Setup Discord cli2 plugin

This adds a cliPlugin to the experimental Discord plugin, along the
lines of the cli 2 plugins for GitHub and Discourse.

Test plan: I set up a Cred instance for SourceCred including our Discord
config, and successfully loaded Cred for it. Not easy to give full
reproduction instructions, not least because it requires a private
Discord bot token.

src/cli2/bundledPlugins.js

@@ -3,6 +3,7 @@
 import type {CliPlugin} from "./cliPlugin";
 import {GithubCliPlugin} from "../plugins/github/cliPlugin";
 import {DiscourseCliPlugin} from "../plugins/discourse/cliPlugin";
+import {DiscordCliPlugin} from "../plugins/experimental-discord/cliPlugin";
 
 /**
  * Returns an object mapping owner-name pairs to CLI plugin
@@ -12,5 +13,6 @@ export function bundledPlugins(): {[pluginId: string]: CliPlugin} {
   return {
     "sourcecred/github": new GithubCliPlugin(),
     "sourcecred/discourse": new DiscourseCliPlugin(),
+    "sourcecred/discord": new DiscordCliPlugin(),
   };
 }

src/plugins/experimental-discord/cliPlugin.js

@@ -0,0 +1,89 @@
+// @flow
+
+import Database from "better-sqlite3";
+
+import type {CliPlugin, PluginDirectoryContext} from "../../cli2/cliPlugin";
+import type {PluginDeclaration} from "../../analysis/pluginDeclaration";
+import {parseConfig, type DiscordConfig, type DiscordToken} from "./config";
+import {declaration} from "./declaration";
+import {join as pathJoin} from "path";
+import fs from "fs-extra";
+import {type TaskReporter} from "../../util/taskReporter";
+import {Fetcher} from "./fetcher";
+import {Mirror} from "./mirror";
+import type {ReferenceDetector} from "../../core/references/referenceDetector";
+import type {WeightedGraph} from "../../core/weightedGraph";
+import {weightsForDeclaration} from "../../analysis/pluginDeclaration";
+import {createGraph} from "./createGraph";
+import * as Model from "./models";
+import {SqliteMirrorRepository} from "./mirrorRepository";
+
+async function loadConfig(
+  dirContext: PluginDirectoryContext
+): Promise<DiscordConfig> {
+  const dirname = dirContext.configDirectory();
+  const path = pathJoin(dirname, "config.json");
+  const contents = await fs.readFile(path);
+  return Promise.resolve(parseConfig(JSON.parse(contents)));
+}
+
+const TOKEN_ENV_VAR_NAME = "SOURCECRED_DISCORD_TOKEN";
+
+function getTokenFromEnv(): DiscordToken {
+  const rawToken = process.env[TOKEN_ENV_VAR_NAME];
+  if (rawToken == null) {
+    throw new Error(`No Discord token provided: set ${TOKEN_ENV_VAR_NAME}`);
+  }
+  return rawToken;
+}
+
+export class DiscordCliPlugin implements CliPlugin {
+  declaration(): PluginDeclaration {
+    return declaration;
+  }
+
+  async load(
+    ctx: PluginDirectoryContext,
+    reporter: TaskReporter
+  ): Promise<void> {
+    const {guildId} = await loadConfig(ctx);
+    const token = getTokenFromEnv();
+    const fetcher = new Fetcher({token});
+    const repo = await repository(ctx, guildId);
+    const mirror = new Mirror(repo, fetcher, guildId);
+    await mirror.update(reporter);
+  }
+
+  async graph(
+    ctx: PluginDirectoryContext,
+    rd: ReferenceDetector
+  ): Promise<WeightedGraph> {
+    const _ = rd; // TODO(#1808): not yet used
+    const {guildId, reactionWeights} = await loadConfig(ctx);
+    const repo = await repository(ctx, guildId);
+    const declarationWeights = weightsForDeclaration(declaration);
+    return await createGraph(
+      guildId,
+      repo,
+      declarationWeights,
+      reactionWeights
+    );
+  }
+
+  async referenceDetector(
+    _unused_ctx: PluginDirectoryContext
+  ): Promise<ReferenceDetector> {
+    // TODO: Implement Discord reference detection
+    // (low priority bc ppl rarely hardlink to Discord messages)
+    return {addressFromUrl: () => undefined};
+  }
+}
+
+async function repository(
+  ctx: PluginDirectoryContext,
+  guild: Model.Snowflake
+): Promise<SqliteMirrorRepository> {
+  const path = pathJoin(ctx.cacheDirectory(), "discordMirror.db");
+  const db = await new Database(path);
+  return new SqliteMirrorRepository(db, guild);
+}