Enable caching for system tests in CI

Setting the `systemTest` Gradle task output as never up-to-date ensures
that all system tests are executed each time they are run in CI. The
`--rerun-tasks` Gradle option that was used previously had the same
effect but also disabled build caching.

Closes gh-29029

buildSrc/src/main/java/org/springframework/boot/build/test/SystemTestPlugin.java

@@ -18,8 +18,10 @@ package org.springframework.boot.build.test;
 
 import org.gradle.api.Plugin;
 import org.gradle.api.Project;
+import org.gradle.api.Task;
 import org.gradle.api.plugins.JavaPlugin;
 import org.gradle.api.plugins.JavaPluginConvention;
+import org.gradle.api.specs.Spec;
 import org.gradle.api.tasks.SourceSet;
 import org.gradle.api.tasks.SourceSetContainer;
 import org.gradle.api.tasks.testing.Test;
@@ -35,6 +37,8 @@ import org.gradle.plugins.ide.eclipse.model.EclipseModel;
  */
 public class SystemTestPlugin implements Plugin<Project> {
 
+	private static final Spec<Task> NEVER = (task) -> false;
+
 	/**
 	 * Name of the {@code systemTest} task.
 	 */
@@ -78,6 +82,13 @@ public class SystemTestPlugin implements Plugin<Project> {
 		systemTest.setTestClassesDirs(systemTestSourceSet.getOutput().getClassesDirs());
 		systemTest.setClasspath(systemTestSourceSet.getRuntimeClasspath());
 		systemTest.shouldRunAfter(JavaPlugin.TEST_TASK_NAME);
+		if (isCi()) {
+			systemTest.getOutputs().upToDateWhen(NEVER);
+		}
+	}
+
+	private boolean isCi() {
+		return Boolean.parseBoolean(System.getenv("CI"));
 	}
 
 }

ci/scripts/run-system-tests.sh

@@ -5,8 +5,8 @@ source $(dirname $0)/common.sh
 
 pushd git-repo > /dev/null
 if [[ -d /opt/openjdk-toolchain ]]; then
-  ./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false --no-daemon --max-workers=4 --rerun-tasks systemTest -PtoolchainVersion=${TOOLCHAIN_JAVA_VERSION} -Porg.gradle.java.installations.auto-detect=false -Porg.gradle.java.installations.auto-download=false -Porg.gradle.java.installations.paths=/opt/openjdk-toolchain/
+  ./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false --no-daemon --max-workers=4 systemTest -PtoolchainVersion=${TOOLCHAIN_JAVA_VERSION} -Porg.gradle.java.installations.auto-detect=false -Porg.gradle.java.installations.auto-download=false -Porg.gradle.java.installations.paths=/opt/openjdk-toolchain/
 else
-  ./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false --no-daemon --max-workers=4 --rerun-tasks systemTest 
+  ./gradlew -Dorg.gradle.internal.launcher.welcomeMessageEnabled=false --no-daemon --max-workers=4 systemTest
 fi
 popd > /dev/null