I am trying to write a function to (wall-clock) timelimit a command's execution time but damned if I cannot eliminate all of the races.
Here is my current iteration of the function:
1 timed_run() {
2 local SLEEP_TIME=$1
3 shift
4
5 set +o monitor
6
7 # start command running
8 # NOTE: if you put the & after the bash -c "" the & will background
9 # setsid which causes a race trying to kill the PGID that setsid tries
10 # to create
11 setsid bash -c "$@" &
12 local child_pid=$!
13 # wait for setsid to have done it's work
14 while ! kill -0 -$child_pid 2>/dev/null; do
15 sleep 1
16 done
17
18 #
19 # start the watchdog
20 # - use setsid to put all of the children of the watchdog into their
21 # own process group so that we can use kill -<pgid> to kill them
22 # all off
27 setsid bash -c "sleep $SLEEP_TIME
28 echo \"$1 was killed due to a $SLEEP_TIME second timeout expiry\"
29 kill -TERM -$child_pid 2>/dev/null
30 sleep 5
41 kill -KILL -$child_pid 2>/dev/null" &
42 local dog_pid=$!
43 # wait for setsid to have done it's work
44 while ! kill -0 -$dog_pid 2>/dev/null; do
45 sleep 1
46 done
47
48 # waiting on the command will end when either it completes or the
49 # watchdog kills it
50 wait $child_pid
51
52 # status of the child command will be set to 143 if the process had
53 # to be killed due to timeout
54 child_status=${PIPESTATUS[0]}
55
56 # kill off the watchdog
57 kill -TERM -$dog_pid
58
59 # return the command's status back to the caller
60 return $child_status
61}
Some notes:
* the setsids are needed to allow kill to kill off a whole group
of processes
* set +o monitor is to disable job control notification messages
(i.e. Killed, Done, etc.)
* the kill -0 calls are needed to ensure that setsid has finished
it's work (i.e. created a new program group) and that a
subsequent kill ... -<pgid> will not be called before the setsid
is done
All of that leaves one race still. The process (group) that setsid
creates could be done (and gone) before the kill -0 is called. But if I
eliminate the kill -0 then I can't be sure that the setsid has created
the process group before calling the kill on it to kill off the
processes.
I have tried an alternate solution where I can be sure that setsid has
done it's work:
1 timed_run() {
2 local SLEEP_TIME=$1
3 shift
4
5 set +o monitor
6
7 # start command running
8 # NOTE: if you put the & after the bash -c "" the & will background
9 # setsid which causes a race trying to kill the PGID that setsid tries
10 # to create
11 setsid bash -c "($@) &"
12 local child_pid=???
17
18 #
19 # start the watchdog
20 # - use setsid to put all of the children of the watchdog into their
21 # own process group so that we can use kill -<pgid> to kill them
22 # all off
27 setsid bash -c "(sleep $SLEEP_TIME
28 echo \"$1 was killed due to a $SLEEP_TIME second timeout expiry\"
29 kill -TERM -$child_pid 2>/dev/null
30 sleep 5
41 kill -KILL -$child_pid 2>/dev/null) &"
42 local dog_pid=???
47
48 # waiting on the command will end when either it completes or the
49 # watchdog kills it
50 wait $child_pid
51
52 # status of the child command will be set to 143 if the process had
53 # to be killed due to timeout
54 child_status=${PIPESTATUS[0]}
55
56 # kill off the watchdog
57 kill -TERM -$dog_pid
58
59 # return the command's status back to the caller
60 return $child_status
61}
Notice that I put the & *inside* the bash commands so that the bash
command is backgrounded and not setsid (as in the previous iteration).
That allows me to get rid of the kill -0 races on lines 14 and 44 but I
lose the ability to learn the pgids that setsid creates which I need to
kill on lines 29/41 or 57.
Now, the pgids that I want to know are the pids of the two bash commands
that the setsids create. So I could almost do:
1 timed_run() {
2 local SLEEP_TIME=$1
3 shift
4
5 set +o monitor
6
7 # start command running
8 # NOTE: if you put the & after the bash -c "" the & will background
9 # setsid which causes a race trying to kill the PGID that setsid tries
10 # to create
11 child_pid=$(setsid bash -c "echo \$\$; ($@) &"
17
18 #
19 # start the watchdog
20 # - use setsid to put all of the children of the watchdog into their
21 # own process group so that we can use kill -<pgid> to kill them
22 # all off
27 dog_pid=$(setsid bash -c "(sleep $SLEEP_TIME
28 echo \"$1 was killed due to a $SLEEP_TIME second timeout expiry\"
29 kill -TERM -$child_pid 2>/dev/null
30 sleep 5
41 kill -KILL -$child_pid 2>/dev/null) &")
47
48 # waiting on the command will end when either it completes or the
49 # watchdog kills it
50 wait $child_pid
51
52 # status of the child command will be set to 143 if the process had
53 # to be killed due to timeout
54 child_status=${PIPESTATUS[0]}
55
56 # kill off the watchdog
57 kill -TERM -$dog_pid
58
59 # return the command's status back to the caller
60 return $child_status
61}
Except that I want whatever stdout/stderr the command that is in $@ on
line 11 produces to stay on stdout/stderr so that timed_run()s caller
can collect it. Probably I can put the echo on line 28 to stderr and
that will solve the problem for that one. I am still left with the
first issue.
Any ideas?
Thanx,
b.
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