If you configure Unix PuTTY to use a proxy, tell it to even proxy

localhost connections, and also enable X forwarding in such a way that
it will attempt to connect to a Unix-domain X server socket, an
assertion will fail when proxy_for_destination() tries to call
sk_getaddr(). Fix by ensuring that Unix-domain sockets are _never_
proxied, since they fundamentally can't be.

[originally from svn r9688]

network.h

@@ -118,6 +118,7 @@ SockAddr sk_nonamelookup(const char *host);
 void sk_getaddr(SockAddr addr, char *buf, int buflen);
 int sk_hostname_is_local(char *name);
 int sk_address_is_local(SockAddr addr);
+int sk_address_is_special_local(SockAddr addr);
 int sk_addrtype(SockAddr addr);
 void sk_addrcopy(SockAddr addr, char *buf);
 void sk_addr_free(SockAddr addr);

proxy.c

@@ -285,6 +285,15 @@ static int proxy_for_destination (SockAddr addr, char *hostname, int port,
     int hostip_len, hostname_len;
     const char *exclude_list;
 
+    /*
+     * Special local connections such as Unix-domain sockets
+     * unconditionally cannot be proxied, even in proxy-localhost
+     * mode. There just isn't any way to ask any known proxy type for
+     * them.
+     */
+    if (addr && sk_address_is_special_local(addr))
+        return 0;                      /* do not proxy */
+
     /*
      * Check the host name and IP against the hard-coded
      * representations of `localhost'.

unix/uxnet.c

@@ -390,6 +390,11 @@ int sk_address_is_local(SockAddr addr)
     }
 }
 
+int sk_address_is_special_local(SockAddr addr)
+{
+    return addr->superfamily == UNIX;
+}
+
 int sk_addrtype(SockAddr addr)
 {
     SockAddrStep step;

windows/winnet.c

@@ -667,6 +667,11 @@ int sk_address_is_local(SockAddr addr)
     }
 }
 
+int sk_address_is_special_local(SockAddr addr)
+{
+    return 0;                /* no Unix-domain socket analogue here */
+}
+
 int sk_addrtype(SockAddr addr)
 {
     SockAddrStep step;