Implement the GitHub plugin adapter (#461)

Summary:
This enables grabbing the GitHub relational view from disk and
converting it to a graph on the client.

Paired with @decentralion.

Test Plan:
For testing, information about the GitHub graph is printed when you
click the “Load data” button in the UI. Do so.

wchargin-branch: github-plugin-adapter

src/v3/app/credExplorer/App.js

@@ -4,6 +4,7 @@ import React from "react";
 import {StyleSheet, css} from "aphrodite/no-important";
 
 import LocalStore from "./LocalStore";
+import {createPluginAdapter as createGithubAdapter} from "../../plugins/github/pluginAdapter";
 
 type Props = {};
 type State = {
@@ -81,7 +82,12 @@ export default class App extends React.Component<Props, State> {
       console.error(`Invalid repository name: ${JSON.stringify(repoName)}`);
       return;
     }
-    console.log(`Would load data for: ${repoOwner}/${repoName}.`);
+    createGithubAdapter(repoOwner, repoName).then((githubAdapter) => {
+      const graph = githubAdapter.graph();
+      const nodeCount = Array.from(graph.nodes()).length;
+      const edgeCount = Array.from(graph.edges()).length;
+      console.log(`Loaded graph: ${nodeCount} nodes, ${edgeCount} edges.`);
+    });
   }
 }
 

src/v3/app/pluginAdapter.js

@@ -0,0 +1,13 @@
+// @flow
+
+import type {Graph, NodeAddressT} from "../core/graph";
+
+export interface Renderer {
+  nodeDescription(NodeAddressT): string;
+}
+
+export interface PluginAdapter {
+  graph(): Graph;
+  renderer(): Renderer;
+  nodePrefix(): NodeAddressT;
+}

src/v3/plugins/github/pluginAdapter.js

@@ -0,0 +1,60 @@
+// @flow
+import type {
+  PluginAdapter as IPluginAdapter,
+  Renderer as IRenderer,
+} from "../../app/pluginAdapter";
+import {type Graph, NodeAddress} from "../../core/graph";
+import {createGraph} from "./createGraph";
+import * as N from "./nodes";
+import {RelationalView} from "./relationalView";
+import {description} from "./render";
+
+export async function createPluginAdapter(
+  repoOwner: string,
+  repoName: string
+): Promise<IPluginAdapter> {
+  const url = `/api/v1/data/data/${repoOwner}/${repoName}/github/view.json`;
+  const response = await fetch(url);
+  if (!response.ok) {
+    return Promise.reject(response);
+  }
+  const json = await response.json();
+  const view = RelationalView.fromJSON(json);
+  const graph = createGraph(view);
+  return new PluginAdapter(view, graph);
+}
+
+class PluginAdapter implements IPluginAdapter {
+  +_view: RelationalView;
+  +_graph: Graph;
+  constructor(view: RelationalView, graph: Graph) {
+    this._view = view;
+    this._graph = graph;
+  }
+  graph() {
+    return this._graph;
+  }
+  renderer() {
+    return new Renderer(this._view);
+  }
+  nodePrefix() {
+    return N._Prefix.base;
+  }
+}
+
+class Renderer implements IRenderer {
+  +_view: RelationalView;
+  constructor(view) {
+    this._view = view;
+  }
+  nodeDescription(node) {
+    // This cast is unsound, and might throw at runtime, but won't have
+    // silent failures or cause problems down the road.
+    const address = N.fromRaw((node: any));
+    const entity = this._view.entity(address);
+    if (entity == null) {
+      throw new Error(`unknown entity: ${NodeAddress.toString(node)}`);
+    }
+    return description(entity);
+  }
+}