passthrough: return error if endpoint is empty and opt.Dialer is nil when building resolver (grpc#5732)

Author: huangchong94

clientconn_parsed_target_test.go

@@ -40,7 +40,6 @@ func (s) TestParsedTarget_Success_WithoutCustomDialer(t *testing.T) {
 		wantParsed resolver.Target
 	}{
 		// No scheme is specified.
-		{target: "", badScheme: true, wantParsed: resolver.Target{Scheme: defScheme, Authority: "", Endpoint: ""}},
 		{target: "://", badScheme: true, wantParsed: resolver.Target{Scheme: defScheme, Authority: "", Endpoint: "://"}},
 		{target: ":///", badScheme: true, wantParsed: resolver.Target{Scheme: defScheme, Authority: "", Endpoint: ":///"}},
 		{target: "://a/", badScheme: true, wantParsed: resolver.Target{Scheme: defScheme, Authority: "", Endpoint: "://a/"}},
@@ -110,6 +109,7 @@ func (s) TestParsedTarget_Success_WithoutCustomDialer(t *testing.T) {
 
 func (s) TestParsedTarget_Failure_WithoutCustomDialer(t *testing.T) {
 	targets := []string{
+		"",
 		"unix://a/b/c",
 		"unix://authority",
 		"unix-abstract://authority/a/b/c",
@@ -179,6 +179,12 @@ func (s) TestParsedTarget_WithCustomDialer(t *testing.T) {
 			wantParsed:        resolver.Target{Scheme: defScheme, Authority: "", Endpoint: "/unix/socket/address"},
 			wantDialerAddress: "/unix/socket/address",
 		},
+		{
+			target:            "",
+			badScheme:         true,
+			wantParsed:        resolver.Target{Scheme: defScheme, Authority: "", Endpoint: ""},
+			wantDialerAddress: "",
+		},
 		{
 			target:            "passthrough://a.server.com/google.com",
 			wantParsed:        resolver.Target{Scheme: "passthrough", Authority: "a.server.com", Endpoint: "google.com"},

internal/resolver/passthrough/passthrough.go

@@ -20,13 +20,20 @@
 // name without scheme back to gRPC as resolved address.
 package passthrough
 
-import "google.golang.org/grpc/resolver"
+import (
+	"errors"
+
+	"google.golang.org/grpc/resolver"
+)
 
 const scheme = "passthrough"
 
 type passthroughBuilder struct{}
 
 func (*passthroughBuilder) Build(target resolver.Target, cc resolver.ClientConn, opts resolver.BuildOptions) (resolver.Resolver, error) {
+	if target.Endpoint == "" && opts.Dialer == nil {
+		return nil, errors.New("passthrough: received empty target in Build()")
+	}
 	r := &passthroughResolver{
 		target: target,
 		cc:     cc,