feat(SpEL): Add applicationMetadata SpEL helper function (#3799)

Function to retrieve application metadata (from `front50`) via SpEL

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

orca-front50/src/main/groovy/com/netflix/spinnaker/orca/front50/pipeline/ApplicationExpressionFunctionProvider.java

@@ -0,0 +1,88 @@
+/*
+ * Copyright 2020 Netflix, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.netflix.spinnaker.orca.front50.pipeline;
+
+import com.fasterxml.jackson.core.type.TypeReference;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.netflix.spinnaker.kork.core.RetrySupport;
+import com.netflix.spinnaker.kork.expressions.ExpressionFunctionProvider;
+import com.netflix.spinnaker.kork.expressions.SpelHelperFunctionException;
+import com.netflix.spinnaker.orca.front50.Front50Service;
+import com.netflix.spinnaker.orca.front50.model.Application;
+import java.util.Map;
+import java.util.Optional;
+import org.jetbrains.annotations.NotNull;
+import org.jetbrains.annotations.Nullable;
+import org.springframework.stereotype.Component;
+
+@Component
+public class ApplicationExpressionFunctionProvider implements ExpressionFunctionProvider {
+  // Static because it's needed during expression eval (which is a static)
+  private static Front50Service front50Service = null;
+  private static ObjectMapper objectMapper = null;
+
+  ApplicationExpressionFunctionProvider(
+      ObjectMapper objectMapper, Optional<Front50Service> front50Service) {
+    front50Service.ifPresent(
+        service -> ApplicationExpressionFunctionProvider.front50Service = service);
+
+    ApplicationExpressionFunctionProvider.objectMapper = objectMapper;
+  }
+
+  @Nullable
+  @Override
+  public String getNamespace() {
+    return null;
+  }
+
+  @NotNull
+  @Override
+  public Functions getFunctions() {
+    return new Functions(
+        new FunctionDefinition(
+            "applicationMetadata",
+            "Get application metadata for a specific application",
+            new FunctionParameter(
+                String.class, "applicationName", "Application to get metadata for")));
+  }
+
+  /**
+   * Function to retrieve application metadata for a specific application
+   *
+   * @param applicationName the application name to look up metadata for
+   * @return returns a map with application metadata
+   */
+  public static Map<String, Object> applicationMetadata(String applicationName) {
+    if (front50Service == null) {
+      throw new SpelHelperFunctionException(
+          "front50 service is missing. It's required when using applicationMetadata function");
+    }
+
+    try {
+      RetrySupport retrySupport = new RetrySupport();
+      Application application =
+          retrySupport.retry(() -> front50Service.get(applicationName), 3, 1000, true);
+
+      return objectMapper.convertValue(application, new TypeReference<Map<String, Object>>() {});
+    } catch (Exception e) {
+      throw new SpelHelperFunctionException(
+          String.format(
+              "Application metadata for application '%s' could not be retrieved", applicationName),
+          e);
+    }
+  }
+}