New facility, platform_start_subprocess.

We already have the ability to start a subprocess and hook it up to a
Socket, for running local proxy commands. Now the same facility is
available as an auxiliary feature, so that a backend can start another
subcommand for a different purpose, and make a separate Socket to
communicate with it.

Just like the local proxy system, this facility captures the
subprocess's stderr, and passes it back to the caller via plug_log. To
make that not look silly, I had to add a system where the "proxy:"
prefix on the usual plug_log messages is reconfigurable, and when you
call platform_start_subprocess(), you get to pass the prefix you want
to use in this case.

windows/handle-socket.c

@@ -388,6 +388,13 @@ Socket *make_handle_socket(HANDLE send_H, HANDLE recv_H, HANDLE stderr_H,
     return &hs->sock;
 }
 
+void handle_socket_set_psb_prefix(Socket *s, const char *prefix)
+{
+    HandleSocket *hs = container_of(s, HandleSocket, sock);
+    assert(hs->sock.vt == &HandleSocket_sockvt);
+    psb_set_prefix(&hs->psb, prefix);
+}
+
 static void sk_handle_deferred_close(Socket *s)
 {
     HandleSocket *hs = container_of(s, HandleSocket, sock);

windows/local-proxy.c

@@ -98,3 +98,21 @@ Socket *platform_new_connection(SockAddr *addr, const char *hostname,
     local_proxy_opener_set_socket(opener, socket);
     return socket;
 }
+
+Socket *platform_start_subprocess(const char *cmd, Plug *plug,
+                                  const char *prefix)
+{
+    Socket *socket = make_deferred_handle_socket(
+        null_deferred_socket_opener(),
+        sk_nonamelookup("<local command>"), 0, plug);
+    char *err = platform_setup_local_proxy(socket, cmd);
+    handle_socket_set_psb_prefix(socket, prefix);
+
+    if (err) {
+        sk_close(socket);
+        socket = new_error_socket_fmt(plug, "%s", err);
+        sfree(err);
+    }
+
+    return socket;
+}

windows/platform.h

@@ -354,6 +354,7 @@ Socket *make_deferred_handle_socket(DeferredSocketOpener *opener,
                                     SockAddr *addr, int port, Plug *plug);
 void setup_handle_socket(Socket *s, HANDLE send_H, HANDLE recv_H,
                          HANDLE stderr_H, bool overlapped);
+void handle_socket_set_psb_prefix(Socket *s, const char *prefix);
 Socket *new_named_pipe_client(const char *pipename, Plug *plug); /* winnpc */
 Socket *new_named_pipe_listener(const char *pipename, Plug *plug); /* winnps */