mcs, tso: delete the tso ms discovery when switching away from API mo…

…de. (#6544)

close #6543

When switching from the PD mode to the API mode, the old tso microservice discovery isn't needed anymore,
and all resources can be released to avoid noisy error logs when the component trying to discover
the non-existent tso microservice.

Signed-off-by: Bin Shi <binshi.bing@gmail.com>

client/client.go

@@ -621,13 +621,13 @@ func (c *client) setServiceMode(newMode pdpb.ServiceMode) {
 	oldTSOClient.Close()
 	// Replace the old TSO service discovery if needed.
 	oldTSOSvcDiscovery := c.tsoSvcDiscovery
-	if newTSOSvcDiscovery != nil {
-		c.tsoSvcDiscovery = newTSOSvcDiscovery
-		// Close the old TSO service discovery safely after both the old client
-		// and service discovery are replaced.
-		if oldTSOSvcDiscovery != nil {
-			oldTSOSvcDiscovery.Close()
-		}
+	// If newTSOSvcDiscovery is nil, that's expected, as it means we are switching to PD service mode and
+	// no tso microservice discovery is needed.
+	c.tsoSvcDiscovery = newTSOSvcDiscovery
+	// Close the old TSO service discovery safely after both the old client and service discovery are replaced.
+	if oldTSOSvcDiscovery != nil {
+		// We are switching from API service mode to PD service mode, so delete the old tso microservice discovery.
+		oldTSOSvcDiscovery.Close()
 	}
 	oldMode := c.serviceMode
 	c.serviceMode = newMode