Disable handle inheritance for Sockets

[Tratcher]

Some ASP.NET Core users are having issues with their server sockets being kept alive by child processes after the parent has existed. This causes issues where clients are still able to connect but no traffic is being served. Handle inheritance for Process.Start has been discussed extensively in https://github.com/dotnet/corefx/issues/306. Failing a general solution there, disabling inheritance specifically for sockets would be an improvement.

While it varies across platforms there are generally two ways to disable this inheritance. The first is to call SetHandleInformation right after creating the socket. Libuv does this on Windows, and ASP.NET Core has added this as a windows specific mitigation. Ubuntu does not appear to be affected but Mac is and it does not have this API. This approach is also not atomic.

The other way to do disable inheritance is to pass a flag directly into the native socket constructor. This isn't exposed in the managed API. This approach is more commonly available across platforms.

[stephentoub]

disabling inheritance specifically for sockets would be an improvement

FYI, we already do this on Linux, where SOCK_CLOEXEC is available:
https://github.com/dotnet/corefx/blob/1d9c45957a4c662a7ae7bbf8ec42063cecb95f14/src/Native/Unix/System.Native/pal_networking.c#L1977-L1980

I've not tried, but I'm assuming on mac we could instead use fcntl with O_CLOEXEC as we do in other places on other file descriptors:
https://github.com/dotnet/corefx/blob/cec9910fcaa0321cd33b6fd43924a2794c5f3bb3/src/Native/Unix/System.Native/pal_io.c#L282-L287
It wouldn't be atomic, but it's probably the best we could do, assuming it works with socket fds. Seems reasonable to do that, and to use SetHandleInformation on Windows to do the equivalent, or we could probably just pass the flag into the native factory method, presumably making it atomic, though I don't know for sure.

[Tratcher]

On Windows WSA_FLAG_NO_HANDLE_INHERIT would be better than SetHandleInformation for atomicity.

[stephentoub]

Yes

[GSPP]

If it's not atomic then it cannot be relied upon and is almost as bad as not doing it IMHO.