[Xcb] [PATCH] Handle EAGAIN errno from poll(2) or select(2)

[Jeremy Huddleston Sequoia]

> On Aug 22, 2015, at 10:30, Josh Triplett <josh at joshtriplett.org> wrote:
> 
> On Sat, Aug 22, 2015 at 02:33:46AM -0700, Jeremy Huddleston Sequoia wrote:
>> 
>>> On Aug 20, 2015, at 09:21, Josh Triplett <josh at joshtriplett.org> wrote:
>>> 
>>> On Thu, Aug 20, 2015 at 12:18:41AM -0700, Jeremy Sequoia wrote:
>>>> Yeah, I thought about sleeping before retrying in the EAGAIN case to
>>>> avoid a possible busy loop.  I can do that if you prefer.
>>>> 
>>>> As I indicated in the commit message, there is know known fallout from
>>>> the lack of EAGAIN handling.  There is no behavioral problem.  Indeed
>>>> the only time someone should ever get back EAGAIN from poll or select
>>>> on darwin is under resource pressure, and its likely the user would
>>>> have bigger concerns than this at that point.
>>>> 
>>>> I just happened to notice this while tracing code to figure out why
>>>> someone on stackoverflow was seeing recv() of the DISPLAY socket
>>>> erring out with EAGAIN and then hanging.
>>> 
>>> If Darwin/OSX returns EAGAIN to a blocking call under *any*
>>> circumstances, including "resource pressure", that's a serious bug.
>>> Don't work around it in XCB or any other library, *especially* because
>>> no other platform should behave the same way.  EAGAIN means "The socket
>>> is marked nonblocking and the receive operation would block, or a
>>> receive timeout had been set and the timeout expired before data was
>>> received."  
>> 
>> No, that is not what EAGAIN means.  From SUSv4 at https://urldefense.proofpoint.com/v2/url?u=http-3A__pubs.opengroup.org_onlinepubs_9699919799_functions_poll.html&d=BQIBAg&c=eEvniauFctOgLOKGJOplqw&r=UaoPsU3Wgwl0YJPmjBVM0jyEVkD-hIP4wNFk_7YgTEE&m=b79atDQl6jtM7bQJnkNie1ThegJwAhDJkHqH6ZBsmeQ&s=8rN43F7_wUVFVOedp3SA7SqafUll4tbQU32iZKnmHM0&e= 
>> 
>> """
>> The poll() function shall fail if:
>> 
>> [EAGAIN]
>> The allocation of internal data structures failed but a subsequent request may succeed.
>> ...
>> """
> 
> Ah, I see; I'd forgotten that the spec actually allows EAGAIN and
> EWOULDBLOCK to be different.  EWOULDBLOCK definitely has the semantics I
> had in mind and that the Linux manpage documents; from
> https://urldefense.proofpoint.com/v2/url?u=http-3A__pubs.opengroup.org_onlinepubs_9699919799_functions_V2-5Fchap02.html-23tag-5F15-5F03&d=BQIBAg&c=eEvniauFctOgLOKGJOplqw&r=UaoPsU3Wgwl0YJPmjBVM0jyEVkD-hIP4wNFk_7YgTEE&m=b79atDQl6jtM7bQJnkNie1ThegJwAhDJkHqH6ZBsmeQ&s=T2bl08Kgddw2duANE9MM75ZPc0SHqKhrvCy9gKYMFPE&e= 
> 
>> Operation would block. An operation on a socket marked as non-blocking has encountered a situation such as no data available that otherwise would have caused the function to suspend execution.
> 
> But sure enough, for EAGAIN it says "Resource temporarily unavailable.
> This is a temporary condition and later calls to the same routine may
> complete normally."  So if an implementation ignores the spec language
> saying "A conforming implementation may assign the same values for
> [EWOULDBLOCK] and [EAGAIN]." and makes them separate, EAGAIN can indeed
> mean the kernel is making its internal problems the application's
> problems and requiring the application to try again.  Sigh.
> 
>>> A blocking call with no timeout should never return EAGAIN;
>>> it should either block or return some fatal error.
>> 
>> Not according to UNIX.
> 
> s/EAGAIN/EWOULDBLOCK/ and the statement holds.

Yep!

>>> Libraries should *definitely* not have to include "wait a bit and try
>>> again" logic; that's the kernel's job.
> 
> I stand by this statement, but evidently the spec allows this particular
> bit of ridiculosity.  Personally, I'd argue that if the kernel has a
> resource allocation failure, it should be returning -ENOMEM.

I agree, but sadly nobody consulted either you or I when writing the SUS.

> Could I talk you into adding a "EAGAIN != EWOULDBLOCK && " before
> checking for EAGAIN?  That way, the "retry immediately on EAGAIN" logic
> will only run on platforms where EAGAIN *doesn't* have the same meaning
> as EWOULDBLOCK's "this is non-blocking and would block".  On platforms
> that define those two identically, the extra logic will constant-fold
> away.

They won't constant fold because we're not checking for EWOULDBLOCK because it doesn't really make sense in this case.  I don't think any implementation of poll(2) or select(2) would return EWOULDBLOCK because it doesn't really make sense to have non-blocking implementations of those syscalls.  The whole point of those syscalls is to block until data is available.

> (I also wonder whether every other application and library includes this
> logic on Darwin, or if other applications and libraries end up just
> exiting with an error in this case.)

I doubt many OS X applications are doing this check.  Error handling is so bad in a lot of code that we're in good shape if we catch the most common errors.  Running out of memory on desktops is mostly unheard of these days, and most desktop systems are likely destined to panic anyways in such a case.  Modern design for embedded systems, however, changes this.  As engineers, we need to now consider systems that have limited memory resources and no swap whereby running out of memory might be a common occurrence.  The kernel can kill runaway or idle processes to reclaim memory and then do the operation again successfully.  I agree with your position above that the kernel should deal with it internally rather than returning EAGAIN, but as code that is designed to work anywhere (or at least anywhere UNIXish), we should try to handle that case.


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