Don't throw NettyWebServer on permission errors

Update `NettyWebServer` so that the `PortInUseException` is not thrown
for permission denied errors.

Fixes gh-19807

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/embedded/netty/NettyWebServer.java

@@ -18,6 +18,7 @@ package org.springframework.boot.web.embedded.netty;
 
 import java.time.Duration;
 
+import io.netty.channel.unix.Errors.NativeIoException;
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 import reactor.netty.ChannelBindException;
@@ -42,6 +43,11 @@ import org.springframework.util.Assert;
  */
 public class NettyWebServer implements WebServer {
 
+	/**
+	 * Permission denied error code from {@code errno.h}.
+	 */
+	private static final int ERROR_NO_EACCES = -13;
+
 	private static final Log logger = LogFactory.getLog(NettyWebServer.class);
 
 	private final HttpServer httpServer;
@@ -68,7 +74,7 @@ public class NettyWebServer implements WebServer {
 			}
 			catch (Exception ex) {
 				ChannelBindException bindException = findBindException(ex);
-				if (bindException != null) {
+				if (bindException != null && !isPermissionDenied(bindException.getCause())) {
 					throw new PortInUseException(bindException.localPort(), ex);
 				}
 				throw new WebServerException("Unable to start Netty", ex);
@@ -78,6 +84,17 @@ public class NettyWebServer implements WebServer {
 		}
 	}
 
+	private boolean isPermissionDenied(Throwable bindExceptionCause) {
+		try {
+			if (bindExceptionCause instanceof NativeIoException) {
+				return ((NativeIoException) bindExceptionCause).expectedErr() == ERROR_NO_EACCES;
+			}
+		}
+		catch (Throwable ex) {
+		}
+		return false;
+	}
+
 	private DisposableServer startHttpServer() {
 		if (this.lifecycleTimeout != null) {
 			return this.httpServer.handle(this.handlerAdapter).bindNow(this.lifecycleTimeout);