resolver: update Resolver.Scheme() docstring to mention requirement of lowercase scheme names

clientconn.go

@@ -1527,9 +1527,12 @@ func (c *channelzChannel) ChannelzMetric() *channelz.ChannelInternalMetric {
 // referenced by users.
 var ErrClientConnTimeout = errors.New("grpc: timed out when dialing")
 
+// getResolver finds the scheme in the cc's resolvers or the global registry.
+// scheme should always be lowercase (typically by virtue of url.Parse()
+// performing proper RFC3986 behavior).
 func (cc *ClientConn) getResolver(scheme string) resolver.Builder {
 	for _, rb := range cc.dopts.resolvers {
-		if scheme == rb.Scheme() {
+		if scheme == strings.ToLower(rb.Scheme()) {
 			return rb
 		}
 	}

resolver/resolver.go

@@ -41,14 +41,15 @@ var (
 
 // TODO(bar) install dns resolver in init(){}.
 
-// Register registers the resolver builder to the resolver map. b.Scheme will be
-// used as the scheme registered with this builder.
+// Register registers the resolver builder to the resolver map. b.Scheme will
+// be used as the scheme registered with this builder. The registry is case
+// insensitive.
 //
 // NOTE: this function must only be called during initialization time (i.e. in
 // an init() function), and is not thread-safe. If multiple Resolvers are
 // registered with the same name, the one registered last will take effect.
 func Register(b Builder) {
-	m[b.Scheme()] = b
+	m[strings.ToLower(b.Scheme())] = b
 }
 
 // Get returns the resolver builder registered with the given scheme.
@@ -291,6 +292,8 @@ type Builder interface {
 	Build(target Target, cc ClientConn, opts BuildOptions) (Resolver, error)
 	// Scheme returns the scheme supported by this resolver.
 	// Scheme is defined at https://github.com/grpc/grpc/blob/master/doc/naming.md.
+	// The returned string should not contain upper-case characters, but will be
+	// converted by gRPC at runtime if needed.
 	Scheme() string
 }
 

resolver_test.go

@@ -0,0 +1,105 @@
+/*
+ *
+ * Copyright 2023 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 grpc
+
+import (
+	"context"
+	"fmt"
+	"net"
+	"testing"
+
+	"google.golang.org/grpc/credentials/insecure"
+	"google.golang.org/grpc/resolver"
+)
+
+type wrapResolverBuilder struct {
+	resolver.Builder
+	scheme string
+}
+
+func (w *wrapResolverBuilder) Scheme() string {
+	return w.scheme
+}
+
+func init() {
+	resolver.Register(&wrapResolverBuilder{Builder: resolver.Get("dns"), scheme: "caseTEST"})
+	resolver.Register(&wrapResolverBuilder{Builder: resolver.Get("dns"), scheme: "caseTest2"})
+	resolver.Register(&wrapResolverBuilder{Builder: resolver.Get("passthrough"), scheme: "caSetest2"})
+}
+
+func (s) TestResolverCaseSensitivity(t *testing.T) {
+	// This should find the "caseTEST" resolver, which is "dns"
+	target := "casetest:///localhost:1234"
+	cc, err := Dial(target, WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		t.Fatalf("Unexpected Dial(%q) error: %v", target, err)
+	}
+	cc.Close()
+
+	// This should find the "caSetest2" resolver instead of the "caseTest2"
+	// resolver, because it was registered later.  "caSetest2" is "passthrough"
+	// and the overwritten one is "dns".  With "passthrough" the dialer should
+	// see the target's address directly, but "dns" would be converted into a
+	// loopback IP (v4 or v6) address.
+	target = "caseTest2:///localhost:1234"
+	addrCh := make(chan string)
+	customDialer := func(ctx context.Context, addr string) (net.Conn, error) {
+		select {
+		case addrCh <- addr:
+		default:
+		}
+		return nil, fmt.Errorf("not dialing with custom dialer")
+	}
+
+	cc, err = Dial(target, WithContextDialer(customDialer), WithTransportCredentials(insecure.NewCredentials()))
+	if err != nil {
+		t.Fatalf("Unexpected Dial(%q) error: %v", target, err)
+	}
+	cc.Connect()
+	if got, want := <-addrCh, "localhost:1234"; got != want {
+		cc.Close()
+		t.Fatalf("Dialer got address %q; wanted %q", got, want)
+	}
+	cc.Close()
+
+	// Clear addrCh for future use.
+	select {
+	case <-addrCh:
+	default:
+	}
+
+	res := &wrapResolverBuilder{Builder: resolver.Get("dns"), scheme: "CaSetest2"}
+	// This should find the injected resolver instead of the "caSetest2"
+	// globally-registered resolver.  Since it is "dns" instead of passthrough,
+	// we can validate by checking for an address that has been resolved
+	// (i.e. is not "localhost:port").
+
+	// WithDisableServiceConfig disables TXT lookups, which can hang for
+	// "localhost".
+	cc, err = Dial(target, WithContextDialer(customDialer), WithResolvers(res),
+		WithTransportCredentials(insecure.NewCredentials()), WithDisableServiceConfig())
+	if err != nil {
+		t.Fatalf("Unexpected Dial(%q) error: %v", target, err)
+	}
+	cc.Connect()
+	defer cc.Close()
+	if got, wantNot := <-addrCh, "localhost:1234"; got == wantNot {
+		t.Fatalf("Dialer got address %q; wanted something other than %q", got, wantNot)
+	}
+}