Merge duplicate implementations of the trivial Plug.

In the course of reworking the socket vtable system, I noticed that
both sshshare.c and x11fwd.c independently invented the idea of a Plug
none of whose methods do anything. We don't need more than one of
those, so let's centralise the idea to somewhere it can be easily
reused.

Recipe

@@ -253,7 +253,7 @@ NONSSH   = telnet raw rlogin ldisc pinger
 SSH      = ssh sshcrc sshdes sshmd5 sshrsa sshrand sshsha sshblowf
          + sshdh sshcrcda sshpubk sshzlib sshdss x11fwd portfwd
          + sshaes sshccp sshsh256 sshsh512 sshbn wildcard pinger ssharcf
-         + sshgssc pgssapi sshshare sshecc aqsync marshal
+         + sshgssc pgssapi sshshare sshecc aqsync marshal nullplug
 WINSSH   = SSH winnoise wincapi winpgntc wingss winshare winnps winnpc
          + winhsock errsock
 UXSSH    = SSH uxnoise uxagentc uxgss uxshare
@@ -344,7 +344,7 @@ psftp    : [U] psftp uxsftp uxcons UXSSH BE_SSH SFTP wildcard UXMISC uxnogtk
 pageant  : [X] uxpgnt uxagentc aqsync pageant sshrsa sshpubk sshdes sshbn
 	 + sshmd5 version tree234 misc sshaes sshsha sshdss sshsh256 sshsh512
 	 + sshecc CONF uxsignal nocproxy nogss be_none x11fwd ux_x11 uxcons
-	 + gtkask gtkmisc UXMISC
+	 + gtkask gtkmisc nullplug UXMISC
 
 ptermapp : [XT] GTKTERM uxmisc misc ldisc settings uxpty uxsel BE_NONE uxstore
          + uxsignal CHARSET uxpterm version time xpmpterm xpmptcfg

network.h

@@ -207,6 +207,11 @@ char *get_hostname(void);
  */
 Socket new_error_socket(const char *errmsg, Plug plug);
 
+/*
+ * Trivial plug that does absolutely nothing. Found in nullplug.c.
+ */
+extern Plug nullplug;
+
 /* ----------------------------------------------------------------------
  * Functions defined outside the network code, which have to be
  * declared in this header file rather than the main putty.h because

nullplug.c

@@ -0,0 +1,42 @@
+/*
+ * nullplug.c: provide a null implementation of the Plug vtable which
+ * ignores all calls. Occasionally useful in cases where we want to
+ * make a network connection just to see if it works, but not do
+ * anything with it afterwards except close it again.
+ */
+
+#include "putty.h"
+
+static void nullplug_socket_log(Plug plug, int type, SockAddr addr, int port,
+                                const char *error_msg, int error_code)
+{
+}
+
+static void nullplug_closing(Plug plug, const char *error_msg, int error_code,
+			     int calling_back)
+{
+}
+
+static void nullplug_receive(Plug plug, int urgent, char *data, int len)
+{
+}
+
+static void nullplug_sent(Plug plug, int bufsize)
+{
+}
+
+static const Plug_vtable nullplug_plugvt = {
+    nullplug_socket_log,
+    nullplug_closing,
+    nullplug_receive,
+    nullplug_sent,
+    NULL
+};
+
+static const Plug_vtable *nullplug_plugvt_ptr = &nullplug_plugvt;
+
+/*
+ * There's a singleton instance of nullplug, because it's not
+ * interesting enough to worry about making more than one of them.
+ */
+Plug nullplug = &nullplug_plugvt_ptr;

sshshare.c

@@ -2031,39 +2031,17 @@ char *ssh_share_sockname(const char *host, int port, Conf *conf)
     return sockname;
 }
 
-static void nullplug_socket_log(Plug plug, int type, SockAddr addr, int port,
-                                const char *error_msg, int error_code) {}
-static void nullplug_closing(Plug plug, const char *error_msg, int error_code,
-			     int calling_back) {}
-static void nullplug_receive(Plug plug, int urgent, char *data, int len) {}
-static void nullplug_sent(Plug plug, int bufsize) {}
-
-static const Plug_vtable nullplug_plugvt = {
-    nullplug_socket_log,
-    nullplug_closing,
-    nullplug_receive,
-    nullplug_sent,
-    NULL
-};
-
-struct nullplug {
-    const Plug_vtable *plugvt;
-};
-
 int ssh_share_test_for_upstream(const char *host, int port, Conf *conf)
 {
     char *sockname, *logtext, *ds_err, *us_err;
     int result;
     Socket sock;
-    struct nullplug np;
-
-    np.plugvt = &nullplug_plugvt;
 
     sockname = ssh_share_sockname(host, port, conf);
 
     sock = NULL;
     logtext = ds_err = us_err = NULL;
-    result = platform_ssh_share(sockname, conf, &np.plugvt, (Plug)NULL, &sock,
+    result = platform_ssh_share(sockname, conf, nullplug, (Plug)NULL, &sock,
                                 &logtext, &ds_err, &us_err, FALSE, TRUE);
 
     sfree(logtext);

x11fwd.c

@@ -52,22 +52,6 @@ static int xdmseen_cmp(void *a, void *b)
            memcmp(sa->clientid, sb->clientid, sizeof(sa->clientid));
 }
 
-/* Do-nothing "plug" implementation, used by x11_setup_display() when it
- * creates a trial connection (and then immediately closes it).
- * XXX: bit out of place here, could in principle live in a platform-
- *      independent network.c or something */
-static void dummy_plug_log(Plug p, int type, SockAddr addr, int port,
-			   const char *error_msg, int error_code) { }
-static void dummy_plug_closing
-     (Plug p, const char *error_msg, int error_code, int calling_back) { }
-static void dummy_plug_receive(Plug p, int urgent, char *data, int len) { }
-static void dummy_plug_sent(Plug p, int bufsize) { }
-static int dummy_plug_accepting(Plug p, accept_fn_t constructor, accept_ctx_t ctx) { return 1; }
-static const Plug_vtable dummy_plug_vtable = {
-    dummy_plug_log, dummy_plug_closing, dummy_plug_receive,
-    dummy_plug_sent, dummy_plug_accepting
-};
-
 struct X11FakeAuth *x11_invent_fake_auth(tree234 *authtree, int authtype)
 {
     struct X11FakeAuth *auth = snew(struct X11FakeAuth);
@@ -306,8 +290,7 @@ struct X11Display *x11_setup_display(const char *display, Conf *conf)
 	if (!err) {
 	    /* Create trial connection to see if there is a useful Unix-domain
 	     * socket */
-            const Plug_vtable *dummy = &dummy_plug_vtable;
-	    Socket s = sk_new(sk_addr_dup(ux), 0, 0, 0, 0, 0, &dummy);
+	    Socket s = sk_new(sk_addr_dup(ux), 0, 0, 0, 0, 0, nullplug);
 	    err = sk_socket_error(s);
 	    sk_close(s);
 	}