consul: avoid extra sync operations when no action required

This PR makes it so the Consul sync logic will ignore operations that
do not specify an action to take (i.e. [de-]register [services|checks]).

Ideally such noops would be discarded at the callsites (i.e. users
of [Create|Update|Remove]Workload], but we can also be defensive
at the commit point.

Also adds 2 trace logging statements which are helpful for diagnosing
sync operations with Consul - when they happen and why.

Fixes #10797

.changelog/10865.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+consul: avoid extra sync operations when no action required
+```

command/agent/consul/service_client.go

@@ -348,6 +348,27 @@ type operations struct {
 	deregChecks   []string
 }
 
+func (o *operations) empty() bool {
+	switch {
+	case o == nil:
+		return true
+	case len(o.regServices) > 0:
+		return false
+	case len(o.regChecks) > 0:
+		return false
+	case len(o.deregServices) > 0:
+		return false
+	case len(o.deregChecks) > 0:
+		return false
+	default:
+		return true
+	}
+}
+
+func (o operations) String() string {
+	return fmt.Sprintf("<%d, %d, %d, %d>", len(o.regServices), len(o.regChecks), len(o.deregServices), len(o.deregChecks))
+}
+
 // AllocRegistration holds the status of services registered for a particular
 // allocations by task.
 type AllocRegistration struct {
@@ -560,11 +581,24 @@ func (c *ServiceClient) hasSeen() bool {
 type syncReason byte
 
 const (
-	syncPeriodic = iota
+	syncPeriodic syncReason = iota
 	syncShutdown
 	syncNewOps
 )
 
+func (sr syncReason) String() string {
+	switch sr {
+	case syncPeriodic:
+		return "periodic"
+	case syncShutdown:
+		return "shutdown"
+	case syncNewOps:
+		return "operations"
+	default:
+		return "unexpected"
+	}
+}
+
 // Run the Consul main loop which retries operations against Consul. It should
 // be called exactly once.
 func (c *ServiceClient) Run() {
@@ -680,6 +714,24 @@ INIT:
 
 // commit operations unless already shutting down.
 func (c *ServiceClient) commit(ops *operations) {
+	c.logger.Trace("commit sync operations", "ops", ops)
+
+	// Ignore empty operations - ideally callers will optimize out syncs with
+	// nothing to do, but be defensive anyway. Sending an empty ops on the chan
+	// will trigger an unnecessary sync with Consul.
+	if ops.empty() {
+		return
+	}
+
+	// Prioritize doing nothing if we are being signaled to shutdown.
+	select {
+	case <-c.shutdownCh:
+		return
+	default:
+	}
+
+	// Send the ops down the ops chan, triggering a sync with Consul. Unless we
+	// receive a signal to shutdown.
 	select {
 	case c.opCh <- ops:
 	case <-c.shutdownCh:
@@ -713,6 +765,8 @@ func (c *ServiceClient) merge(ops *operations) {
 
 // sync enqueued operations.
 func (c *ServiceClient) sync(reason syncReason) error {
+	c.logger.Trace("execute sync", "reason", reason)
+
 	sreg, creg, sdereg, cdereg := 0, 0, 0, 0
 	var err error
 

command/agent/consul/service_client_test.go

@@ -1,6 +1,7 @@
 package consul
 
 import (
+	"fmt"
 	"testing"
 	"time"
 
@@ -599,3 +600,27 @@ func TestSyncLogic_proxyUpstreamsDifferent(t *testing.T) {
 		}
 	})
 }
+
+func TestSyncReason_String(t *testing.T) {
+	t.Parallel()
+
+	require.Equal(t, "periodic", fmt.Sprintf("%s", syncPeriodic))
+	require.Equal(t, "shutdown", fmt.Sprintf("%s", syncShutdown))
+	require.Equal(t, "operations", fmt.Sprintf("%s", syncNewOps))
+	require.Equal(t, "unexpected", fmt.Sprintf("%s", syncReason(128)))
+}
+
+func TestSyncOps_empty(t *testing.T) {
+	t.Parallel()
+
+	try := func(ops *operations, exp bool) {
+		require.Equal(t, exp, ops.empty())
+	}
+
+	try(&operations{regServices: make([]*api.AgentServiceRegistration, 1)}, false)
+	try(&operations{regChecks: make([]*api.AgentCheckRegistration, 1)}, false)
+	try(&operations{deregServices: make([]string, 1)}, false)
+	try(&operations{deregChecks: make([]string, 1)}, false)
+	try(&operations{}, true)
+	try(nil, true)
+}