Re: [PATCH] compat: Add another rudimentary poll() emulation

From: Erik Faye-Lund <kusmabite@googlemail.com>
Date: May 27, 2010, 12:00 UTC
Message-ID: <AANLkTinXwM1fem6E3RgnLW9vqLD1fV7JvaQnxzZVRakA@mail.gmail.com>
In-Reply-To: <AANLkTikfaWVEPkHBRF8WHNWmyXIH9f7wRju1wSRC_lwm@mail.gmail.com>

On Thu, May 27, 2010 at 1:39 PM, Marko Kreen <markokr@gmail.com> wrote:

Show 26 quoted lines

> On 5/27/10, Erik Faye-Lund <kusmabite@googlemail.com> wrote:
>> On Thu, May 27, 2010 at 12:10 PM, Jonathan Nieder <jrnieder@gmail.com> wrote:
>>  > Implement the subset of poll() semantics needed by git in terms of
>>  > select(), for use by the Interix port.  Inspired by commit 6ed807f
>>  > (Windows: A rudimentary poll() emulation, 2007-12-01).
>>  >
>>
>>
>> A possible problem with this approach is that the maximum number of
>>  file descriptors poll can handle limited by RLIMIT_NOFILE, whereas the
>>  maximum number of file descriptors select can handle is limited by
>>  FD_SETSIZE.
>>
>>  I don't think this is a big problem in reality, though - both values
>>  seem to be pretty high in most implementations. And IIRC git-daemon is
>>  the only one who needs more than 2, and it doesn't even check
>>  RLIMIT_NOFILE.
>>
>>  If we decide to go this route, perhaps it'd make sense to change to
>>  this code for Windows also? Our Windows-implementation of poll() has
>>  some annoying limitations...
>
> Example of poll() compat without FD_SETSIZE limit:
>
>  http://github.com/markokr/plproxy-dev/blob/master/src/poll_compat.c
>

How does this code convince FD_SET() that the buffer has increased? It looks to me like it depends on a specific FD_SET() implementation... For instance, Windows' FD_SET() implementation is like this:

#define FD_SET(fd, set) do { \
    if (((fd_set FAR *)(set))->fd_count < FD_SETSIZE) \
        ((fd_set FAR *)(set))->fd_array[((fd_set FAR
*)(set))->fd_count++]=(fd);\
} while(0)

...so unless another set is passed in, it won't add any more fds once fd_count reaches FD_SETSIZE.

Also, FD_SETSIZE is 64 on Windows. IIRC it's 1024 on Linux, so it is much more likely that we encounter this issue on Windows than on Linux, at least ;)

-- 
Erik "kusma" Faye-Lund