[JENKINS-48899] Do not downgrade detached plugins when upgrading Jenkins (#3229)

* Reproduce JENKINS-48899

* Do not downgrade installed detached plugins

core/src/main/java/hudson/PluginManager.java

@@ -706,13 +706,14 @@ public abstract class PluginManager extends AbstractModelObject implements OnMas
                     // If this was a plugin that was detached some time in the past i.e. not just one of the
                     // plugins that was bundled "for fun".
                     if (ClassicPluginStrategy.isDetachedPlugin(name)) {
-                        // If it's already installed and the installed version is older
-                        // than the bundled version, then we upgrade. The bundled version is the min required version
-                        // for "this" version of Jenkins, so we must upgrade.
                         VersionNumber installedVersion = getPluginVersion(rootDir, name);
                         VersionNumber bundledVersion = getPluginVersion(dir, name);
-                        if (installedVersion != null && bundledVersion != null && installedVersion.isOlderThan(bundledVersion)) {
-                            return true;
+                        // If the plugin is already installed, we need to decide whether to replace it with the bundled version.
+                        if (installedVersion != null && bundledVersion != null) {
+                            // If the installed version is older than the bundled version, then it MUST be upgraded.
+                            // If the installed version is newer than the bundled version, then it MUST NOT be upgraded.
+                            // If the versions are equal we just keep the installed version.
+                            return installedVersion.isOlderThan(bundledVersion);
                         }
                     }
 

test/src/test/java/jenkins/install/LoadDetachedPluginsTest.java

@@ -120,6 +120,25 @@ public class LoadDetachedPluginsTest {
         });
     }
 
+    @Issue("JENKINS-48899")
+    @Test
+    @LocalData
+    public void upgradeFromJenkins2WithNewerPlugin() {
+        // @LocalData has command-launcher 1.2 installed, which should not be downgraded to the detached version: 1.0.
+        VersionNumber since = new VersionNumber("2.0");
+        rr.then(r -> {
+            List<DetachedPlugin> detachedPlugins = ClassicPluginStrategy.getDetachedPlugins(since);
+            assertThat("Plugins have been detached since the pre-upgrade version",
+                    detachedPlugins.size(), greaterThan(1));
+            assertThat("Plugins detached between the pre-upgrade version and the current version should be installed",
+                    getInstalledDetachedPlugins(r, detachedPlugins).size(), equalTo(detachedPlugins.size()));
+            Plugin commandLauncher = r.jenkins.getPlugin("command-launcher");
+            assertThat("Installed detached plugins should not be overwritten by older versions",
+                    commandLauncher.getWrapper().getVersionNumber(), equalTo(new VersionNumber("1.2")));
+            assertNoFailedPlugins(r);
+        });
+    }
+
     @Test
     public void newInstallation() {
         rr.then(r -> {