net: make concurrent resolver lookups independent

The current resolver uses a global lookupGroup which merges LookupIPAddr
calls together for lookups for the same hostname if used concurrently.
As a result only one of the resolvers is actually used to perform the
DNS lookup but the result is shared by all the resolvers.

This commit limits the scope of the lookupGroup to the resolver itself
allowing each resolver to make its own requests without sharing the
result with other resolvers.

Fixes #22908

Change-Id: Ibba896eebb05e59f18ce4132564ea1f2b4b6c6d9
Reviewed-on: https://go-review.googlesource.com/80775
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/lookup.go

@@ -133,13 +133,25 @@ type Resolver struct {
 	// If nil, the default dialer is used.
 	Dial func(ctx context.Context, network, address string) (Conn, error)
 
+	// lookupGroup merges LookupIPAddr calls together for lookups for the same
+	// host. The lookupGroup key is the LookupIPAddr.host argument.
+	// The return values are ([]IPAddr, error).
+	lookupGroup singleflight.Group
+
 	// TODO(bradfitz): optional interface impl override hook
 	// TODO(bradfitz): Timeout time.Duration?
 }
 
 func (r *Resolver) preferGo() bool     { return r != nil && r.PreferGo }
 func (r *Resolver) strictErrors() bool { return r != nil && r.StrictErrors }
 
+func (r *Resolver) getLookupGroup() *singleflight.Group {
+	if r == nil {
+		return &DefaultResolver.lookupGroup
+	}
+	return &r.lookupGroup
+}
+
 // LookupHost looks up the given host using the local resolver.
 // It returns a slice of that host's addresses.
 func LookupHost(host string) (addrs []string, err error) {
@@ -204,7 +216,7 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 	lookupGroupCtx, lookupGroupCancel := context.WithCancel(context.Background())
 
 	dnsWaitGroup.Add(1)
-	ch, called := lookupGroup.DoChan(host, func() (interface{}, error) {
+	ch, called := r.getLookupGroup().DoChan(host, func() (interface{}, error) {
 		defer dnsWaitGroup.Done()
 		return testHookLookupIP(lookupGroupCtx, resolverFunc, host)
 	})
@@ -221,7 +233,7 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 		// let the lookup continue uncanceled, and let later
 		// lookups with the same key share the result.
 		// See issues 8602, 20703, 22724.
-		if lookupGroup.ForgetUnshared(host) {
+		if r.getLookupGroup().ForgetUnshared(host) {
 			lookupGroupCancel()
 		} else {
 			go func() {
@@ -244,12 +256,6 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 	}
 }
 
-// lookupGroup merges LookupIPAddr calls together for lookups
-// for the same host. The lookupGroup key is is the LookupIPAddr.host
-// argument.
-// The return values are ([]IPAddr, error).
-var lookupGroup singleflight.Group
-
 // lookupIPReturn turns the return values from singleflight.Do into
 // the return values from LookupIP.
 func lookupIPReturn(addrsi interface{}, err error, shared bool) ([]IPAddr, error) {

src/net/lookup_test.go

@@ -15,6 +15,7 @@ import (
 	"runtime"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 )
@@ -925,3 +926,59 @@ func TestLookupHostCancel(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+type lookupCustomResolver struct {
+	*Resolver
+	mu     sync.RWMutex
+	dialed bool
+}
+
+func (lcr *lookupCustomResolver) dial() func(ctx context.Context, network, address string) (Conn, error) {
+	return func(ctx context.Context, network, address string) (Conn, error) {
+		lcr.mu.Lock()
+		lcr.dialed = true
+		lcr.mu.Unlock()
+		return Dial(network, address)
+	}
+}
+
+// TestConcurrentPreferGoResolversDial tests that multiple resolvers with the
+// PreferGo option used concurrently are all dialed properly.
+func TestConcurrentPreferGoResolversDial(t *testing.T) {
+	// The windows implementation of the resolver does not use the Dial
+	// function.
+	if runtime.GOOS == "windows" {
+		t.Skip("skip on windows")
+	}
+
+	testenv.MustHaveExternalNetwork(t)
+	testenv.SkipFlakyNet(t)
+
+	defer dnsWaitGroup.Wait()
+
+	resolvers := make([]*lookupCustomResolver, 2)
+	for i := range resolvers {
+		cs := lookupCustomResolver{Resolver: &Resolver{PreferGo: true}}
+		cs.Dial = cs.dial()
+		resolvers[i] = &cs
+	}
+
+	var wg sync.WaitGroup
+	wg.Add(len(resolvers))
+	for i, resolver := range resolvers {
+		go func(r *Resolver, index int) {
+			defer wg.Done()
+			_, err := r.LookupIPAddr(context.Background(), "google.com")
+			if err != nil {
+				t.Fatalf("lookup failed for resolver %d: %q", index, err)
+			}
+		}(resolver.Resolver, i)
+	}
+	wg.Wait()
+
+	for i, resolver := range resolvers {
+		if !resolver.dialed {
+			t.Errorf("custom resolver %d not dialed during lookup", i)
+		}
+	}
+}