grpcutil: add ValidateTargetURI function

Fixes #8747

Add an internal API to validate gRPC target URI strings. The function
checks that the target parses as a valid RFC 3986 URI and that a
resolver is registered for its scheme.

RELEASE NOTES: n/a

Author: HueCodes

internal/grpcutil/target.go

@@ -0,0 +1,45 @@
+/*
+ *
+ * Copyright 2025 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 grpcutil
+
+import (
+	"fmt"
+	"net/url"
+
+	"google.golang.org/grpc/resolver"
+)
+
+// ValidateTargetURI validates that target is a valid RFC 3986 URI
+// and that a resolver is registered for its scheme.
+func ValidateTargetURI(target string) error {
+	u, err := url.Parse(target)
+	if err != nil {
+		return fmt.Errorf("invalid target URI %q: %w", target, err)
+	}
+
+	if u.Scheme == "" {
+		return fmt.Errorf("target URI %q has no scheme", target)
+	}
+
+	if resolver.Get(u.Scheme) == nil {
+		return fmt.Errorf("no resolver registered for scheme %q", u.Scheme)
+	}
+
+	return nil
+}

internal/grpcutil/target_test.go

@@ -0,0 +1,86 @@
+/*
+ *
+ * Copyright 2025 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 grpcutil
+
+import (
+	"strings"
+	"testing"
+
+	_ "google.golang.org/grpc/resolver/dns"         // Register dns resolver
+	_ "google.golang.org/grpc/resolver/passthrough" // Register passthrough resolver
+)
+
+func TestValidateTargetURI(t *testing.T) {
+	tests := []struct {
+		name       string
+		target     string
+		wantErr    bool
+		errContain string
+	}{
+		{
+			name:    "valid dns scheme",
+			target:  "dns:///example.com:443",
+			wantErr: false,
+		},
+		{
+			name:    "valid passthrough scheme",
+			target:  "passthrough:///localhost:8080",
+			wantErr: false,
+		},
+		{
+			name:       "missing scheme",
+			target:     "/path/to/socket",
+			wantErr:    true,
+			errContain: "has no scheme",
+		},
+		{
+			name:       "host:port parsed as scheme",
+			target:     "example.com:443",
+			wantErr:    true,
+			errContain: "no resolver registered for scheme",
+		},
+		{
+			name:       "unregistered scheme",
+			target:     "unknown:///example.com:443",
+			wantErr:    true,
+			errContain: "no resolver registered for scheme",
+		},
+		{
+			name:       "invalid URI with control character",
+			target:     "dns:///example\x00.com",
+			wantErr:    true,
+			errContain: "invalid target URI",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := ValidateTargetURI(tt.target)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("ValidateTargetURI(%q) error = %v, wantErr %v", tt.target, err, tt.wantErr)
+				return
+			}
+			if tt.wantErr && tt.errContain != "" {
+				if !strings.Contains(err.Error(), tt.errContain) {
+					t.Errorf("ValidateTargetURI(%q) error = %v, want error containing %q", tt.target, err, tt.errContain)
+				}
+			}
+		})
+	}
+}