gracefulswitch: add ParseConfig and make UpdateClientConnState call SwitchTo if needed

balancer/balancer.go

@@ -54,13 +54,14 @@ var (
 // an init() function), and is not thread-safe. If multiple Balancers are
 // registered with the same name, the one registered last will take effect.
 func Register(b Builder) {
-	if strings.ToLower(b.Name()) != b.Name() {
+	name := strings.ToLower(b.Name())
+	if name != b.Name() {
 		// TODO: Skip the use of strings.ToLower() to index the map after v1.59
 		// is released to switch to case sensitive balancer registry. Also,
 		// remove this warning and update the docstrings for Register and Get.
 		logger.Warningf("Balancer registered with name %q. grpc-go will be switching to case sensitive balancer registries soon", b.Name())
 	}
-	m[strings.ToLower(b.Name())] = b
+	m[name] = b
 }
 
 // unregisterForTesting deletes the balancer with the given name from the

balancer/rls/config_test.go

@@ -322,7 +322,7 @@ func (s) TestParseConfigErrors(t *testing.T) {
 				"childPolicy": [{"grpclb": {"childPolicy": [{"pickfirst": {}}]}}],
 				"childPolicyConfigTargetFieldName": "serviceName"
 			}`),
-			wantErr: "invalid loadBalancingConfig: no supported policies found",
+			wantErr: "no supported policies found in config",
 		},
 		{
 			desc: "no child policy",

balancer_wrapper.go

@@ -21,7 +21,6 @@ package grpc
 import (
 	"context"
 	"fmt"
-	"strings"
 	"sync"
 
 	"google.golang.org/grpc/balancer"
@@ -66,7 +65,8 @@ type ccBalancerWrapper struct {
 }
 
 // newCCBalancerWrapper creates a new balancer wrapper in idle state. The
-// underlying balancer is not created until the switchTo() method is invoked.
+// underlying balancer is not created until the updateClientConnState() method
+// is invoked.
 func newCCBalancerWrapper(cc *ClientConn) *ccBalancerWrapper {
 	ctx, cancel := context.WithCancel(cc.ctx)
 	ccb := &ccBalancerWrapper{
@@ -97,6 +97,11 @@ func (ccb *ccBalancerWrapper) updateClientConnState(ccs *balancer.ClientConnStat
 		if ctx.Err() != nil || ccb.balancer == nil {
 			return
 		}
+		name := gracefulswitch.ChildName(ccs.BalancerConfig)
+		if ccb.curBalancerName != name {
+			ccb.curBalancerName = name
+			channelz.Infof(logger, ccb.cc.channelz, "Channel switches to new LB policy %q", name)
+		}
 		err := ccb.balancer.UpdateClientConnState(*ccs)
 		if logger.V(2) && err != nil {
 			logger.Infof("error from balancer.UpdateClientConnState: %v", err)
@@ -120,54 +125,6 @@ func (ccb *ccBalancerWrapper) resolverError(err error) {
 	})
 }
 
-// switchTo is invoked by grpc to instruct the balancer wrapper to switch to the
-// LB policy identified by name.
-//
-// ClientConn calls newCCBalancerWrapper() at creation time. Upon receipt of the
-// first good update from the name resolver, it determines the LB policy to use
-// and invokes the switchTo() method. Upon receipt of every subsequent update
-// from the name resolver, it invokes this method.
-//
-// the ccBalancerWrapper keeps track of the current LB policy name, and skips
-// the graceful balancer switching process if the name does not change.
-func (ccb *ccBalancerWrapper) switchTo(name string) {
-	ccb.serializer.Schedule(func(ctx context.Context) {
-		if ctx.Err() != nil || ccb.balancer == nil {
-			return
-		}
-		// TODO: Other languages use case-sensitive balancer registries. We should
-		// switch as well. See: https://github.com/grpc/grpc-go/issues/5288.
-		if strings.EqualFold(ccb.curBalancerName, name) {
-			return
-		}
-		ccb.buildLoadBalancingPolicy(name)
-	})
-}
-
-// buildLoadBalancingPolicy performs the following:
-//   - retrieve a balancer builder for the given name. Use the default LB
-//     policy, pick_first, if no LB policy with name is found in the registry.
-//   - instruct the gracefulswitch balancer to switch to the above builder. This
-//     will actually build the new balancer.
-//   - update the `curBalancerName` field
-//
-// Must be called from a serializer callback.
-func (ccb *ccBalancerWrapper) buildLoadBalancingPolicy(name string) {
-	builder := balancer.Get(name)
-	if builder == nil {
-		channelz.Warningf(logger, ccb.cc.channelz, "Channel switches to new LB policy %q, since the specified LB policy %q was not registered", PickFirstBalancerName, name)
-		builder = newPickfirstBuilder()
-	} else {
-		channelz.Infof(logger, ccb.cc.channelz, "Channel switches to new LB policy %q", name)
-	}
-
-	if err := ccb.balancer.SwitchTo(builder); err != nil {
-		channelz.Errorf(logger, ccb.cc.channelz, "Channel failed to build new LB policy %q: %v", name, err)
-		return
-	}
-	ccb.curBalancerName = builder.Name()
-}
-
 // close initiates async shutdown of the wrapper.  cc.mu must be held when
 // calling this function.  To determine the wrapper has finished shutting down,
 // the channel should block on ccb.serializer.Done() without cc.mu held.

clientconn.go

@@ -692,6 +692,7 @@ func (cc *ClientConn) waitForResolvedAddrs(ctx context.Context) error {
 var emptyServiceConfig *ServiceConfig
 
 func init() {
+	balancer.Register(pickfirstBuilder{})
 	cfg := parseServiceConfig("{}")
 	if cfg.Err != nil {
 		panic(fmt.Sprintf("impossible error parsing empty service config: %v", cfg.Err))
@@ -777,7 +778,7 @@ func (cc *ClientConn) updateResolverStateAndUnlock(s resolver.State, err error)
 
 	var balCfg serviceconfig.LoadBalancingConfig
 	if cc.sc != nil && cc.sc.lbConfig != nil {
-		balCfg = cc.sc.lbConfig.cfg
+		balCfg = cc.sc.lbConfig
 	}
 	bw := cc.balancerWrapper
 	cc.mu.Unlock()
@@ -1074,17 +1075,6 @@ func (cc *ClientConn) applyServiceConfigAndBalancer(sc *ServiceConfig, configSel
 	} else {
 		cc.retryThrottler.Store((*retryThrottler)(nil))
 	}
-
-	var newBalancerName string
-	if cc.sc == nil || (cc.sc.lbConfig == nil && cc.sc.LB == nil) {
-		// No service config or no LB policy specified in config.
-		newBalancerName = PickFirstBalancerName
-	} else if cc.sc.lbConfig != nil {
-		newBalancerName = cc.sc.lbConfig.name
-	} else { // cc.sc.LB != nil
-		newBalancerName = *cc.sc.LB
-	}
-	cc.balancerWrapper.switchTo(newBalancerName)
 }
 
 func (cc *ClientConn) resolveNow(o resolver.ResolveNowOptions) {

internal/balancer/gracefulswitch/config.go

@@ -0,0 +1,83 @@
+/*
+ *
+ * Copyright 2024 gRPC authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ */
+
+package gracefulswitch
+
+import (
+	"encoding/json"
+	"fmt"
+
+	"google.golang.org/grpc/balancer"
+	"google.golang.org/grpc/serviceconfig"
+)
+
+type lbConfig struct {
+	serviceconfig.LoadBalancingConfig
+
+	childBuilder balancer.Builder
+	childConfig  serviceconfig.LoadBalancingConfig
+}
+
+func ChildName(l serviceconfig.LoadBalancingConfig) string {
+	return l.(*lbConfig).childBuilder.Name()
+}
+
+// ParseConfig parses a child config list and returns a LB config for the
+// gracefulswitch Balancer.
+//
+// cfg is expected to be a json.RawMessage containing a JSON array of LB policy
+// names + configs as the format of the "loadBalancingConfig" field in
+// ServiceConfig.  It returns a type that should be passed to
+// UpdateClientConnState in the BalancerConfig field.
+func ParseConfig(cfg json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+	var lbCfg []map[string]json.RawMessage
+	if err := json.Unmarshal(cfg, &lbCfg); err != nil {
+		return nil, err
+	}
+	for i, e := range lbCfg {
+		if len(e) != 1 {
+			return nil, fmt.Errorf("expected a JSON struct with one entry; received entry %v at index %d", e, i)
+		}
+
+		var name string
+		var jsonCfg json.RawMessage
+		for name, jsonCfg = range e {
+		}
+
+		builder := balancer.Get(name)
+		if builder == nil {
+			// Skip unregistered balancer names.
+			continue
+		}
+
+		parser, ok := builder.(balancer.ConfigParser)
+		if !ok {
+			// This is a valid child with no config.
+			return &lbConfig{childBuilder: builder}, nil
+		}
+
+		cfg, err := parser.ParseConfig(jsonCfg)
+		if err != nil {
+			return nil, fmt.Errorf("error parsing config for policy %q: %v", name, err)
+		}
+
+		return &lbConfig{childBuilder: builder, childConfig: cfg}, nil
+	}
+
+	return nil, fmt.Errorf("no supported policies found in config: %v", string(cfg))
+}

internal/balancer/gracefulswitch/gracefulswitch.go

@@ -94,14 +94,23 @@
 // process is not complete when this method returns. This method must be called
 // synchronously alongside the rest of the balancer.Balancer methods this
 // Graceful Switch Balancer implements.
+//
+// Deprecated: use ParseConfig and pass a parsed config to UpdateClientConnState
+// to cause the Balancer to automatically change to the new child when necessary.
 func (gsb *Balancer) SwitchTo(builder balancer.Builder) error {
+	_, err := gsb.switchTo(builder)
+	return err
+}
+
+func (gsb *Balancer) switchTo(builder balancer.Builder) (*balancerWrapper, error) {
 	gsb.mu.Lock()
 	if gsb.closed {
 		gsb.mu.Unlock()
-		return errBalancerClosed
+		return nil, errBalancerClosed
 	}
 	bw := &balancerWrapper{
-		gsb: gsb,
+		builder: builder,
+		gsb:     gsb,
 		lastState: balancer.State{
 			ConnectivityState: connectivity.Connecting,
 			Picker:            base.NewErrPicker(balancer.ErrNoSubConnAvailable),
@@ -129,7 +138,7 @@
 			gsb.balancerCurrent = nil
 		}
 		gsb.mu.Unlock()
-		return balancer.ErrBadResolverState
+		return nil, balancer.ErrBadResolverState
 	}
 
 	// This write doesn't need to take gsb.mu because this field never gets read
@@ -138,7 +147,7 @@
 	// bw.Balancer field will never be forwarded to until this SwitchTo()
 	// function returns.
 	bw.Balancer = newBalancer
-	return nil
+	return bw, nil
 }
 
 // Returns nil if the graceful switch balancer is closed.
@@ -152,12 +161,33 @@
 }
 
 // UpdateClientConnState forwards the update to the latest balancer created.
+//
+// If the state's BalancerConfig is the config returned by a call to
+// gracefulswitch.ParseConfig, then this function will automatically SwitchTo
+// the balancer indicated by the config before forwarding its config to it, if
+// necessary.
 func (gsb *Balancer) UpdateClientConnState(state balancer.ClientConnState) error {
 	// The resolver data is only relevant to the most recent LB Policy.
 	balToUpdate := gsb.latestBalancer()
+
+	gsbCfg, ok := state.BalancerConfig.(*lbConfig)
+	if ok {
+		// Switch to the child in the config unless it is already active.
+		if balToUpdate == nil || gsbCfg.childBuilder.Name() != balToUpdate.builder.Name() {
+			var err error
+			balToUpdate, err = gsb.switchTo(gsbCfg.childBuilder)
+			if err != nil {
+				return fmt.Errorf("could not switch to new child balancer: %w", err)
+			}
+		}
+		// Unwrap the child balancer's config.
+		state.BalancerConfig = gsbCfg.childConfig
+	}
+
 	if balToUpdate == nil {
 		return errBalancerClosed
 	}
+
 	// Perform this call without gsb.mu to prevent deadlocks if the child calls
 	// back into the channel. The latest balancer can never be closed during a
 	// call from the channel, even without gsb.mu held.
@@ -169,6 +199,10 @@
 	// The resolver data is only relevant to the most recent LB Policy.
 	balToUpdate := gsb.latestBalancer()
 	if balToUpdate == nil {
+		gsb.cc.UpdateState(balancer.State{
+			ConnectivityState: connectivity.TransientFailure,
+			Picker:            base.NewErrPicker(err),
+		})
 		return
 	}
 	// Perform this call without gsb.mu to prevent deadlocks if the child calls
@@ -261,7 +295,8 @@
 // graceful switch logic.
 type balancerWrapper struct {
 	balancer.Balancer
-	gsb *Balancer
+	gsb     *Balancer
+	builder balancer.Builder
 
 	lastState balancer.State
 	subconns  map[balancer.SubConn]bool // subconns created by this balancer

pickfirst.go

@@ -38,19 +38,15 @@ const (
 	logPrefix             = "[pick-first-lb %p] "
 )
 
-func newPickfirstBuilder() balancer.Builder {
-	return &pickfirstBuilder{}
-}
-
 type pickfirstBuilder struct{}
 
-func (*pickfirstBuilder) Build(cc balancer.ClientConn, opt balancer.BuildOptions) balancer.Balancer {
+func (pickfirstBuilder) Build(cc balancer.ClientConn, opt balancer.BuildOptions) balancer.Balancer {
 	b := &pickfirstBalancer{cc: cc}
 	b.logger = internalgrpclog.NewPrefixLogger(logger, fmt.Sprintf(logPrefix, b))
 	return b
 }
 
-func (*pickfirstBuilder) Name() string {
+func (pickfirstBuilder) Name() string {
 	return PickFirstBalancerName
 }
 
@@ -63,7 +59,7 @@ type pfConfig struct {
 	ShuffleAddressList bool `json:"shuffleAddressList"`
 }
 
-func (*pickfirstBuilder) ParseConfig(js json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
+func (pickfirstBuilder) ParseConfig(js json.RawMessage) (serviceconfig.LoadBalancingConfig, error) {
 	var cfg pfConfig
 	if err := json.Unmarshal(js, &cfg); err != nil {
 		return nil, fmt.Errorf("pickfirst: unable to unmarshal LB policy config: %s, error: %v", string(js), err)
@@ -243,7 +239,3 @@ func (i *idlePicker) Pick(balancer.PickInfo) (balancer.PickResult, error) {
 	i.subConn.Connect()
 	return balancer.PickResult{}, balancer.ErrNoSubConnAvailable
 }
-
-func init() {
-	balancer.Register(newPickfirstBuilder())
-}