Optional plugins missing from update center should not fail installat…

…ion (#296)

Co-authored-by: Tim Jacomb <21194782+timja@users.noreply.github.com>

plugin-management-library/src/main/java/io/jenkins/tools/pluginmanager/impl/PluginManager.java

@@ -912,16 +912,26 @@ public List<Plugin> resolveDependenciesFromJson(Plugin plugin, JSONObject plugin
             String pluginName = dependency.getString("name");
             String pluginVersion = dependency.getString("version");
             Plugin dependentPlugin = new Plugin(pluginName, pluginVersion, null, null);
-            if (useLatestSpecified && plugin.isLatest() || useLatestAll) {
-                VersionNumber latestPluginVersion = getLatestPluginVersion(pluginName);
-                dependentPlugin.setVersion(latestPluginVersion);
-                dependentPlugin.setLatest(true);
-            }
             dependentPlugin.setOptional(dependency.getBoolean("optional"));
-
-            dependentPlugins.add(dependentPlugin);
             dependentPlugin.setParent(plugin);
-        }
+
+            try {
+                if (useLatestSpecified && plugin.isLatest() || useLatestAll) {
+                    VersionNumber latestPluginVersion = getLatestPluginVersion(pluginName);
+                    dependentPlugin.setVersion(latestPluginVersion);
+                    dependentPlugin.setLatest(true);
+                }
+                dependentPlugins.add(dependentPlugin);
+            } catch (PluginNotFoundException e) {
+                if (!dependentPlugin.getOptional()) {
+                    throw e;
+                }
+                logVerbose(String.format(
+                            "Unable to find optional plugin %s in update center %s. " +
+                            "Ignoring until it becomes required.",
+                            pluginName, jenkinsUcLatest));
+            }
+    }
 
         logVerbose(dependentPlugins.isEmpty() ? String.format("%n%s has no dependencies", plugin.getName()) :
                 String.format("%n%s depends on: %n", plugin.getName()) +