RFR 8203369 : Check for both EAGAIN and EWOULDBLOCK error codes

[Ivan Gerasimov]

Hi David!


On 5/24/18 9:45 PM, David Holmes wrote:
>
> I looked but did not review - it was just an observation :)
>

Well, thank you anyway :)

>>
>>> It seems pointless to double up these condition checks everywhere 
>>> just in case there is some platform (do we know of one?) where this 
>>> may be necessary.
>>>
>> That's exactly what man pages suggest: "...a portable application 
>> should check for both..."
>
> Yes but that's the native code that calls the library methods. That 
> doesn't necessarily mean we have to propagate the ambiguity through 
> our own native wrappers and/or Java code.
>
Ah, I didn't immediately understood you're talking about constants in 
UnixConstants.java and LinuxWatchService.java.
This part might probably be skipped (because we know that on Linux the 
constants have the same values), but I thought it's better it add it for 
consistency.

In other parts of the fix we do treat the constants uniformly and 
propagate some non-ambiguous value to Java, like returning 
IOS_UNAVAILABLE in most cases.

>> And yes, there exist such platforms.
>>
>>> I also wonder whether a smart compiler might not flag code where the 
>>> errors do infact have the same value:
>>>
>>> if (errno == 11 || errno == 11) ...
>>>
>> At least gcc -O completely removes the second redundant test, so no 
>> observable changes is expected on supported platforms.
>
> I'm more worried about a new compiler warning - especially if you 
> happened to use them in a switch! - resulting in future build failures.
>

What compiler do you mean: gcc or javac?

If gcc, then we already have the same test for both constants in code 
with no issues.
For example, java.base/unix/native/libnet/SocketInputStream.c, in 
NET_ReadWithTimeout():
         result = NET_NonBlockingRead(fd, bufP, len);
         if (result == -1 && ((errno == EAGAIN) || (errno == 
EWOULDBLOCK))) {


If javac, then, I was thinking about it too, but I don't have a good a 
universal solution to propose right now.
If one day someone needs to use these (platform dependent by definition) 
constants in switch, one will need to invent something to workaround the 
fact that some constants may have the same values on some platforms.
With respect to EAGAIN and EWOULDBLOCK, it will be caught early enough 
because it will fail during the very first build on any currently 
supported Unix platform.


With kind regards,
Ivan


> Cheers,
> David
>
>> With kind regards,
>> Ivan
>>
>>> Cheers,
>>> David
>>>
>>> On 25/05/2018 6:57 AM, Ivan Gerasimov wrote:
>>>> Hello!
>>>>
>>>> On Unix systems several system calls (including pread, read, readv, 
>>>> recvfrom, recvmsg, send, sendfile, sendmsg, sendto) may set errno 
>>>> to either EAGAIN or EWOULDBLOCK on the same condition.
>>>>
>>>> On Linux these two constants are the same, but they are not 
>>>> required to be the same.
>>>>
>>>> For example, here's an extract from the Linux man page of send():
>>>> EAGAIN or EWOULDBLOCK
>>>> The  socket  is marked nonblocking and the requested operation 
>>>> would block.  POSIX.1-2001 allows either error to be returned for 
>>>> this case, and does not require these constants to have the same 
>>>> value, so a portable application should check for both possibilities.
>>>>
>>>> We should check for both error codes when appropriate.
>>>>
>>>> Would you please help review the fix?
>>>>
>>>> BUGURL: https://bugs.openjdk.java.net/browse/JDK-8203369
>>>> WEBREV: http://cr.openjdk.java.net/~igerasim/8203369/00/webrev/
>>>>
>>>> Thanks!
>>>>
>>>
>>
>

-- 
With kind regards,
Ivan Gerasimov