Fix IllegalStateException when cache is disabled via command line (#424)

Author: cowwoc

src/main/java/org/apache/maven/buildcache/BuildCacheMojosExecutionStrategy.java

@@ -152,11 +152,12 @@ public void execute(
             }
 
             try {
-                if (!restored && !forkedExecution) {
+                if (cacheState == INITIALIZED && !restored && !forkedExecution) {
                     // Move pre-existing artifacts to staging directory to prevent caching stale files
                     // from previous builds (e.g., after source changes or from cache restored
                     // with clock skew). This ensures save() only sees fresh files built during this session.
                     // Skip for forked executions since they don't cache and shouldn't modify artifacts.
+                    // Skip when cache is disabled to avoid accessing uninitialized cache configuration.
                     try {
                         cacheController.stagePreExistingArtifacts(session, project);
                     } catch (IOException e) {
@@ -193,7 +194,8 @@ public void execute(
                 // Always restore staged files after build completes (whether save ran or not).
                 // Files that were rebuilt are discarded; files that weren't rebuilt are restored.
                 // Skip for forked executions since they don't stage artifacts.
-                if (!restored && !forkedExecution) {
+                // Skip when cache is disabled since staging was not performed.
+                if (cacheState == INITIALIZED && !restored && !forkedExecution) {
                     cacheController.restoreStagedArtifacts(session, project);
                 }
             }

src/test/java/org/apache/maven/buildcache/its/SkipBuildExtensionTest.java

@@ -18,6 +18,7 @@
  */
 package org.apache.maven.buildcache.its;
 
+import java.util.Arrays;
 import java.util.List;
 
 import org.apache.maven.buildcache.its.junit.IntegrationTest;
@@ -56,6 +57,29 @@ void multipleGoals(Verifier verifier) throws VerificationException {
         verifyNoTextInLog(verifier, "Build cache is disabled for 'clean' goal.");
     }
 
+    /**
+     * Verifies that running with -Dmaven.build.cache.enabled=false does not cause
+     * IllegalStateException and the build completes successfully.
+     * <p>
+     * This tests the fix for the regression where stagePreExistingArtifacts() was called
+     * without checking if the cache was initialized, causing IllegalStateException when
+     * cache is disabled via command line.
+     *
+     * @see <a href="https://github.com/apache/maven-build-cache-extension/pull/394#issuecomment-3714680789">PR #394 comment</a>
+     */
+    @Test
+    void cacheDisabledViaCommandLine(Verifier verifier) throws VerificationException {
+        verifier.setAutoclean(false);
+        verifier.addCliOption("-Dmaven.build.cache.enabled=false");
+
+        verifier.setLogFileName("../log-cache-disabled.txt");
+        verifier.executeGoals(Arrays.asList("clean", "install"));
+        verifier.verifyErrorFreeLog();
+
+        // Verify cache was actually disabled
+        verifier.verifyTextInLog("Cache disabled by command line flag");
+    }
+
     private static void verifyNoTextInLog(Verifier verifier, String text) throws VerificationException {
         Assertions.assertNull(findFirstLineContainingTextsInLogs(verifier, text));
     }