dns: stop polling for updates; use UpdateState API

internal/resolver/dns/dns_resolver.go

@@ -32,9 +32,7 @@ import (
 	"sync"
 	"time"
 
-	"google.golang.org/grpc/backoff"
 	"google.golang.org/grpc/grpclog"
-	internalbackoff "google.golang.org/grpc/internal/backoff"
 	"google.golang.org/grpc/internal/grpcrand"
 	"google.golang.org/grpc/resolver"
 )
@@ -45,7 +43,6 @@ func init() {
 
 const (
 	defaultPort       = "443"
-	defaultFreq       = time.Minute * 30
 	defaultDNSSvrPort = "53"
 	golang            = "GO"
 	// txtPrefix is the prefix string to be prepended to the host name for txt record lookup.
@@ -95,13 +92,10 @@ var customAuthorityResolver = func(authority string) (netResolver, error) {
 
 // NewBuilder creates a dnsBuilder which is used to factory DNS resolvers.
 func NewBuilder() resolver.Builder {
-	return &dnsBuilder{minFreq: defaultFreq}
+	return &dnsBuilder{}
 }
 
-type dnsBuilder struct {
-	// minimum frequency of polling the DNS server.
-	minFreq time.Duration
-}
+type dnsBuilder struct{}
 
 // Build creates and starts a DNS resolver that watches the name resolution of the target.
 func (b *dnsBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOption) (resolver.Resolver, error) {
@@ -111,33 +105,20 @@ func (b *dnsBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts
 	}
 
 	// IP address.
-	if net.ParseIP(host) != nil {
-		host, _ = formatIP(host)
-		addr := []resolver.Address{{Addr: host + ":" + port}}
-		i := &ipResolver{
-			cc: cc,
-			ip: addr,
-			rn: make(chan struct{}, 1),
-			q:  make(chan struct{}),
-		}
-		cc.NewAddress(addr)
-		go i.watcher()
-		return i, nil
+	if ipAddr, ok := formatIP(host); ok {
+		addr := []resolver.Address{{Addr: ipAddr + ":" + port}}
+		cc.UpdateState(resolver.State{Addresses: addr})
+		return deadResolver{}, nil
 	}
 
 	// DNS address (non-IP).
 	ctx, cancel := context.WithCancel(context.Background())
-	bc := backoff.DefaultConfig
-	bc.MaxDelay = b.minFreq
 	d := &dnsResolver{
-		freq:                 b.minFreq,
-		backoff:              internalbackoff.Exponential{Config: bc},
 		host:                 host,
 		port:                 port,
 		ctx:                  ctx,
 		cancel:               cancel,
 		cc:                   cc,
-		t:                    time.NewTimer(0),
 		rn:                   make(chan struct{}, 1),
 		disableServiceConfig: opts.DisableServiceConfig,
 	}
@@ -153,6 +134,7 @@ func (b *dnsBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts
 
 	d.wg.Add(1)
 	go d.watcher()
+	d.ResolveNow(resolver.ResolveNowOption{})
 	return d, nil
 }
 
@@ -167,53 +149,23 @@ type netResolver interface {
 	LookupTXT(ctx context.Context, name string) (txts []string, err error)
 }
 
-// ipResolver watches for the name resolution update for an IP address.
-type ipResolver struct {
-	cc resolver.ClientConn
-	ip []resolver.Address
-	// rn channel is used by ResolveNow() to force an immediate resolution of the target.
-	rn chan struct{}
-	q  chan struct{}
-}
-
-// ResolveNow resend the address it stores, no resolution is needed.
-func (i *ipResolver) ResolveNow(opt resolver.ResolveNowOption) {
-	select {
-	case i.rn <- struct{}{}:
-	default:
-	}
-}
+// deadResolver is a resolver that does nothing.
+type deadResolver struct{}
 
-// Close closes the ipResolver.
-func (i *ipResolver) Close() {
-	close(i.q)
-}
+func (deadResolver) ResolveNow(_ resolver.ResolveNowOption) {}
 
-func (i *ipResolver) watcher() {
-	for {
-		select {
-		case <-i.rn:
-			i.cc.NewAddress(i.ip)
-		case <-i.q:
-			return
-		}
-	}
-}
+func (deadResolver) Close() {}
 
 // dnsResolver watches for the name resolution update for a non-IP target.
 type dnsResolver struct {
-	freq       time.Duration
-	backoff    internalbackoff.Exponential
-	retryCount int
-	host       string
-	port       string
-	resolver   netResolver
-	ctx        context.Context
-	cancel     context.CancelFunc
-	cc         resolver.ClientConn
+	host     string
+	port     string
+	resolver netResolver
+	ctx      context.Context
+	cancel   context.CancelFunc
+	cc       resolver.ClientConn
 	// rn channel is used by ResolveNow() to force an immediate resolution of the target.
 	rn chan struct{}
-	t  *time.Timer
 	// wg is used to enforce Close() to return after the watcher() goroutine has finished.
 	// Otherwise, data race will be possible. [Race Example] in dns_resolver_test we
 	// replace the real lookup functions with mocked ones to facilitate testing.
@@ -225,7 +177,7 @@ type dnsResolver struct {
 }
 
 // ResolveNow invoke an immediate resolution of the target that this dnsResolver watches.
-func (d *dnsResolver) ResolveNow(opt resolver.ResolveNowOption) {
+func (d *dnsResolver) ResolveNow(resolver.ResolveNowOption) {
 	select {
 	case d.rn <- struct{}{}:
 	default:
@@ -236,7 +188,6 @@ func (d *dnsResolver) ResolveNow(opt resolver.ResolveNowOption) {
 func (d *dnsResolver) Close() {
 	d.cancel()
 	d.wg.Wait()
-	d.t.Stop()
 }
 
 func (d *dnsResolver) watcher() {
@@ -245,27 +196,11 @@ func (d *dnsResolver) watcher() {
 		select {
 		case <-d.ctx.Done():
 			return
-		case <-d.t.C:
 		case <-d.rn:
-			if !d.t.Stop() {
-				// Before resetting a timer, it should be stopped to prevent racing with
-				// reads on it's channel.
-				<-d.t.C
-			}
 		}
 
-		result, sc := d.lookup()
-		// Next lookup should happen within an interval defined by d.freq. It may be
-		// more often due to exponential retry on empty address list.
-		if len(result) == 0 {
-			d.retryCount++
-			d.t.Reset(d.backoff.Backoff(d.retryCount))
-		} else {
-			d.retryCount = 0
-			d.t.Reset(d.freq)
-		}
-		d.cc.NewServiceConfig(sc)
-		d.cc.NewAddress(result)
+		state := d.lookup()
+		d.cc.UpdateState(*state)
 
 		// Sleep to prevent excessive re-resolutions. Incoming resolution requests
 		// will be queued in d.rn.
@@ -343,15 +278,17 @@ func (d *dnsResolver) lookupHost() []resolver.Address {
 	return newAddrs
 }
 
-func (d *dnsResolver) lookup() ([]resolver.Address, string) {
-	newAddrs := d.lookupSRV()
+func (d *dnsResolver) lookup() *resolver.State {
+	srv := d.lookupSRV()
+	state := &resolver.State{
+		Addresses: append(d.lookupHost(), srv...),
+	}
 	// Support fallback to non-balancer address.
-	newAddrs = append(newAddrs, d.lookupHost()...)
-	if d.disableServiceConfig {
-		return newAddrs, ""
+	if !d.disableServiceConfig {
+		sc := canaryingSC(d.lookupTXT())
+		state.ServiceConfig = d.cc.ParseServiceConfig(sc)
 	}
-	sc := d.lookupTXT()
-	return newAddrs, canaryingSC(sc)
+	return state
 }
 
 // formatIP returns ok = false if addr is not a valid textual representation of an IP address.