Add logic to convert from Legacy Events to CloudEvents. (GoogleCloudPlatform#61)

* Add explanatory comments to the new code in BackgroundFunctionExecutor.

* Add logic to convert from Legacy Events to CloudEvents.

This allows received Legacy Events to be handled by function code that expects a
CloudEvent input. The translation logic and tests are based closely on the
corresponding code in https://github.com/GoogleCloudPlatform/functions-framework-dotnet.

The new code also passes the relevant conformance tests in
https://github.com/GoogleCloudPlatform/functions-framework-conformance, once
GoogleCloudPlatform/functions-framework-conformance#30
is applied. For now I'm running those tests manually. I plan at least to commit
the script and test functions that allow me to do so.

Author: eamonnmcmanus

invoker/core/src/main/java/com/google/cloud/functions/invoker/BackgroundFunctionExecutor.java

@@ -32,6 +32,7 @@
 import io.cloudevents.core.message.impl.UnknownEncodingMessageReader;
 import java.io.BufferedReader;
 import java.io.IOException;
+import java.io.Reader;
 import java.lang.reflect.Type;
 import java.time.OffsetDateTime;
 import java.time.format.DateTimeFormatter;
@@ -171,17 +172,21 @@ static Optional<Type> backgroundFunctionTypeArgument(
 
   private static Event parseLegacyEvent(HttpServletRequest req) throws IOException {
     try (BufferedReader bodyReader = req.getReader()) {
-      // A Type Adapter is required to set the type of the JsonObject because CloudFunctionsContext
-      // is abstract and Gson default behavior instantiates the type provided.
-      TypeAdapter<CloudFunctionsContext> typeAdapter =
-          CloudFunctionsContext.typeAdapter(new Gson());
-      Gson gson = new GsonBuilder()
-          .registerTypeAdapter(CloudFunctionsContext.class, typeAdapter)
-          .registerTypeAdapter(Event.class, new Event.EventDeserializer())
-          .create();
-      return gson.fromJson(bodyReader, Event.class);
+      return parseLegacyEvent(bodyReader);
     }
   }
+  
+  static Event parseLegacyEvent(Reader reader) throws IOException {
+    // A Type Adapter is required to set the type of the JsonObject because CloudFunctionsContext
+    // is abstract and Gson default behavior instantiates the type provided.
+    TypeAdapter<CloudFunctionsContext> typeAdapter =
+        CloudFunctionsContext.typeAdapter(new Gson());
+    Gson gson = new GsonBuilder()
+        .registerTypeAdapter(CloudFunctionsContext.class, typeAdapter)
+        .registerTypeAdapter(Event.class, new Event.EventDeserializer())
+        .create();
+    return gson.fromJson(reader, Event.class);
+  }
 
   private static Context contextFromCloudEvent(CloudEvent cloudEvent) {
     OffsetDateTime timestamp = Optional.ofNullable(cloudEvent.getTime()).orElse(OffsetDateTime.now());
@@ -291,7 +296,7 @@ void serviceCloudEvent(CloudEvent cloudEvent) throws Exception {
     }
   }
 
-  private static class CloudEventFunctionExecutor extends FunctionExecutor<Void>{
+  private static class CloudEventFunctionExecutor extends FunctionExecutor<Void> {
     private final ExperimentalCloudEventsFunction function;
 
     CloudEventFunctionExecutor(ExperimentalCloudEventsFunction function) {
@@ -301,8 +306,8 @@ private static class CloudEventFunctionExecutor extends FunctionExecutor<Void>{
 
     @Override
     void serviceLegacyEvent(Event legacyEvent) throws Exception {
-      throw new UnsupportedOperationException(
-          "Conversion from legacy events to CloudEvents not yet implemented");
+      CloudEvent cloudEvent = GcfEvents.convertToCloudEvent(legacyEvent);
+      function.accept(cloudEvent);
     }
 
     @Override

invoker/core/src/main/java/com/google/cloud/functions/invoker/CloudFunctionsContext.java

@@ -17,6 +17,7 @@
 import com.google.auto.value.AutoValue;
 import com.google.cloud.functions.Context;
 import com.google.gson.Gson;
+import com.google.gson.GsonBuilder;
 import com.google.gson.TypeAdapter;
 import java.lang.annotation.Retention;
 import java.lang.annotation.RetentionPolicy;
@@ -46,6 +47,9 @@ abstract class CloudFunctionsContext implements Context {
   @Nullable
   public abstract String resource();
 
+  // TODO: expose this in the Context interface (as a default method).
+  abstract Map<String, String> params();
+
   @Override
   public abstract Map<String, String> attributes();
 
@@ -55,6 +59,7 @@ public static TypeAdapter<CloudFunctionsContext> typeAdapter(Gson gson) {
 
   static Builder builder() {
     return new AutoValue_CloudFunctionsContext.Builder()
+        .setParams(Collections.emptyMap())
         .setAttributes(Collections.emptyMap());
   }
 
@@ -64,8 +69,51 @@ abstract static class Builder {
     abstract Builder setTimestamp(String x);
     abstract Builder setEventType(String x);
     abstract Builder setResource(String x);
+    abstract Builder setParams(Map<String, String> x);
     abstract Builder setAttributes(Map<String, String> value);
 
     abstract CloudFunctionsContext build();
   }
+
+  /**
+   * Depending on the event type, the {@link Context#resource()} field is either a JSON string (complete
+   * with encosing quotes) or a JSON object. This class allows us to redeserialize that JSON representation
+   * into its components.
+   */
+  @AutoValue
+  abstract static class Resource {
+    abstract @Nullable String service();
+    abstract String name();
+    abstract @Nullable String type();
+
+    static TypeAdapter<Resource> typeAdapter(Gson gson) {
+      return new AutoValue_CloudFunctionsContext_Resource.GsonTypeAdapter(gson);
+    }
+
+    static Resource from(String s) {
+      Gson baseGson = new Gson();
+      if (s.startsWith("\"") && s.endsWith("\"")) {
+        String name = baseGson.fromJson(s, String.class);
+        return builder().setName(name).build();
+      }
+      if (s.startsWith("{") && (s.endsWith("}") || s.endsWith("}\n"))) {
+        TypeAdapter<Resource> typeAdapter = typeAdapter(baseGson);
+        Gson gson = new GsonBuilder().registerTypeAdapter(Resource.class, typeAdapter).create();
+        return gson.fromJson(s, Resource.class);
+      }
+      throw new IllegalArgumentException("Unexpected resource syntax: " + s);
+    }
+
+    static Builder builder() {
+      return new AutoValue_CloudFunctionsContext_Resource.Builder();
+    }
+
+    @AutoValue.Builder
+    abstract static class Builder {
+      abstract Builder setService(String x);
+      abstract Builder setName(String x);
+      abstract Builder setType(String x);
+      abstract Resource build();
+    }
+  }
 }

invoker/core/src/main/java/com/google/cloud/functions/invoker/Event.java

@@ -14,6 +14,7 @@
 
 package com.google.cloud.functions.invoker;
 
+import com.google.auto.value.AutoValue;
 import com.google.gson.JsonDeserializationContext;
 import com.google.gson.JsonDeserializer;
 import com.google.gson.JsonElement;
@@ -25,23 +26,15 @@
  * Represents an event that should be handled by a background function. This is an internal format
  * which is later converted to actual background function parameter types.
  */
-class Event {
-
-  private JsonElement data;
-  private CloudFunctionsContext context;
-
-  Event(JsonElement data, CloudFunctionsContext context) {
-    this.data = data;
-    this.context = context;
+@AutoValue
+abstract class Event {
+  static Event of(JsonElement data, CloudFunctionsContext context) {
+    return new AutoValue_Event(data, context);
   }
 
-  JsonElement getData() {
-    return data;
-  }
+  abstract JsonElement getData();
 
-  CloudFunctionsContext getContext() {
-    return context;
-  }
+  abstract CloudFunctionsContext getContext();
 
   /** Custom deserializer that supports both GCF beta and GCF GA event formats. */
   static class EventDeserializer implements JsonDeserializer<Event> {
@@ -67,7 +60,7 @@ public Event deserialize(
             jsonDeserializationContext.deserialize(
                 adjustContextResource(rootCopy), CloudFunctionsContext.class);
       }
-      return new Event(data, context);
+      return Event.of(data, context);
     }
 
     /**