[JENKINS-47426] getPropertyKey is not consistent over time (#3080)

* JENKINS-47426: getPropertyKey is not consistent over time
- add passing test for the case the rootUrl is set
- add failing test for the case the rootUrl is not set yet, the propertyKey is not consistent
- add a warning message when the flow passes in the fallback mode

* - fixing the getEncryptedValue using a static memory

* - add issue annotations

* - modifications proposed by Jesse: use legacyId instead of randomId, stable over restart

* - small correction for a test case

core/src/main/java/hudson/cli/ClientAuthenticationCache.java

@@ -38,7 +38,7 @@ public class ClientAuthenticationCache implements Serializable {
 
     private static final HMACConfidentialKey MAC = new HMACConfidentialKey(ClientAuthenticationCache.class, "MAC");
     private static final Logger LOGGER = Logger.getLogger(ClientAuthenticationCache.class.getName());
-
+    
     /**
      * Where the store should be placed.
      */
@@ -110,9 +110,12 @@ public class ClientAuthenticationCache implements Serializable {
      */
     @VisibleForTesting
     String getPropertyKey() {
-        String url = Jenkins.getActiveInstance().getRootUrl();
+        Jenkins j = Jenkins.getActiveInstance();
+        String url = j.getRootUrl();
         if (url!=null)  return url;
-        return Secret.fromString("key").getEncryptedValue();
+        
+        LOGGER.log(Level.WARNING, "The instance is not configured using a rootUrl, the key that represents your instance will not be stable");
+        return j.getLegacyInstanceId();
     }
 
     /**

test/src/test/java/hudson/cli/ClientAuthenticationCacheTest.java

@@ -112,7 +112,25 @@ public class ClientAuthenticationCacheTest {
         assertEquals(r.getURL().toString(), cache.getPropertyKey());
         JenkinsLocationConfiguration.get().setUrl(null);
         String key = cache.getPropertyKey();
-        assertTrue(key, Secret.decrypt(key) != null);
+        assertEquals(r.jenkins.getLegacyInstanceId(), key);
+    }
+    
+    @Test
+    @Issue("JENKINS-47426")
+    public void getPropertyKey_mustBeEquivalentOverTime() throws Exception {
+        ClientAuthenticationCache cache = new ClientAuthenticationCache(null);
+
+        String key1 = cache.getPropertyKey();
+        String key2 = cache.getPropertyKey();
+
+        assertEquals("Two calls to the getPropertyKey() must be equivalent over time, with rootUrl", key1, key2);
+
+        JenkinsLocationConfiguration.get().setUrl(null);
+
+        key1 = cache.getPropertyKey();
+        key2 = cache.getPropertyKey();
+
+        assertEquals("Two calls to the getPropertyKey() must be equivalent over time, without rootUrl", key1, key2);
     }
 
     private void assertCLI(int code, @CheckForNull String output, File jar, String... args) throws Exception {