Avoid spawning thread for trivial getnameinfo calls

When calling getnameinfo we spawn a thread because it may do a slow,
blocking reverse-DNS lookup. Spawning a thread is relatively fast (~20µs
on my Linux machine) but still an order of magnitude slower than when
getnameinfo is simply translating to a numeric IP or port, which, at
least in my tests on Linux, doesn't even make a syscall.

This commit adds a fast path for when reverse DNS isn't required: either
host isn't being fetched or NI_NUMERICHOST is set AND either the
service name isn't required or NI_NUMERICSERV is set. The service name
should only need to read /etc/services, which should be fast-ish, but
is still I/O so I kept the existing behaviour (it could be on a network
fs I guess).

I tested with:

    s = TCPSocket.open("www.ruby-lang.org", 80)
    500_000.times { Socket.unpack_sockaddr_in(s.getpeername) }

Before: 12.935s
After: 0.338s

Author: jhawthorn

ext/socket/raddrinfo.c

@@ -578,6 +578,10 @@ rb_getaddrinfo(const char *hostp, const char *portp, const struct addrinfo *hint
 
 #endif
 
+#define GETNAMEINFO_WONT_BLOCK(host, serv, flags) \
+    ((!(host) || ((flags) & NI_NUMERICHOST)) && \
+     (!(serv) || ((flags) & NI_NUMERICSERV)))
+
 #if GETADDRINFO_IMPL == 0
 
 int
@@ -615,6 +619,10 @@ rb_getnameinfo(const struct sockaddr *sa, socklen_t salen,
                char *host, size_t hostlen,
                char *serv, size_t servlen, int flags)
 {
+    if (GETNAMEINFO_WONT_BLOCK(host, serv, flags)) {
+        return getnameinfo(sa, salen, host, hostlen, serv, servlen, flags);
+    }
+
     struct getnameinfo_arg arg;
     int ret;
     arg.sa = sa;
@@ -743,6 +751,10 @@ rb_getnameinfo(const struct sockaddr *sa, socklen_t salen,
     struct getnameinfo_arg *arg;
     int err = 0, gni_errno = 0;
 
+    if (GETNAMEINFO_WONT_BLOCK(host, serv, flags)) {
+        return getnameinfo(sa, salen, host, hostlen, serv, servlen, flags);
+    }
+
 start:
     retry = 0;