Reserve layer names for future use

Update layered jar support so that the name `ext` and any name
starting `springboot` are reserved.

See gh-19767

Author: philwebb

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/main/java/org/springframework/boot/loader/tools/Layer.java

@@ -41,6 +41,8 @@ public class Layer {
 	public Layer(String name) {
 		Assert.hasText(name, "Name must not be empty");
 		Assert.isTrue(PATTERN.matcher(name).matches(), "Malformed layer name '" + name + "'");
+		Assert.isTrue(!name.equalsIgnoreCase("ext") && !name.toLowerCase().startsWith("springboot"),
+				"Layer name '" + name + "' is reserved");
 		this.name = name;
 	}
 

spring-boot-project/spring-boot-tools/spring-boot-loader-tools/src/test/java/org/springframework/boot/loader/tools/LayerTests.java

@@ -59,4 +59,14 @@ void toStringReturnsName() {
 		assertThat(new Layer("test")).hasToString("test");
 	}
 
+	@Test
+	void createWhenUsingReservedNameThrowsException() {
+		assertThatIllegalArgumentException().isThrownBy(() -> new Layer("ext"))
+				.withMessage("Layer name 'ext' is reserved");
+		assertThatIllegalArgumentException().isThrownBy(() -> new Layer("ExT"))
+				.withMessage("Layer name 'ExT' is reserved");
+		assertThatIllegalArgumentException().isThrownBy(() -> new Layer("springbootloader"))
+				.withMessage("Layer name 'springbootloader' is reserved");
+	}
+
 }