fix: fail build on WARN/ERROR log events via internal logging hook

Use Gradle 8.x’s internal logging API to abort the build if any log event of severity WARN or higher is emitted. This helps prevent harmless warnings from blocking Log4j adoption in strict environments.

⚠️ This approach relies on unsupported APIs and may break with Gradle 9.x, but it is currently the most effective option.

Author: ppkarwasz

settings.gradle

@@ -21,6 +21,42 @@ pluginManagement {
         mavenCentral()
     }
 }
+
+/*
+ * Fail the build if any WARN/ERROR log events occur.
+ * Some projects forbid *any* build-time logging at these levels;
+ * this ensures Log4j upgrades don't get blocked by harmless noise.
+ *
+ * See: https://github.com/apache/logging-log4j2/issues/3779
+ */
+import org.gradle.internal.logging.events.LogEvent
+import org.gradle.internal.logging.LoggingManagerInternal
+def events = []
+def listener = { e ->
+    if (e instanceof LogEvent && e.logLevel.ordinal() >= LogLevel.WARN.ordinal()) {
+        events << e
+    }
+}
+
+gradle.settingsEvaluated {
+    def loggingManager = gradle.services.get(LoggingManagerInternal)
+    loggingManager.addOutputEventListener(listener)
+    /*
+     * Note: This relies on Gradle’s internal logging APIs.
+     * It may break in Gradle 9.x or later, but that’s expected —
+     * using internal hooks is unsupported until Gradle provides a stable alternative.
+     */
+    gradle.buildFinished {
+        loggingManager.removeOutputEventListener(listener)
+        if (!events.isEmpty()) {
+            println "\n* Build failed due to ${events.size()} WARN/ERROR log event(s):"
+            events.each { println "[${it.logLevel}] ${it.message}" }
+            throw new GradleException("Build aborted: disallowed WARN/ERROR log events detected.")
+        }
+    }
+}
+
+
 dependencyResolutionManagement {
     repositoriesMode.set(RepositoriesMode.FAIL_ON_PROJECT_REPOS)
     repositories {