTERM and
then, after a further delay, SIGKILL.

Now `close_wait_program' calls `close' on the spawn ID associated with
the pipeline, but this call doesn't affect the pipeline as its input has
been redirected from `/dev/null'.  As the next step `wait' is called on
the same spawn ID and returns successfully right away with a result like
{0 exp8 0 0} in `wres', where no PID is indicated, consistently with the
null PID result of the original `spawn' command that assigned the spawn
ID (`exp8' here) to the pipeline.  The return from the `wait' command
causes code to be executed for the pending force-kills to be killed.

At this point the process situation is like below:

  PID TTY      STAT   TIME COMMAND
 6908 pts/3    Sl     0:00 expect -- .../share/dejagnu/runtest.exp --tool gcc --target_board remote-localhost execute.exp=postmod-1.c
 6976 pts/3    S      0:00  \_ ssh -p 2222 -l macro localhost sh -c '.../gcc/testsuite/gcc/postmod-1.exe ; echo XYZ${?}ZYX'
 6977 pts/3    Z      0:00  \_ [cat] <defunct>
 6991 pts/3    Z      0:00  \_ [sh] <defunct>

so `cat' and `sh' have already terminated, the former presumably due to
SIGINT sent previously and the latter having been the force-kills just
killed, and only await being wait(2)ed for, however `ssh' is still live
and in the interruptible sleep, presumably awaiting communication with
the remote end.

Since there is nothing else to do for `close_wait_program' it returns
success to `local_exec', which then calls `close' on the pipeline to
clean up after it.  But that in turn causes wait(2) to be called on the
individual PIDs comprising the pipeline and when the PID associated with
`ssh' the call hangs indefinitely preventing the whole testsuite from
proceeding.

A similar situation triggers with GDB testing where a Tcl command
pipeline channel is opened in `remote_spawn' instead, and then closed,
after `close_wait_program' has been called, in `standard_close'.

So the solution to the problem is twofold.  First pending force-kills
are not killed after `wait' if there are more than one PID in the list
passed to `close_wait_program'.  This follows the observation that if
there was only one PID on the list, then the process must have been
created directly by `spawn' rather than by assigning a spawn ID to a
pipeline and the return from `wait' would mean the process associated
with the PID must have already been cleaned up after, so it is only when
there are more there is a possibility any may have been left behind
live.

Second if a pipeline has been used, then the channel associated with the
pipeline is set to the nonblocking mode in case any of the processes
that may have left live is stuck in the noninterruptible sleep (aka D)
state.  Such a process would necessarily ignore even SIGKILL so long as
it remains in that state and would cause wait(2) called by `close' to
hang possibly indefinitely, and we want the testsuite to proceed rather
than hang even in bad circumstances.

Finally it appears to be safe to leave pending force-kills to complete
their job after `wait' has been called in `close_wait_program', because
based on the observation made here the command does not actually call
wait(2) if issued on a spawn ID associated with a pipeline created by
`open' rather than a process created by `spawn'.  Instead the PIDs from
a pipeline are supposed to be cleaned up after by calling wait(2) from
the `close' command call made on the pipeline channel.  If on the other
hand the channel is set to the nonblocking mode before `close', then
even that command does not call wait(2) on the associated PIDs.

Therefore the PIDs on the list passed are not subject to PID reuse and
the force-kills won't accidentally kill an unrelated process, as a PID
cannot be allocated by the kernel for a new process until any previous
process's status has been consumed from its PID by wait(2).  And then
PIDs of any children that have actually terminated one way or another
are wait(2)ed for by Tcl automatically in the event loop, so no mess is
left behind.

	* lib/remote.exp (close_wait_program): Only kill the pending
	force-kills if the PID list has a single entry.
	(local_exec): Set the channel about to be closed to the
	nonblocking mode if we didn't see an EOF.
	(standard_close): Likewise, unconditionally.

Signed-off-by: Maciej W. Rozycki <macro@wdc.com>