Improve blocking queries on services that do not exist (hashicorp#4810)

## Background

When making a blocking query on a missing service (was never registered, or is not registered anymore) the query returns as soon as any service is updated.
On clusters with frequent updates (5~10 updates/s in our DCs) these queries virtually do not block, and clients with no protections againt this waste ressources on the agent and server side. Clients that do protect against this get updates later than they should because of the backoff time they implement between requests.

## Implementation

While reducing the number of unnecessary updates we still want :
* Clients to be notified as soon as when the last instance of a service disapears.
* Clients to be notified whenever there's there is an update for the service.
* Clients to be notified as soon as the first instance of the requested service is added.

To reduce the number of unnecessary updates we need to block when a request to a missing service is made. However in the following case :

1. Client `client1` makes a query for service `foo`, gets back a node and X-Consul-Index 42
2. `foo` is unregistered 
3. `client1`  makes a query for `foo` with `index=42` -> `foo` does not exist, the query blocks and `client1` is not notified of the change on `foo` 

We could store the last raft index when each service was last alive to know wether we should block on the incoming query or not, but that list could grow indefinetly. 
We instead store the last raft index when a service was unregistered and use it when a query targets a service that does not exist. 
When a service `srv` is unregistered this "missing service index" is always greater than any X-Consul-Index held by the clients while `srv` was up, allowing us to immediatly notify them.

1. Client `client1` makes a query for service `foo`, gets back a node and `X-Consul-Index: 42`
2. `foo` is unregistered, we set the "missing service index" to 43 
3. `client1` makes a blocking query for `foo` with `index=42` -> `foo` does not exist, we check against the "missing service index" and return immediatly with `X-Consul-Index: 43`
4. `client1` makes a blocking query for `foo` with `index=43` -> we block
5. Other changes happen in the cluster, but foo still doesn't exist and "missing service index" hasn't changed, the query is still blocked
6. `foo` is registered again on index 62 -> `foo` exists and its index is greater than 43, we unblock the query

Author: Aestek

agent/consul/state/catalog.go

@@ -13,6 +13,10 @@ import (
 
 const (
 	servicesTableName = "services"
+
+	// serviceLastExtinctionIndexName keeps track of the last raft index when the last instance
+	// of any service was unregistered. This is used by blocking queries on missing services.
+	serviceLastExtinctionIndexName = "service_last_extinction"
 )
 
 // nodesTableSchema returns a new table schema used for storing node
@@ -841,19 +845,30 @@ func (s *Store) ServicesByNodeMeta(ws memdb.WatchSet, filters map[string]string)
 }
 
 // maxIndexForService return the maximum Raft Index for a service
-// If the index is not set for the service, it will return:
-// - maxIndex(nodes, services) if checks is false
-// - maxIndex(nodes, services, checks) if checks is true
-func maxIndexForService(tx *memdb.Txn, serviceName string, checks bool) uint64 {
-	transaction, err := tx.First("index", "id", serviceIndexName(serviceName))
-	if err == nil {
-		if idx, ok := transaction.(*IndexEntry); ok {
-			return idx.Value
-		}
+// If the index is not set for the service, it will return the missing
+// service index.
+// The service_last_extinction is set to the last raft index when a service
+// was unregistered (or 0 if no services were ever unregistered). This
+// allows blocking queries to
+//   * return when the last instance of a service is removed
+//   * block until an instance for this service is available, or another
+//     service is unregistered.
+func maxIndexForService(tx *memdb.Txn, serviceName string, serviceExists, checks bool) uint64 {
+	if !serviceExists {
+		res, err := tx.First("index", "id", serviceLastExtinctionIndexName)
+		if missingIdx, ok := res.(*IndexEntry); ok && err == nil {
+			return missingIdx.Value
+		}
+	}
+
+	res, err := tx.First("index", "id", serviceIndexName(serviceName))
+	if idx, ok := res.(*IndexEntry); ok && err == nil {
+		return idx.Value
 	}
 	if checks {
 		return maxIndexTxn(tx, "nodes", "services", "checks")
 	}
+
 	return maxIndexTxn(tx, "nodes", "services")
 }
 
@@ -873,9 +888,6 @@ func (s *Store) serviceNodes(ws memdb.WatchSet, serviceName string, connect bool
 	tx := s.db.Txn(false)
 	defer tx.Abort()
 
-	// Get the table index.
-	idx := maxIndexForService(tx, serviceName, false)
-
 	// Function for lookup
 	var f func() (memdb.ResultIterator, error)
 	if !connect {
@@ -905,6 +917,10 @@ func (s *Store) serviceNodes(ws memdb.WatchSet, serviceName string, connect bool
 	if err != nil {
 		return 0, nil, fmt.Errorf("failed parsing service nodes: %s", err)
 	}
+
+	// Get the table index.
+	idx := maxIndexForService(tx, serviceName, len(results) > 0, false)
+
 	return idx, results, nil
 }
 
@@ -914,9 +930,6 @@ func (s *Store) ServiceTagNodes(ws memdb.WatchSet, service string, tags []string
 	tx := s.db.Txn(false)
 	defer tx.Abort()
 
-	// Get the table index.
-	idx := maxIndexForService(tx, service, false)
-
 	// List all the services.
 	services, err := tx.Get("services", "service", service)
 	if err != nil {
@@ -925,9 +938,11 @@ func (s *Store) ServiceTagNodes(ws memdb.WatchSet, service string, tags []string
 	ws.Add(services.WatchCh())
 
 	// Gather all the services and apply the tag filter.
+	serviceExists := false
 	var results structs.ServiceNodes
 	for service := services.Next(); service != nil; service = services.Next() {
 		svc := service.(*structs.ServiceNode)
+		serviceExists = true
 		if !serviceTagsFilter(svc, tags) {
 			results = append(results, svc)
 		}
@@ -938,6 +953,9 @@ func (s *Store) ServiceTagNodes(ws memdb.WatchSet, service string, tags []string
 	if err != nil {
 		return 0, nil, fmt.Errorf("failed parsing service nodes: %s", err)
 	}
+	// Get the table index.
+	idx := maxIndexForService(tx, service, serviceExists, false)
+
 	return idx, results, nil
 }
 
@@ -1214,6 +1232,11 @@ func (s *Store) deleteServiceTxn(tx *memdb.Txn, idx uint64, nodeName, serviceID
 					return fmt.Errorf("[FAILED] deleting serviceIndex %s: %s", svc.ServiceName, err)
 				}
 			}
+
+			if err := tx.Insert("index", &IndexEntry{serviceLastExtinctionIndexName, idx}); err != nil {
+				return fmt.Errorf("failed updating missing service index: %s", err)
+			}
+
 		}
 	} else {
 		return fmt.Errorf("Could not find any service %s: %s", svc.ServiceName, err)
@@ -1438,7 +1461,7 @@ func (s *Store) ServiceChecksByNodeMeta(ws memdb.WatchSet, serviceName string,
 	defer tx.Abort()
 
 	// Get the table index.
-	idx := maxIndexForService(tx, serviceName, true)
+	idx := maxIndexForService(tx, serviceName, true, true)
 	// Return the checks.
 	iter, err := tx.Get("checks", "service", serviceName)
 	if err != nil {
@@ -1627,9 +1650,6 @@ func (s *Store) checkServiceNodes(ws memdb.WatchSet, serviceName string, connect
 	tx := s.db.Txn(false)
 	defer tx.Abort()
 
-	// Get the table index.
-	idx := maxIndexForService(tx, serviceName, true)
-
 	// Function for lookup
 	var f func() (memdb.ResultIterator, error)
 	if !connect {
@@ -1654,6 +1674,10 @@ func (s *Store) checkServiceNodes(ws memdb.WatchSet, serviceName string, connect
 	for service := iter.Next(); service != nil; service = iter.Next() {
 		results = append(results, service.(*structs.ServiceNode))
 	}
+
+	// Get the table index.
+	idx := maxIndexForService(tx, serviceName, len(results) > 0, true)
+
 	return s.parseCheckServiceNodes(tx, ws, idx, serviceName, results, err)
 }
 
@@ -1663,9 +1687,6 @@ func (s *Store) CheckServiceTagNodes(ws memdb.WatchSet, serviceName string, tags
 	tx := s.db.Txn(false)
 	defer tx.Abort()
 
-	// Get the table index.
-	idx := maxIndexForService(tx, serviceName, true)
-
 	// Query the state store for the service.
 	iter, err := tx.Get("services", "service", serviceName)
 	if err != nil {
@@ -1674,13 +1695,18 @@ func (s *Store) CheckServiceTagNodes(ws memdb.WatchSet, serviceName string, tags
 	ws.Add(iter.WatchCh())
 
 	// Return the results, filtering by tag.
+	serviceExists := false
 	var results structs.ServiceNodes
 	for service := iter.Next(); service != nil; service = iter.Next() {
 		svc := service.(*structs.ServiceNode)
+		serviceExists = true
 		if !serviceTagsFilter(svc, tags) {
 			results = append(results, svc)
 		}
 	}
+
+	// Get the table index.
+	idx := maxIndexForService(tx, serviceName, serviceExists, true)
 	return s.parseCheckServiceNodes(tx, ws, idx, serviceName, results, err)
 }
 

agent/consul/state/catalog_test.go

@@ -2848,16 +2848,68 @@ func TestIndexIndependence(t *testing.T) {
 	ensureServiceVersion(t, s, ws, "service_shared", 17, 0)
 
 	testRegisterService(t, s, 18, "node1", "service_new")
-	// Since service does not exists anymore, its index should be last insert
-	// The behaviour is the same as all non-existing services, meaning
-	// we properly did collect garbage
-	ensureServiceVersion(t, s, ws, "service_shared", 18, 0)
+
+	// Since service does not exists anymore, its index should be that of
+	// the last deleted service
+	ensureServiceVersion(t, s, ws, "service_shared", 17, 0)
+
 	// No index should exist anymore, it must have been garbage collected
 	ensureIndexForService(t, s, ws, "service_shared", 0)
 	if !watchFired(ws) {
 		t.Fatalf("bad")
 	}
+}
+
+func TestMissingServiceIndex(t *testing.T) {
+	s := testStateStore(t)
+
+	// Querying with no matches gives an empty response
+	ws := memdb.NewWatchSet()
+	idx, res, err := s.CheckServiceNodes(ws, "service1")
+	require.Nil(t, err)
+	require.Nil(t, res)
+
+	// index should be 0 for a non existing service at startup
+	require.Equal(t, uint64(0), idx)
+
+	testRegisterNode(t, s, 0, "node1")
+
+	// node operations should not affect missing service index
+	ensureServiceVersion(t, s, ws, "service1", 0, 0)
+
+	testRegisterService(t, s, 10, "node1", "service1")
+	ensureServiceVersion(t, s, ws, "service1", 10, 1)
 
+	s.DeleteService(11, "node1", "service1")
+	// service1 is now missing, its index is now that of the last index a service was
+	// deleted at
+	ensureServiceVersion(t, s, ws, "service1", 11, 0)
+
+	testRegisterService(t, s, 12, "node1", "service2")
+	ensureServiceVersion(t, s, ws, "service2", 12, 1)
+
+	// missing service index does not change even though another service have been
+	// registered
+	ensureServiceVersion(t, s, ws, "service1", 11, 0)
+	ensureServiceVersion(t, s, ws, "i_do_not_exist", 11, 0)
+
+	// registering a service on another node does not affect missing service
+	// index
+	testRegisterNode(t, s, 13, "node2")
+	testRegisterService(t, s, 14, "node2", "service3")
+	ensureServiceVersion(t, s, ws, "service3", 14, 1)
+	ensureServiceVersion(t, s, ws, "service1", 11, 0)
+
+	// unregistering a service bumps missing service index
+	s.DeleteService(15, "node2", "service3")
+	ensureServiceVersion(t, s, ws, "service3", 15, 0)
+	ensureServiceVersion(t, s, ws, "service2", 12, 1)
+	ensureServiceVersion(t, s, ws, "service1", 15, 0)
+	ensureServiceVersion(t, s, ws, "i_do_not_exist", 15, 0)
+
+	// registering again a missing service correctly updates its index
+	testRegisterService(t, s, 16, "node1", "service1")
+	ensureServiceVersion(t, s, ws, "service1", 16, 1)
 }
 
 func TestStateStore_CheckServiceNodes(t *testing.T) {