net: treat a nil *Resolver as a zero one, as documented

Add accessors that handle nil without crashing.

Fixes #24330

Change-Id: If5fbbb6015ca8d65f620a06bad6e52de8cd896ad
Reviewed-on: https://go-review.googlesource.com/101315
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>

src/net/conf.go

@@ -126,7 +126,7 @@ func (c *conf) hostLookupOrder(r *Resolver, hostname string) (ret hostLookupOrde
 		}()
 	}
 	fallbackOrder := hostLookupCgo
-	if c.netGo || (r != nil && r.PreferGo) {
+	if c.netGo || r.preferGo() {
 		fallbackOrder = hostLookupFilesDNS
 	}
 	if c.forceCgoLookupHost || c.resolv.unknownOpt || c.goos == "android" {

src/net/dnsclient_unix.go

@@ -377,7 +377,7 @@ func (r *Resolver) lookup(ctx context.Context, name string, qtype dnsmessage.Typ
 		if err == nil {
 			break
 		}
-		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.StrictErrors {
+		if nerr, ok := err.(Error); ok && nerr.Temporary() && r.strictErrors() {
 			// If we hit a temporary error with StrictErrors enabled,
 			// stop immediately instead of trying more names.
 			break
@@ -565,7 +565,7 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 		for range qtypes {
 			racer := <-lane
 			if racer.error != nil {
-				if nerr, ok := racer.error.(Error); ok && nerr.Temporary() && r.StrictErrors {
+				if nerr, ok := racer.error.(Error); ok && nerr.Temporary() && r.strictErrors() {
 					// This error will abort the nameList loop.
 					hitStrictError = true
 					lastErr = racer.error

src/net/lookup.go

@@ -137,6 +137,9 @@ type Resolver struct {
 	// 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 }
+
 // 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) {

src/net/lookup_test.go

@@ -814,3 +814,27 @@ func TestLookupContextCancel(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+// Issue 24330: treat the nil *Resolver like a zero value. Verify nothing
+// crashes if nil is used.
+func TestNilResolverLookup(t *testing.T) {
+	if testenv.Builder() == "" {
+		testenv.MustHaveExternalNetwork(t)
+	}
+	if runtime.GOOS == "nacl" {
+		t.Skip("skip on nacl")
+	}
+	var r *Resolver = nil
+	ctx := context.Background()
+
+	// Don't care about the results, just that nothing panics:
+	r.LookupAddr(ctx, "8.8.8.8")
+	r.LookupCNAME(ctx, "google.com")
+	r.LookupHost(ctx, "google.com")
+	r.LookupIPAddr(ctx, "google.com")
+	r.LookupMX(ctx, "gmail.com")
+	r.LookupNS(ctx, "google.com")
+	r.LookupPort(ctx, "tcp", "smtp")
+	r.LookupSRV(ctx, "service", "proto", "name")
+	r.LookupTXT(ctx, "gmail.com")
+}

src/net/lookup_unix.go

@@ -61,7 +61,7 @@ func (r *Resolver) dial(ctx context.Context, network, server string) (Conn, erro
 	// addresses, which Dial will use without a DNS lookup.
 	var c Conn
 	var err error
-	if r.Dial != nil {
+	if r != nil && r.Dial != nil {
 		c, err = r.Dial(ctx, network, server)
 	} else {
 		var d Dialer
@@ -75,7 +75,7 @@ func (r *Resolver) dial(ctx context.Context, network, server string) (Conn, erro
 
 func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string, err error) {
 	order := systemConf().hostLookupOrder(r, host)
-	if !r.PreferGo && order == hostLookupCgo {
+	if !r.preferGo() && order == hostLookupCgo {
 		if addrs, err, ok := cgoLookupHost(ctx, host); ok {
 			return addrs, err
 		}
@@ -86,7 +86,7 @@ func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string,
 }
 
 func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
-	if r.PreferGo {
+	if r.preferGo() {
 		return r.goLookupIP(ctx, host)
 	}
 	order := systemConf().hostLookupOrder(r, host)
@@ -102,7 +102,7 @@ func (r *Resolver) lookupIP(ctx context.Context, host string) (addrs []IPAddr, e
 }
 
 func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int, error) {
-	if !r.PreferGo && systemConf().canUseCgo() {
+	if !r.preferGo() && systemConf().canUseCgo() {
 		if port, err, ok := cgoLookupPort(ctx, network, service); ok {
 			if err != nil {
 				// Issue 18213: if cgo fails, first check to see whether we
@@ -118,7 +118,7 @@ func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int
 }
 
 func (r *Resolver) lookupCNAME(ctx context.Context, name string) (string, error) {
-	if !r.PreferGo && systemConf().canUseCgo() {
+	if !r.preferGo() && systemConf().canUseCgo() {
 		if cname, err, ok := cgoLookupCNAME(ctx, name); ok {
 			return cname, err
 		}
@@ -308,7 +308,7 @@ func (r *Resolver) lookupTXT(ctx context.Context, name string) ([]string, error)
 }
 
 func (r *Resolver) lookupAddr(ctx context.Context, addr string) ([]string, error) {
-	if !r.PreferGo && systemConf().canUseCgo() {
+	if !r.preferGo() && systemConf().canUseCgo() {
 		if ptrs, err, ok := cgoLookupPTR(ctx, addr); ok {
 			return ptrs, err
 		}

src/net/lookup_windows.go

@@ -136,7 +136,7 @@ func (r *Resolver) lookupIP(ctx context.Context, name string) ([]IPAddr, error)
 }
 
 func (r *Resolver) lookupPort(ctx context.Context, network, service string) (int, error) {
-	if r.PreferGo {
+	if r.preferGo() {
 		return lookupPortMap(network, service)
 	}