campaignd: Reload configuration on SIGHUP

It's a bit tricky right now because whoever changes the configuration
must make sure to do it within the flush interval or their changes may
be lost before they get to send SIGHUP to campaignd. Another issue is
that the campaigns list may change (new uploads, bumped download counts,
etc.) between editing the config file and sending SIGHUP, and any
changes in the on-memory version of the list will be irreversibly lost.

Hopefully soon I'll add some socket commands to address these
limitations.

src/campaign_server/campaign_server.cpp

@@ -62,11 +62,17 @@ static lg::log_domain log_campaignd("campaignd");
 
 namespace {
 
-void exit_sighup(int signal)
+/**
+ * Whether to reload the server configuration as soon as possible
+ * (e.g. after SIGHUP).
+ */
+sig_atomic_t need_reload = 0;
+
+void flag_sighup(int signal)
 {
 	assert(signal == SIGHUP);
-	LOG_CS << "SIGHUP caught, exiting without cleanup immediately.\n";
-	exit(128 + SIGHUP);
+	LOG_CS << "SIGHUP caught, scheduling config reload.\n";
+	need_reload = 1;
 }
 
 void exit_sigint(int signal)
@@ -114,7 +120,7 @@ server::server(const std::string& cfg_file, size_t min_threads, size_t max_threa
 	, server_manager_(load_config())
 {
 #ifndef _MSC_VER
-	signal(SIGHUP, exit_sighup);
+	signal(SIGHUP, flag_sighup);
 #endif
 	signal(SIGINT, exit_sigint);
 	signal(SIGTERM, exit_sigterm);
@@ -269,6 +275,15 @@ void server::run()
 
 	for(;;)
 	{
+		if(need_reload) {
+			load_config(); // TODO: handle port number config changes
+
+			need_reload = 0;
+			last_ts = 0;
+
+			LOG_CS << "Reloaded configuration\n";
+		}
+
 		try {
 			std::string admin_cmd;