[Tarantool-patches] [PATCH v2 2/3] box/info: report replication.X.downstream.lag value

[Cyrill Gorcunov via Tarantool-patches <tarantool-patches@dev.tarantool.org>]

This is basically a reflection of replication.X.upstream.lag value.
The upstream lag can be considered as transaction RTT in direction
from master to replica, in turn downstream lag is the reverse and
represents RTT from replica to master.

An example of output is (on replica node)

 | 2:
 |   id: 2
 |   uuid: 8bb22366-cd21-492e-98df-693884be11bd
 |   lsn: 0
 |   downstream:
 |     status: follow
 |     idle: 0.55381065199617
 |     vclock: {1: 119}
 |     lag: 0.00019168853759766

In case if there some old replicas which are not sending
timestamp in vclock encoding we simply don't show lag
field for backward compatibility sake.

Closes #5447

@TarantoolBot document
Title: Document box.info.replication[n].downstream.lag

replication[n].downstream.lag is time difference between
local time of the replica `n` sending current vector clock
state and the time this message was received by a master
node.

Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>
---
 src/box/lua/info.c |  8 ++++++++
 src/box/relay.cc   | 16 ++++++++++++++++
 src/box/relay.h    |  8 ++++++++
 3 files changed, 32 insertions(+)

diff --git a/src/box/lua/info.c b/src/box/lua/info.c
index c4c9fa0a0..1e533fe8d 100644
--- a/src/box/lua/info.c
+++ b/src/box/lua/info.c
@@ -133,16 +133,24 @@ lbox_pushrelay(lua_State *L, struct relay *relay)
 
 	switch(relay_get_state(relay)) {
 	case RELAY_FOLLOW:
+	{
+		double lag = relay_lag(relay);
 		lua_pushstring(L, "follow");
 		lua_settable(L, -3);
 		lua_pushstring(L, "vclock");
 		lbox_pushvclock(L, relay_vclock(relay));
 		lua_settable(L, -3);
+		if (lag != 0) {
+			lua_pushstring(L, "lag");
+			lua_pushnumber(L, relay_lag(relay));
+			lua_settable(L, -3);
+		}
 		lua_pushstring(L, "idle");
 		lua_pushnumber(L, ev_monotonic_now(loop()) -
 			       relay_last_row_time(relay));
 		lua_settable(L, -3);
 		break;
+	}
 	case RELAY_STOPPED:
 	{
 		lua_pushstring(L, "stopped");
diff --git a/src/box/relay.cc b/src/box/relay.cc
index df04f8198..f23e43b30 100644
--- a/src/box/relay.cc
+++ b/src/box/relay.cc
@@ -138,6 +138,8 @@ struct relay {
 	struct stailq pending_gc;
 	/** Time when last row was sent to peer. */
 	double last_row_time;
+	/** Number of seconds this master is prior the remote replica. */
+	double lag;
 	/** Relay sync state. */
 	enum relay_state state;
 
@@ -179,6 +181,12 @@ relay_last_row_time(const struct relay *relay)
 	return relay->last_row_time;
 }
 
+double
+relay_lag(const struct relay *relay)
+{
+	return relay->lag;
+}
+
 static void
 relay_send(struct relay *relay, struct xrow_header *packet);
 static void
@@ -219,6 +227,7 @@ relay_start(struct relay *relay, int fd, uint64_t sync,
 	relay->sync = sync;
 	relay->state = RELAY_FOLLOW;
 	relay->last_row_time = ev_monotonic_now(loop());
+	relay->lag = 0;
 }
 
 void
@@ -558,6 +567,13 @@ relay_reader_f(va_list ap)
 			/* vclock is followed while decoding, zeroing it. */
 			vclock_create(&relay->recv_vclock);
 			xrow_decode_vclock_xc(&xrow, &relay->recv_vclock);
+			/*
+			 * Old versions may send not a timestamp but
+			 * zeroified memory field. We can use +0 as
+			 * a sign that there is nothing encoded.
+			 */
+			if (xrow.tm != 0)
+				relay->lag = ev_now(loop()) - xrow.tm;
 			fiber_cond_signal(&relay->reader_cond);
 		}
 	} catch (Exception *e) {
diff --git a/src/box/relay.h b/src/box/relay.h
index b32e2ea2a..ec9d16925 100644
--- a/src/box/relay.h
+++ b/src/box/relay.h
@@ -93,6 +93,14 @@ relay_vclock(const struct relay *relay);
 double
 relay_last_row_time(const struct relay *relay);
 
+/**
+ * Returns relay's lag
+ * @param relay relay
+ * @returns relay's lag
+ */
+double
+relay_lag(const struct relay *relay);
+
 /**
  * Send a Raft update request to the relay channel. It is not
  * guaranteed that it will be delivered. The connection may break.
-- 
2.29.2