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

On Thu, May 27, 2010 at 1:39 PM, Marko Kreen [off-list ref] wrote:

On 5/27/10, Erik Faye-Lund [off-list ref] wrote:

quoted

On Thu, May 27, 2010 at 12:10 PM, Jonathan Nieder [off-list ref] 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