serve: Add a killswitch that causes the server to uncleanly exit on hang

We define a hang as 120 seconds for now; that should be OK (famous last words).
When I say unclean, I mean it; the control socket is left hanging around too.

This is a workaround for the fact that the client can hang the whole server by
sending a write request header specifying > 0 bytes, then uncleanly going away.
On the server side, we acquire the IO mutex, and then try to read > 0 bytes from
the socket; the data never arrives, and when the client reconnects, its requests
never get a response (since we're waiting on that mutex). Getting rid of that
mutex (which isn't actually needed, except for migration) would be better.

src/client.h

@@ -1,6 +1,9 @@
 #ifndef CLIENT_H
 #define CLIENT_H
 
+#include <signal.h>
+#include <time.h>
+
 /** CLIENT_MAX_WAIT_SECS
  * This is the length of time an inbound migration will wait for a fresh
  * write before assuming the source has Gone Away.  Note: it is *not*
@@ -9,6 +12,19 @@
  */
 #define CLIENT_MAX_WAIT_SECS 5
 
+/** CLIENT_HANDLER_TIMEOUT
+ * This is the length of time (in seconds) any request can be outstanding for.
+ * If we spend longer than this in a request, the whole server is killed.
+ */
+#define CLIENT_HANDLER_TIMEOUT 120
+
+/** CLIENT_KILLSWITCH_SIGNAL
+ * The signal number we use to kill the server when *any* killswitch timer
+ * fires. We don't actually need to install a signal handler for it, the default
+ * behaviour is perfectly fine.
+ */
+#define CLIENT_KILLSWITCH_SIGNAL ( SIGRTMIN + 1 )
+
 
 struct client {
 	/* When we call pthread_join, if the thread is already dead
@@ -20,16 +36,20 @@ struct client {
 	 */
 	int     stopped;
 	int     socket;
-	
+
 	int     fileno;
 	char*   mapped;
 
 	struct self_pipe * stop_signal;
-	
+
 	struct server* serve; /* FIXME: remove above duplication */
 
 	/* Have we seen a REQUEST_DISCONNECT message? */
 	int     disconnect;
+
+	/* kill the whole server if a request has been outstanding too long */
+	timer_t killswitch;
+
 };
 
 
@@ -39,3 +59,4 @@ void client_destroy( struct client * client );
 void client_signal_stop( struct client * client );
 
 #endif
+