[JENKINS-33843] Permit disabling optional dependencies (#4001)

* Implied dependencies on detached plugins were not being marked optional despite being listed in optionalDependencies, confusing the calculation of optionalDependents in resolveDependentPlugins.

* Javadoc improvements based on code exploration.

* [JENKINS-33843] Do not consider optional dependencies for purposes of disabling plugins.

* PluginManagerInstalledGUITest updated to reflect new behavior.

core/src/main/java/hudson/PluginWrapper.java

@@ -255,6 +255,7 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
 
     /**
      * Get the list of components that depend on this plugin.
+     * Note that the list will include elements of {@link #getOptionalDependents}.
      * @return The list of components that depend on this plugin.
      */
     public @Nonnull Set<String> getDependents() {
@@ -273,6 +274,16 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
         return getDependents();
     }
 
+    /**
+     * Like {@link #getDependents} but excluding optional dependencies.
+     * @since TODO
+     */
+    public @Nonnull Set<String> getMandatoryDependents() {
+        Set<String> s = new HashSet<>(dependents);
+        s.removeAll(optionalDependents);
+        return s;
+    }
+
     /**
      * @return The list of components that depend optionally on this plugin.
      */
@@ -290,6 +301,7 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
 
     /**
      * Does this plugin have anything that depends on it.
+     * Note that optional dependents are included.
      * @return {@code true} if something (Jenkins core, or another plugin) depends on this
      * plugin, otherwise {@code false}.
      */
@@ -297,6 +309,17 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
         return (isBundled || !dependents.isEmpty());
     }
 
+    /**
+     * Like {@link #hasDependents} but excluding optional dependencies.
+     * @since TODO
+     */
+    public boolean hasMandatoryDependents() {
+        if (isBundled) {
+            return true;
+        }
+        return dependents.stream().anyMatch(d -> !optionalDependents.contains(d));
+    }
+
     /**
      * @deprecated Please use {@link hasDependents}.
      */
@@ -324,10 +347,19 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
 
     /**
      * Does this plugin depend on any other plugins.
+     * Note that this include optional dependencies.
      * @return {@code true} if this plugin depends on other plugins, otherwise {@code false}.
      */
     public boolean hasDependencies() {
-        return (dependencies != null && !dependencies.isEmpty());
+        return !dependencies.isEmpty();
+    }
+
+    /**
+     * Like {@link #hasDependencies} but omitting optional dependencies.
+     * @since TODO
+     */
+    public boolean hasMandatoryDependencies() {
+        return dependencies.stream().anyMatch(d -> !d.optional);
     }
 
     @ExportedBean
@@ -394,6 +426,9 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
 		this.active = !disableFile.exists();
 		this.dependencies = dependencies;
 		this.optionalDependencies = optionalDependencies;
+        for (Dependency d : optionalDependencies) {
+            assert d.optional : d + " included among optionalDependencies of " + shortName + " but was not marked optional";
+        }
         this.archive = archive;
     }
 
@@ -438,11 +473,24 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
         return getBaseName(fileName);
     }
 
+    /**
+     * Gets all dependencies of this plugin on other plugins.
+     * Note that the list will <em>usually</em> include the members of {@link #getOptionalDependencies}
+     * (missing optional dependencies will however be omitted).
+     */
     @Exported
     public List<Dependency> getDependencies() {
         return dependencies;
     }
 
+    /**
+     * Like {@link #getDependencies} but omits optional dependencies.
+     * @since TODO
+     */
+    public List<Dependency> getMandatoryDependencies() {
+        return dependencies.stream().filter(d -> !d.optional).collect(Collectors.toList());
+    }
+
     public List<Dependency> getOptionalDependencies() {
         return optionalDependencies;
     }
@@ -753,7 +801,12 @@ public class PluginWrapper implements Comparable<PluginWrapper>, ModelObject {
     public void setHasCycleDependency(boolean hasCycle){
         hasCycleDependency = hasCycle;
     }
-    
+
+    /**
+     * Is this plugin bundled in the WAR?
+     * Normally false as noted in {@link PluginManager#loadBundledPlugins}:
+     * this does <em>not</em> apply to “detached” plugins.
+     */
     @Exported
     public boolean isBundled() {
         return isBundled;