Interface to the pselect()
system call¶
This module defines a class PSelecter
which can be used to
call the system call pselect()
and which can also be used in a
with
statement to block given signals until
PSelecter.pselect()
is called.
AUTHORS:
- Jeroen Demeyer (2013-02-07): initial version (trac ticket #14079)
Waiting for subprocesses¶
One possible use is to wait with a timeout until any child process
exits, as opposed to os.wait()
which doesn’t have a timeout or
multiprocessing.Process.join()
which waits for one specific process.
Since SIGCHLD
is ignored by default, we first need to install a
signal handler for SIGCHLD
. It doesn’t matter what it does, as long
as the signal isn’t ignored:
sage: import signal
sage: def dummy_handler(sig, frame):
....: pass
....:
sage: _ = signal.signal(signal.SIGCHLD, dummy_handler)
We wait for a child created using the subprocess
module:
sage: from sage.ext.pselect import PSelecter
sage: from subprocess import *
sage: with PSelecter([signal.SIGCHLD]) as sel:
....: p = Popen(["sleep", "1"])
....: _ = sel.sleep()
....:
sage: p.poll() # p should be finished
0
Now using the multiprocessing
module:
sage: from sage.ext.pselect import PSelecter
sage: from multiprocessing import *
sage: import time
sage: with PSelecter([signal.SIGCHLD]) as sel:
....: p = Process(target=time.sleep, args=(1,))
....: p.start()
....: _ = sel.sleep()
....: p.is_alive() # p should be finished
....:
False
-
class
sage.ext.pselect.
PSelecter
¶ Bases:
object
This class gives an interface to the
pselect
system call.It can be used in a
with
statement to block given signals such that they can only occur during thepselect()
orsleep()
calls.As an example, we block the
SIGHUP
andSIGALRM
signals and then raise aSIGALRM
signal. The interrupt will only be seen during thesleep()
call:sage: from sage.ext.pselect import PSelecter sage: import signal, time sage: with PSelecter([signal.SIGHUP, signal.SIGALRM]) as sel: ....: os.kill(os.getpid(), signal.SIGALRM) ....: time.sleep(0.5) # Simply sleep, no interrupt detected ....: try: ....: _ = sel.sleep(1) # Interrupt seen here ....: except AlarmInterrupt: ....: print("Interrupt OK") ....: Interrupt OK
Warning
If
SIGCHLD
is blocked inside thewith
block, then you should not usePopen().wait()
orProcess().join()
because those might block, even if the process has actually exited. Use non-blocking alternatives such asPopen.poll()
ormultiprocessing.active_children()
instead.-
pselect
(rlist=[], wlist=[], xlist=[], timeout=None)¶ Wait until one of the given files is ready, or a signal has been received, or until
timeout
seconds have past.INPUT:
rlist
– (default:[]
) a list of files to wait for reading.wlist
– (default:[]
) a list of files to wait for writing.xlist
– (default:[]
) a list of files to wait for exceptions.timeout
– (default:None
) a timeout in seconds, whereNone
stands for no timeout.
OUTPUT: A 4-tuple
(rready, wready, xready, tmout)
where the first three are lists of file descriptors which are ready, that is a subset of(rlist, wlist, xlist)
. The fourth is a boolean which isTrue
if and only if the command timed out. Ifpselect
was interrupted by a signal, the output is([], [], [], False)
.See also
Use the
sleep()
method instead if you don’t care about file descriptors.EXAMPLES:
The file
/dev/null
should always be available for reading and writing:sage: from sage.ext.pselect import PSelecter sage: f = open(os.devnull, "r+") sage: sel = PSelecter() sage: sel.pselect(rlist=[f]) ([<open file '/dev/null', mode 'r+' at ...>], [], [], False) sage: sel.pselect(wlist=[f]) ([], [<open file '/dev/null', mode 'r+' at ...>], [], False)
A list of various files, all of them should be ready for reading. Also create a pipe, which should be ready for writing, but not reading (since nothing has been written):
sage: f = open(os.devnull, "r") sage: g = open('/usr/bin/python2.7', "r") sage: (pr, pw) = os.pipe() sage: r, w, x, t = PSelecter().pselect([f,g,pr,pw], [pw], [pr,pw]) sage: len(r), len(w), len(x), t (2, 1, 0, False)
Checking for exceptions on the pipe should simply time out:
sage: sel.pselect(xlist=[pr,pw], timeout=0.2) ([], [], [], True)
TESTS:
It is legal (but silly) to list the same file multiple times:
sage: r, w, x, t = PSelecter().pselect([f,g,f,f,g]) sage: len(r) 5
Invalid input:
sage: PSelecter().pselect([None]) Traceback (most recent call last): ... TypeError: an integer is required
Open a file and close it, but save the (invalid) file descriptor:
sage: f = open(os.devnull, "r") sage: n = f.fileno() sage: f.close() sage: PSelecter().pselect([n]) Traceback (most recent call last): ... IOError: ...
-
sleep
(timeout=None)¶ Wait until a signal has been received, or until
timeout
seconds have past.This is implemented as a special case of
pselect()
with empty lists of file descriptors.INPUT:
timeout
– (default:None
) a timeout in seconds, whereNone
stands for no timeout.
OUTPUT: A boolean which is
True
if the call timed out, False if it was interrupted.EXAMPLES:
A simple wait with timeout:
sage: from sage.ext.pselect import PSelecter sage: sel = PSelecter() sage: sel.sleep(timeout=0.1) True
0 or negative time-outs are allowed,
sleep
should then return immediately:sage: sel.sleep(timeout=0) True sage: sel.sleep(timeout=-123.45) True
-
-
sage.ext.pselect.
get_fileno
(f)¶ Return the file descriptor of
f
.INPUT:
f
– an object with a.fileno
method or an integer, which is a file descriptor.
OUTPUT: A C
long
representing the file descriptor.EXAMPLES:
sage: from sage.ext.pselect import get_fileno sage: get_fileno(open(os.devnull)) # random 5 sage: get_fileno(42) 42 sage: get_fileno(None) Traceback (most recent call last): ... TypeError: an integer is required sage: get_fileno(-1) Traceback (most recent call last): ... ValueError: Invalid file descriptor sage: get_fileno(2^30) Traceback (most recent call last): ... ValueError: Invalid file descriptor