New Plink operating mode: 'plink -shareexists'.

A Plink invocation of the form 'plink -shareexists <session>' tests
for a currently live connection-sharing upstream for the session in
question. <session> can be any syntax you'd use with Plink to make the
actual connection (a host/port number, a bare saved session name,
-load, whatever).

I envisage this being useful for things like adaptive proxying - e.g.
if you want to connect to host A which you can't route to directly,
and you might already have a connection to either of hosts B or C
which are viable proxies, then you could write a proxy shell script
which checks whether you already have an upstream for B or C and goes
via whichever one is currently active.

Testing for the upstream's existence has to be done by actually
connecting to its socket, because on Unix the mere existence of a
Unix-domain socket file doesn't guarantee that there's a process
listening to it. So we make a test connection, and then immediately
disconnect; hence, that shows up in the upstream's event log.

windows/winplink.c

@@ -303,6 +303,7 @@ int main(int argc, char **argv)
     int errors;
     int got_host = FALSE;
     int use_subsystem = 0;
+    int just_test_share_exists = FALSE;
     unsigned long now, next, then;
 
     sklist = NULL;
@@ -364,6 +365,8 @@ int main(int argc, char **argv)
             } else if (!strcmp(p, "-pgpfp")) {
                 pgp_fingerprints();
                 exit(1);
+	    } else if (!strcmp(p, "-shareexists")) {
+                just_test_share_exists = TRUE;
 	    } else {
 		fprintf(stderr, "plink: unknown option \"%s\"\n", p);
 		errors = 1;
@@ -596,6 +599,19 @@ int main(int argc, char **argv)
     logctx = log_init(NULL, conf);
     console_provide_logctx(logctx);
 
+    if (just_test_share_exists) {
+        if (!back->test_for_upstream) {
+            fprintf(stderr, "Connection sharing not supported for connection "
+                    "type '%s'\n", back->name);
+            return 1;
+        }
+        if (back->test_for_upstream(conf_get_str(conf, CONF_host),
+                                    conf_get_int(conf, CONF_port), conf))
+            return 0;
+        else
+            return 1;
+    }
+
     /*
      * Start up the connection.
      */

windows/winser.c

@@ -453,6 +453,7 @@ Backend serial_backend = {
     serial_provide_logctx,
     serial_unthrottle,
     serial_cfg_info,
+    NULL /* test_for_upstream */,
     "serial",
     PROT_SERIAL,
     0