[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

>  To support 3.2 and 2.7:
>  (1) select.error must be caught as in the original patch,
>  (2) e.args[0] must be used - select.error doesn't have 'errno'
> attribute.
> Should I prepare the patch for 3.2 and 2.7?

It shouldn't be necessary, I've just committed fixes.
Thanks for noticing!

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[issue7978] SocketServer doesn't handle syscall interruption

[Roundup Robot]

Roundup Robot  added the comment:

New changeset f8e7fcd581ff by Antoine Pitrou in branch '3.2':
Fix the patch for issue #7978: select() raises select.error before 3.3, not 
OSError.
http://hg.python.org/cpython/rev/f8e7fcd581ff

New changeset 4298d6e79ecb by Antoine Pitrou in branch '2.7':
Fix the patch for issue #7978: select() raises select.error before 3.3, not 
OSError.
http://hg.python.org/cpython/rev/4298d6e79ecb

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[issue7978] SocketServer doesn't handle syscall interruption

[Jerzy Kozera]

Jerzy Kozera  added the comment:

I forgot to mention my patch is 3.3-only, sorry - it depends on changes from 
#12555 (http://hg.python.org/cpython/rev/41a1de81ef2b#l18.21 to be precise). To 
support 3.2 and 2.7:
 (1) select.error must be caught as in the original patch,
 (2) e.args[0] must be used - select.error doesn't have 'errno' attribute.
Should I prepare the patch for 3.2 and 2.7?

Regarding not updating the timeout, it was already mentioned above. Though as 
an afterthought, it might be worrying that if the process is receiving repeated 
signals with interval between them less than timeout, we might fall into 
infinite loop of select() when it should timeout, but that is probably very 
obscure case.

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Changes by Antoine Pitrou :


--
type: enhancement -> behavior

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

> You must recompute the timeout when select() is interrupted: see issue 
> #12338.

Ah, right. It's a minor issue, though. I would suggest opening a separate issue 
for it. The patch is now committed to all 3 branches (I'm finally convinced 
this is a bug worth fixing), so I'm closing this issue.

--
assignee: gregory.p.smith -> 
resolution:  -> fixed
stage: patch review -> committed/rejected
status: open -> closed
type: behavior -> enhancement
versions: +Python 2.7

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[issue7978] SocketServer doesn't handle syscall interruption

[STINNER Victor]

STINNER Victor  added the comment:

You must recompute the timeout when select() is interrupted: see issue #12338.

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[issue7978] SocketServer doesn't handle syscall interruption

[Roundup Robot]

Roundup Robot  added the comment:

New changeset d941d1fcc6e6 by Antoine Pitrou in branch '2.7':
Issue #7978: socketserver now restarts the select() call when EINTR is returned.
http://hg.python.org/cpython/rev/d941d1fcc6e6

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[issue7978] SocketServer doesn't handle syscall interruption

[Roundup Robot]

Roundup Robot  added the comment:

New changeset 5493d44b56d8 by Antoine Pitrou in branch '3.2':
Issue #7978: socketserver now restarts the select() call when EINTR is returned.
http://hg.python.org/cpython/rev/5493d44b56d8

New changeset 97a0c6230ece by Antoine Pitrou in branch 'default':
Issue #7978: socketserver now restarts the select() call when EINTR is returned.
http://hg.python.org/cpython/rev/97a0c6230ece

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Jerzy's latest patch looks ok to me.
This is a slight behaviour change so I'm not sure it should go in 3.2/2.7.

--
stage:  -> patch review
versions: +Python 3.3 -Python 2.7, Python 3.1

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[issue7978] SocketServer doesn't handle syscall interruption

[Jerzy Kozera]

Jerzy Kozera  added the comment:

I've updated the patch according to suggestions from Gregory P. Smith. Thanks 
to a change from #12555 (PEP 3151) now just checking for OSError is enough.

(I've decided to use mocked select() instead of calling alarm() to avoid 
depending on timing.)

--
nosy: +Jerzy.Kozera
Added file: http://bugs.python.org/file25144/socketserver_eintr_20120406.diff

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[issue7978] SocketServer doesn't handle syscall interruption

[Giampaolo Rodola']

Changes by Giampaolo Rodola' :


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[issue7978] SocketServer doesn't handle syscall interruption

[Vetoshkin Nikita]

Vetoshkin Nikita  added the comment:

I don't think we should block signals - we can sleep on select for about 
forever.

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[issue7978] SocketServer doesn't handle syscall interruption

[STINNER Victor]

STINNER Victor  added the comment:

I think that #12224 is a duplicate of this issue.

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[issue7978] SocketServer doesn't handle syscall interruption

[STINNER Victor]

STINNER Victor  added the comment:

Using signalfd() (require Linux 2.6.22+), specified signals are written in a 
file and don't interrupt system calls (select). Bonus: we can wait for a signal 
using select!

Using pthread_sigmask(), you can also block signals before calling select, but 
it's not atomic. Or you can use pselect() which is pthread_sigmask()+select() 
in an atomic fashion.

pthread_sigmask() is already in Python 3.3, I plan to add signalfd() in a few 
weeks into Python 3.3. For pselect(), I don't know if we need it.

pthread_sigmask() is available on most POSIX systems, not on Windows. Can 
select() be interrupted by CTRL+c on Windows?

I don't say that we should not restart manually select() on EINTR, just that 
better solutions do exist today.

--
nosy: +haypo

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[issue7978] SocketServer doesn't handle syscall interruption

[Gregory P. Smith]

Gregory P. Smith  added the comment:

I've added some code comments on http://bugs.python.org/review/7978/show

overall I think the patch is right, I pointed out one thing to clean up and I 
think the unittest can be greatly simplified by using stubbed out mock 
select.select() instead of fork+signals+sleeping.

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[issue7978] SocketServer doesn't handle syscall interruption

[Vetoshkin Nikita]

Vetoshkin Nikita  added the comment:

Any news on this? Could we possibly apply patch as is? If I'm not mistaken 
timeout issue is the only one left unresolved.

About timeout. The most elegant way, would be to use select's syscall timeout 
parameter, but man 2 select says: "On  Linux,  select()  modifies  timeout to 
reflect the amount of time not slept; most other implementations do not do 
this. (POSIX.1-2001 permits either behavior.)  This causes problems both when 
Linux code which reads timeout is ported to other operating systems, and when 
code is ported to Linux that reuses a struct timeval for  multiple select()s in 
a loop without reinitializing it. Consider timeout to be undefined after 
select() returns."

I'm ready to adapt Yaniv's patch to 2.7

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[issue7978] SocketServer doesn't handle syscall interruption

[Kevin Chen]

Changes by Kevin Chen :


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nosy: +kchen

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[issue7978] SocketServer doesn't handle syscall interruption

[Terry J. Reedy]

Changes by Terry J. Reedy :


--
versions:  -Python 2.5, Python 2.6

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[issue7978] SocketServer doesn't handle syscall interruption

[Yaniv Aknin]

Yaniv Aknin  added the comment:

While bda's usecase is indeed fixed is recent versions, I want to point out the 
issue still exists in a recent py3k checkout. I've updated my patch to apply 
cleanly against recent py3k (and made less structural changes to 
test_socketserver.py; I still think it has an archaic unittest structure but I 
want this patch to remain relevant a bit longer, maybe fix the structure at a 
later time).

Again, I argue that resolving the general case (a generic untilConcludes to be 
exposed in stdlib) is hard and that we have a patch review worthy (IMO) 
solution at hand. Let's apply it (and possibly open a separate ticket for the 
generic case, if that's deemed interesting).

--
Added file: 
http://bugs.python.org/file17314/socketserver_eintr_py3k_updated.diff

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[issue7978] SocketServer doesn't handle syscall interruption

[Yaniv Aknin]

Changes by Yaniv Aknin :


Removed file: http://bugs.python.org/file16841/socketserver_eintr_py3k.diff

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[issue7978] SocketServer doesn't handle syscall interruption

[Christophe Simonis]

Changes by Christophe Simonis :


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nosy: +Christophe Simonis

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

> In BaseServer, a threading.Event is used in shutdown, so it can block
> until server_forever is finished (after checking __serving). Since the
> SIGTERM interrupts the select system call, the event set is never
> reached, and shutdown hangs waiting on the event.

This precise use case is already fixed (in SVN trunk and in the 2.6
branch) since the select() loop is now wrapped in a try..finally.
I just ran your test case and killing -TERM works ok.

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[issue7978] SocketServer doesn't handle syscall interruption

[Bryce Allen]

Bryce Allen  added the comment:

I encountered this issue when trying to exit cleanly on SIGTERM, which I use to 
terminate background daemons running serve_forever.

In BaseServer, a threading.Event is used in shutdown, so it can block until 
server_forever is finished (after checking __serving). Since the SIGTERM 
interrupts the select system call, the event set is never reached, and shutdown 
hangs waiting on the event.

I've attached an example of the pattern I was trying to use in my server. There 
are several ways around the issue, but looking at the API it seems like this 
_should_ work, and in my experience all servers have clean-up code so it's a 
very common case.

--
nosy: +bda
Added file: http://bugs.python.org/file17120/sockServe.py

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[issue7978] SocketServer doesn't handle syscall interruption

[Yaniv Aknin]

Yaniv Aknin  added the comment:

pitrou, re. test code: I actually started with the test code, so that part is 
done.

I opted to use a forked SocketServer rather than threads. I'm not an expert on 
the low-level details of a multi-threaded process receiving threads, but it 
seems to me like there's a chicken and egg issue here: suppose we have threadA 
(blocking on select()) and threadB (managing the test and doing something like 
os.kill(os.getpid(), SIGFOO). I think by definition the thread running when the 
signal is received is threadB (since it sent the signal), and I'm not sure 
threadA will be interrupted properly that way (threads are sneaky, now aren't 
they). So I cleaned up test_socketserver.py a bit, split it to a base test case 
and concrete test cases, and added a separate test case which uses os.fork() 
rather than threads and tests that way.

The attached patch is against recent py3k and includes both neologix's patch 
(adapted to py3k) and the added test code. If this looks good, I'll easily 
backport it to 2.x/trunk.

spiv, re. handling timeout in select: You're very right, and I'm not sure how 
we should handle it if we want to take the "untilConcludes" code that is 
currently shared between subprocess.py and socketserver.py and move it outside. 
For subprocess' and socketserver's specific cases, the naive implementation 
which doesn't handle timeouts is OK, because they don't use the timeout 
parameter anyway.

Moving it outside warrants more discussion. Remember this isn't just about 
select() - it's a general user-space-restarter for any of the myriad of "slow" 
system calls that might be interrupted. There is some tension between elegance 
and even feasibility of correct implementation vs. the wrapper hiding 
functionality of some of these system calls. You could argue that if the user 
of the wrapper is unhappy about the lost functionality, they can make their own 
wrapper. 

Unless we can reach a resolution quickly, my gut feeling is that the current 
patch should be applied as-is and this issue be closed. A new issue along the 
lines of "Extract duplicate interruption-restart code to a relevant module and 
review stdlib for unprotected select() calls" should be opened instead. Your 
thoughts?

--
Added file: http://bugs.python.org/file16841/socketserver_eintr_py3k.diff

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[issue7978] SocketServer doesn't handle syscall interruption

[Andrew Bennetts]

Andrew Bennetts  added the comment:

Note that a trivial untilConcludes isn't correct for select if a timeout was 
passed.  If a select(..., 60) was interrupted after 59 seconds, you probably 
want to restart it with a timeout of 1 second, not 60.

The SocketServer_eintr.diff patch has this flaw.

--
nosy: +spiv

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[issue7978] SocketServer doesn't handle syscall interruption

[Vetoshkin Nikita]

Vetoshkin Nikita  added the comment:

I vote for making a global wrapper. As neologix pointed - many other modules 
can be (or are) affected. Futher found bugs could be fixed using that global 
wrapper.

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Changes by Antoine Pitrou :


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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Ok, I think the wrapper approach is a good one. Now remains to decide whether 
it should stay private to socketserver or be exposed as a generic method (for 
example in the "os" module as proposed).

By the way, adding a test would be nice, too. I'm not sure it what form it 
should be done; perhaps register a signal handler and trigger the signal from a 
sub-thread while the main thread is iterating in a select loop?

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

Ok, I think the wrapper approach is a good one. Now remains to decide whether 
it should stay private to socketserver or be exposed as a generic method (for 
example in the "os" module as proposed).

By the way, adding a patch would be nice, too. I'm not sure it what form it 
should be done; perhaps register a signal handler and trigger the signal from a 
sub-thread while the main thread is iterating in a select loop?

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[issue7978] SocketServer doesn't handle syscall interruption

[Charles-Francois Natali]

Charles-Francois Natali  added the comment:

> If we agree so far, I believe that an implementation of untilConcludes 
> *should* be added to stdlib ("signal.restartable_call", anyone?).

Definitely, it's better to have this handler written once and correct than 
having various implementations scattered around libraries.

> If people agree, I'd be happy to produce the patch (trunk+py3k, doc+test).

Go ahead :-)

> According to "man 7 signal" select must be explicitely restarted, regardless 
> of the SA_RESTART flag.

man sigaction states this:

SA_RESTART 

Provide behaviour compatible with BSD signal semantics by making _certain_ 
system calls restartable across signals. 

Indeed: you can't really rely on SA_RESTART, since certain syscalls are not 
restartable, and this flag isn't really portable.
select is even specific since the file descriptor sets can be modified, and the 
timeout too.

That's why it's definitely better to take the EINTR-wrapper approach.

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[issue7978] SocketServer doesn't handle syscall interruption

[Vetoshkin Nikita]

Vetoshkin Nikita  added the comment:

According to "man 7 signal" select must be explicitely restarted, regardless of 
the SA_RESTART flag.

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[issue7978] SocketServer doesn't handle syscall interruption

[Gregory P. Smith]

Gregory P. Smith  added the comment:

wrapping select in an eintr handler that loops like SocketServer_eintr.diff is 
fine.

--
assignee:  -> gregory.p.smith
nosy: +gregory.p.smith

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[issue7978] SocketServer doesn't handle syscall interruption

[Yaniv Aknin]

Yaniv Aknin  added the comment:

First, let me cast my vote in favour of leaving select() itself alone, it's 
excessive to fix this issue with changing select().

Second, while exarkun's comments are accurate, I understand where pitrou is 
coming from. EINTR is not a pathological case, but it's not commonplace, 
either. I believe that on modern Unices (I tried Linux and OpenSolaris), the 
default handler for SIGCHLD will restart interrupted calls. By default, SIGUSR1 
and most sane others, will kill the process anyway. So I can understand the 
purity of the argument that if a process sets a signal handler, it might just 
as well set SA_RESTART and be done with it.

That said, Imagine I write a forking server with SocketServer which sets a 
signal handler for SIGCHLD, and I don't set SA_RESTART because I don't think I 
need to. I believe this is a valid real-world scenario where (a) I don't expect 
an EINTER-related exception to be raised from SocketServer, (b) SocketServer 
can actually do something about it, and (c) I doubt the old EINTR behaviour is 
relied upon by anyone (now I'm /certain/ someone relying on this will 
pop-up...). I think its a safer bet on behalf of SocketServer (and subprocess, 
etc) to use something like untilConcludes.

If we agree so far, I believe that an implementation of untilConcludes *should* 
be added to stdlib ("signal.restartable_call", anyone?). If people agree, I'd 
be happy to produce the patch (trunk+py3k, doc+test). Other than SocketServer 
and subprocess, anything else that should use "untilConcludes"?

Finally, to answer pitrou's second question, I'm pretty sure untilConcludes as 
presented will not hurt other signal handlers. The interrupt handler will be 
executed before returning to untilConcludes' loop and manually restarting the 
call, so KeyboardInterrupt (or anything else signalistic) will behave as 
expected.

I'm attaching a toy 3.x program to play with select() and EINTR, I used it in 
writing this comment. Gee, it even has an OptionParser CLI to save you some 
commenting-out.

--
nosy: +Yaniv.Aknin
Added file: http://bugs.python.org/file16813/eintr_safety.py

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[issue7978] SocketServer doesn't handle syscall interruption

[Vetoshkin Nikita]

Vetoshkin Nikita  added the comment:

> What kind of signals can be received in real-life?

We use SIGUSR1 to reopen log files after rotation. sighandler works just fine, 
but after that Paste crashes.

I suppose that implementing silent syscall restart at select.select() level is 
a bad idea (explanation is given already), but some helper like untilConcludes 
in socket module might be useful.

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[issue7978] SocketServer doesn't handle syscall interruption

[Jean-Paul Calderone]

Jean-Paul Calderone  added the comment:

> What kind of signals can be received in real-life?

There are lots of possible answers.  Here's one.  You launch a child process 
with os.fork() (and perhaps os.exec*() as well).  Eventually you'll get a 
SIGCHLD in the parent when the child exits.  If you've also installed a 
Python-level SIGCHLD handler with signal.signal (and not flagged the handler 
with SA_RESTART using signal.siginterrupt) then if you're in select.select() 
when the SIGCHLD is received, select.select() will fail with select.error EINTR.

If your Python SIGCHLD handler deals with the SIGCHLD completely before 
returning, then this lets you support child processes and use SocketServer at 
the same time.

Changing the behavior of select.select() ("a fix at select level would be 
better") should be *very* carefully considered, and probably rejected after 
such consideration.  There are completely legitimate use cases which require 
select.select() to fail with EINTR, and these should remain supported (not to 
mention that existing code relying on this behavior shouldn't be broken by a 
Python upgrade in a quite insidious way).

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[issue7978] SocketServer doesn't handle syscall interruption

[Charles-Francois Natali]

Charles-Francois Natali  added the comment:

In real life, you can receive for example SIGSTOP (strace, gdb, shell), but 
mostly SIGCHLD (any process spawning children), etc. The attached patch just 
restarts calls when EINTR is received, as is done in subprocess module. The 
mailing list is a good idea.

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[issue7978] SocketServer doesn't handle syscall interruption

[Antoine Pitrou]

Antoine Pitrou  added the comment:

What kind of signals can be received in real-life?
(I'm assuming that most signals are of the kind that you only receive if 
someone else deliberately sends it to you, in which case you are supposed to be 
prepared to handle it)

Also, does your patch still allow Python to handle the signal properly (invoke 
a registered signal handler, or perhaps raise KeyboardInterrupt)?

You could raise the issue on the python-dev mailing-list for more advice.

--
nosy: +exarkun, pitrou

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[issue7978] SocketServer doesn't handle syscall interruption

[Charles-Francois Natali]

Charles-Francois Natali  added the comment:

A patch to make select calls EINTR-safe is attached, but:
- there are many modules that suffer from the same problem, so maybe a fix at 
select level would be better
- if not, it could be a good idea to add this EINTR-retry handler to a given 
module (signal, os?) so that every module doesn't have to reimplement it (I 
know that at least subprocess does have one).

Thoughts ?

--
keywords: +patch
nosy: +neologix
Added file: http://bugs.python.org/file16755/SocketServer_eintr.diff

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[issue7978] SocketServer doesn't handle syscall interruption

[Vetoshkin Nikita]

Vetoshkin Nikita  added the comment:

Wrapping select in (taken from twisted sources) can help:
def untilConcludes(f, *a, **kw):
while True:
try:
return f(*a, **kw)
except (IOError, OSError), e:
if e.args[0] == errno.EINTR:
continue
raise

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