agent: Allow threads to interrupt main select loop with SIGCONT.

* agent/gpg-agent.c (interrupt_main_thread_loop): New function on
non-windows platforms, allows other threads to interrupt the main loop
if there's something that the main loop might be interested in.

--

For example, the main loop might be interested in changes in program
state that affect the timers it expects to see.

I don't know how to do this on Windows platforms, but i welcome any
proposed improvements.

Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>

Gbp-Pq: Topic gpg-agent-idling
Gbp-Pq: Name agent-Allow-threads-to-interrupt-main-select-loop-wi.patch

agent/agent.h

@@ -370,6 +370,7 @@ void *get_agent_scd_notify_event (void);
 #endif
 void agent_sighup_action (void);
 int map_pk_openpgp_to_gcry (int openpgp_algo);
+void interrupt_main_thread_loop (void);
 
 /*-- command.c --*/
 gpg_error_t agent_inq_pinentry_launched (ctrl_t ctrl, unsigned long pid,

agent/gpg-agent.c

@@ -417,6 +417,9 @@ static int have_homedir_inotify;
  * works reliable.  */
 static int reliable_homedir_inotify;
 
+/* Record the pid of the main thread, for easier signalling */
+static pid_t main_thread_pid = (pid_t)(-1);
+
 /* Number of active connections.  */
 static int active_connections;
 
@@ -2472,6 +2475,10 @@ handle_signal (int signo)
       agent_sigusr2_action ();
       break;
 
+      /* nothing to do here, just take an extra cycle on the select loop */
+    case SIGCONT:
+      break;
+
     case SIGTERM:
       if (!shutdown_pending)
         log_info ("SIGTERM received - shutting down ...\n");
@@ -2810,6 +2817,13 @@ start_connection_thread_ssh (void *arg)
 }
 
 
+void interrupt_main_thread_loop (void)
+{
+#ifndef HAVE_W32_SYSTEM
+  kill (main_thread_pid, SIGCONT);
+#endif
+}
+
 /* helper function for readability: test whether a given struct
    timespec is set to all-zeros */
 static inline int
@@ -2879,8 +2893,10 @@ handle_connections (gnupg_fd_t listen_fd,
   npth_sigev_add (SIGUSR1);
   npth_sigev_add (SIGUSR2);
   npth_sigev_add (SIGINT);
+  npth_sigev_add (SIGCONT);
   npth_sigev_add (SIGTERM);
   npth_sigev_fini ();
+  main_thread_pid = getpid ();
 #else
 # ifdef HAVE_W32CE_SYSTEM
   /* Use a dummy event. */