fix(server): wait before closing pubsub database connection

Add `drain` parameter to `destroy` function to optionally await the `drain` event on the pubsub database client before calling `end`.
Expose manager on `pubsweet-server` package root.

packages/server/src/graphql/pubsub.js

@@ -42,12 +42,18 @@ module.exports = {
     return pubsub
   },
 
-  destroy: () => {
-    if (client) {
+  /**
+   * @param drain {boolean} whether to wait for the database client to emit 'drain' before closing
+   * @returns {Promise<void>}
+   */
+  destroy: async ({ drain } = {}) => {
+    if (pubsub) {
+      if (drain) {
+        await new Promise(resolve => pubsub.client.on('drain', resolve))
+      }
+      await pubsub.client.end()
       pubsub = null
-      return client.end()
     }
-    return Promise.resolve()
   },
 
   /**

packages/server/src/index.js

@@ -8,6 +8,7 @@ module.exports.Collection = require('./models/Collection')
 module.exports.helpers = require('./helpers/authorization')
 
 module.exports.db = require('./db')
+module.exports.pubsubManager = require('./graphql/pubsub')
 module.exports.NotFoundError = require('./errors/NotFoundError')
 
 module.exports.startServer = require('./start-server')

packages/server/test/graphql/pubsub_test.js

+const { getPubsub, destroy } = require('../../src/graphql/pubsub')
+
+describe('pubsub manager', () => {
+  afterEach(() => destroy())
+
+  it('can call destroy before connect', () =>
+    expect(destroy()).resolves.toBeUndefined())
+
+  it('waits for client to drain before closing', async () => {
+    const pubsub = await getPubsub()
+    const cb = jest.fn()
+    pubsub.subscribe('test_channel', cb)
+    pubsub.publish('test_channel', { test: 'content' })
+    const destroyPromise = destroy({ drain: true })
+
+    expect(cb).not.toHaveBeenCalled()
+    await destroyPromise
+    expect(cb).toHaveBeenCalledWith({ test: 'content' })
+  })
+})