sendMsg is likely not GC-safe and may crash

[vdukhovni]

The sendMsg implementation is likely not GC-safe. It uses pointer values outside withForeignPtr. The underlying ByteStrings that own the pointers may be deallocated before the pointers are used.

I think it is necessary to retain and touch# all the foreign pointers of each ByteString. Similar issues may be present in some of the other new functions.

[vdukhovni]

For example the [Cmsg] argument of recvMsg is likely also used in an unsafe way.

[vdukhovni]

Also withIOVecfromBS, which is then used in sendMany... The issue (if I'm not mistaken) is pervasive.

[kazu-yamamoto]

Nice catch!

@vdukhovni Would you review #451?

[kazu-yamamoto]

Closing via #451.