GHC does not use select()/poll() correctly on non-Linux platforms

From my discovery at https://phabricator.haskell.org/D42#30542:

Why does the existing code work on platforms that are not Linux? In my select man page it says:

On Linux, select() modifies timeout to reflect the amount of  time  not
slept;  most  other implementations do not do this.  (POSIX.1-2001 per‐
mits 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.

The existing select loop seems to rely on the fact that &tv is updated as described here.

Same for man 2 poll.

E.g. man 2 select on FreeBSD 11 says explicitly:

BUGS
     Version 2 of the Single UNIX Specification (``SUSv2'') allows systems to
     modify the original timeout in place.  Thus, it is unwise to assume that
     the timeout value will be unmodified by the select() system call.
     FreeBSD does not modify the return value, which can cause problems for
     applications ported from other systems.

I have tested this now on FreeBSD, and indeed it doesn't work as expected.

With GHC 7.10.2:

import System.IO
main = hWaitForInput stdin (1 * 1000)

ghc --make test.hs -rtsopts

[root@ ~]# time ./test           

real	0m1.386s
user	0m0.004s
sys	0m0.000s
[root@ ~]# time ./test +RTS -V0.01

real	0m1.386s
user	0m0.001s
sys	0m0.000s
[root@ ~]# time ./test +RTS -V0.001

real	0m1.678s
user	0m0.003s
sys	0m0.002s
[root@ ~]# time ./test +RTS -V0.0001

real	0m11.311s
user	0m0.032s
sys	0m0.139s

See how when we increase the timer signal, the sleep suddenly takes 10x longer than it should.

That's because it triggers the case where EINTR is received in https://github.com/ghc/ghc/blob/f46369b8a1bf90a3bdc30f2b566c3a7e03672518%5E/libraries/base/cbits/inputReady.c#L48, letting us use the same unmodified 1-second struct timeval *timeout again and again.

This demo of the bug works for GHC 7.10 and 8.0.1; in 8.0.2 hWaitForInput is broken (https://ghc.haskell.org/trac/ghc/ticket/12912#ticket:13497#comment:134309) so the demo doesn't work there.

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Convenience: Here is the call chain of hWaitForInput