[Patch] tabled: replace event_loopbreak with pipe

[Pete Zaitcev <zaitcev@redhat.com>]

As it turned out, event_loopbreak() does not awaken the thread that
exectutes event_dispatch(), but our code expected that it would.
One easily noticeable effect was that there was a noticeable delay
between the state transition to DB master and listening on sockets.
I knew the mysterious delay existed for a while, but never got around
to investigate. For ncld API, I moved the processing of CLD packets
to its own thread and suddenly everything else froze. Apparently the
existing code only works because of extra packets of CLD protocol.

For a fix, dispose of event_loopbreak and use a loopback pipe.
Also gone is state_tdb_new. That thing was just disgusting.

Notice that we still have one event_loopbreak remaining, because it
works correctly thanks to UNIX signal awakening the polling thread.

Signed-Off-By: Pete Zaitcev <zaitcev@redhat.com>

---
 server/server.c |   90 +++++++++++++++++++++++++++++++++-------------
 server/tabled.h |    4 +-
 2 files changed, 68 insertions(+), 26 deletions(-)

commit fb1aaf280f14e020e6a0110b828f4879b5eaa11e
Author: Master <zaitcev@lembas.zaitcev.lan>
Date:   Wed Feb 3 19:48:42 2010 -0700

    Replace event_loopbreak with ev_pipe.

diff --git a/server/server.c b/server/server.c
index e5580c5..b205340 100644
--- a/server/server.c
+++ b/server/server.c
@@ -89,7 +89,6 @@ static error_t parse_opt (int key, char *arg, struct argp_state *state);
 static const struct argp argp = { options, parse_opt, NULL, doc };
 
 static bool server_running = true;
-static bool dump_stats;
 static bool use_syslog = true;
 int debugging = 0;
 
@@ -99,6 +98,12 @@ struct server tabled_srv = {
 
 struct tabledb tdb;
 
+enum {
+	TT_CMD_DUMP,
+	TT_CMD_TDBST_MASTER,
+	TT_CMD_TDBST_SLAVE
+};
+
 struct compiled_pat patterns[] = {
 	[pat_auth] =
 	{ "^AWS (\\w+):(\\S+)", 0, },
@@ -361,8 +366,8 @@ static void term_signal(int signo)
 
 static void stats_signal(int signo)
 {
-	dump_stats = true;
-	event_loopbreak();
+	static const unsigned char cmd = TT_CMD_DUMP;
+	write(tabled_srv.ev_pipe[1], &cmd, 1);
 }
 
 #define X(stat) \
@@ -1353,6 +1358,7 @@ static void tdb_checkpoint(int fd, short events, void *userdata)
 
 static void tdb_state_cb(enum db_event event)
 {
+	unsigned char cmd;
 
 	switch (event) {
 	case TDB_EV_ELECTED:
@@ -1369,25 +1375,20 @@ static void tdb_state_cb(enum db_event event)
 		 * This callback runs on the context of the replication
 		 * manager thread, and calling any of our functions thus
 		 * turns our program into a multi-threaded one. Instead
-		 * we do a loopbreak and postpone the processing.
+		 * we signal them main thread to do the processing.
 		 */
 		if (tabled_srv.state_tdb != ST_TDB_INIT &&
 		    tabled_srv.state_tdb != ST_TDB_OPEN) {
 			if (event == TDB_EV_MASTER)
-				tabled_srv.state_tdb_new = ST_TDB_MASTER;
+				cmd = TT_CMD_TDBST_MASTER;
 			else
-				tabled_srv.state_tdb_new = ST_TDB_SLAVE;
-			if (debugging) {
-				applog(LOG_DEBUG, "TDB state > %s",
-				       state_name_tdb[tabled_srv.state_tdb_new]);
-			}
-			event_loopbreak();
+				cmd = TT_CMD_TDBST_SLAVE;
+			write(tabled_srv.ev_pipe[1], &cmd, 1);
 		}
 		break;
 	default:
 		applog(LOG_WARNING, "API confusion with TDB, event 0x%x", event);
 		tabled_srv.state_tdb = ST_TDB_OPEN;  /* wrong, stub for now */
-		tabled_srv.state_tdb_new = ST_TDB_INIT;
 	}
 }
 
@@ -1727,6 +1728,8 @@ static void tdb_state_process(enum st_tdb new_state)
 {
 	unsigned int db_flags;
 
+	if (debugging)
+		applog(LOG_DEBUG, "TDB state > %s", state_name_tdb[new_state]);
 	if ((new_state == ST_TDB_MASTER || new_state == ST_TDB_SLAVE) &&
 	    tabled_srv.state_tdb == ST_TDB_ACTIVE) {
 
@@ -1744,6 +1747,38 @@ static void tdb_state_process(enum st_tdb new_state)
 	}
 }
 
+static void internal_event(int fd, short events, void *userdata)
+{
+	unsigned char cmd;
+	ssize_t rrc;
+
+	rrc = read(tabled_srv.ev_pipe[0], &cmd, 1);
+	if (rrc < 0) {
+		applog(LOG_WARNING, "pipe read error: %s", strerror(errno));
+		abort();
+	}
+	if (rrc < 1) {
+		applog(LOG_WARNING, "pipe short read");
+		abort();
+	}
+
+	if (cmd == TT_CMD_DUMP) {
+		stats_dump();
+	} else if (cmd == TT_CMD_TDBST_MASTER) {
+		if (tabled_srv.state_tdb != ST_TDB_MASTER) {
+			tdb_state_process(ST_TDB_MASTER);
+			tabled_srv.state_tdb = ST_TDB_MASTER;
+		}
+	} else if (cmd == TT_CMD_TDBST_SLAVE) {
+		if (tabled_srv.state_tdb != ST_TDB_SLAVE) {
+			tdb_state_process(ST_TDB_SLAVE);
+			tabled_srv.state_tdb = ST_TDB_SLAVE;
+		}
+	} else {
+		applog(LOG_WARNING, "internal error: command 0x%x", cmd);
+	}
+}
+
 int main (int argc, char *argv[])
 {
 	error_t aprc;
@@ -1820,9 +1855,22 @@ int main (int argc, char *argv[])
 	signal(SIGTERM, term_signal);
 	signal(SIGUSR1, stats_signal);
 
+	/*
+	 * Prepare the libevent paraphernalia
+	 */
 	tabled_srv.evbase_main = event_init();
 	event_base_rep = event_base_new();
 	evtimer_set(&tabled_srv.chkpt_timer, tdb_checkpoint, NULL);
+	if (pipe(tabled_srv.ev_pipe) < 0) {
+		applogerr("pipe");
+		goto err_evpipe;
+	}
+	event_set(&tabled_srv.pevt, tabled_srv.ev_pipe[0], EV_READ | EV_PERSIST,
+		  internal_event, NULL);
+	if (event_add(&tabled_srv.pevt, NULL) < 0) {
+		applog(LOG_WARNING, "pevt event_add");
+		goto err_pevt;
+	}
 
 	/* set up server networking */
 	rc = net_open();
@@ -1839,21 +1887,9 @@ int main (int argc, char *argv[])
 	applog(LOG_INFO, "initialized (%s)",
 	   (tabled_srv.flags & SFL_FOREGROUND)? "fg": "bg");
 
-	while (server_running) {
+	while (server_running)
 		event_dispatch();
 
-		if (dump_stats) {
-			dump_stats = false;
-			stats_dump();
-		}
-
-		if (tabled_srv.state_tdb_new != ST_TDB_INIT &&
-		    tabled_srv.state_tdb_new != tabled_srv.state_tdb) {
-			tdb_state_process(tabled_srv.state_tdb_new);
-			tabled_srv.state_tdb = tabled_srv.state_tdb_new;
-		}
-	}
-
 	applog(LOG_INFO, "shutting down");
 
 	rc = 0;
@@ -1871,6 +1907,10 @@ err_out_net:
 		tdb_fini(&tdb);
 	}
 /* err_tdb_init: */
+err_pevt:
+	close(tabled_srv.ev_pipe[0]);
+	close(tabled_srv.ev_pipe[1]);
+err_evpipe:
 	unlink(tabled_srv.pid_file);
 	close(tabled_srv.pid_fd);
 err_out:
diff --git a/server/tabled.h b/server/tabled.h
index a2ffd8b..91cccae 100644
--- a/server/tabled.h
+++ b/server/tabled.h
@@ -226,6 +226,8 @@ struct server {
 	int			pid_fd;		/* fd of pid_file */
 	GMutex			*bigmutex;
 	struct event_base	*evbase_main;
+	int			ev_pipe[2];
+	struct event		pevt;
 
 	char			*config;	/* config file (static) */
 
@@ -248,7 +250,7 @@ struct server {
 	int			num_stor;	/* number of storage_node's  */
 	uint64_t		object_count;
 
-	enum st_tdb		state_tdb, state_tdb_new;
+	enum st_tdb		state_tdb;
 	enum st_net		state_net;
 
 	struct event		chkpt_timer;	/* db4 checkpoint timer */