Avoid silently swallowing connection errors

When a connection error takes place, the test setup would silently
swallow it (missing callback for `connection.connect()`)

Rewrite the whole thing so that it's easier to follow, to make sure
it awaits the promises, and to use a type interface that aliases
over `mysql` and `mysql2` instead of `any`.

test/koa-mysql/app/src/app.ts

@@ -1,7 +1,7 @@
 import Koa from "koa"
 import Router from "@koa/router"
 import bodyParser from "koa-bodyparser"
-import mysql, { Connection } from "mysql"
+import mysql from "mysql"
 import mysql2 from "mysql2"
 
 const mysqlConfig = {
@@ -11,6 +11,68 @@ const mysqlConfig = {
   database: process.env.MYSQL_DATABASE
 }
 
+type MaybeMysqlError = mysql.MysqlError | mysql2.QueryError | null | undefined
+
+interface MysqlConnection {
+  connect(callback: (err: MaybeMysqlError) => void): void
+  query(
+    sql: string,
+    callback: (err: MaybeMysqlError, rows: any, fields: any) => void
+  ): void
+  end(callback: (err: MaybeMysqlError) => void): void
+}
+
+function connectionPromise(mysqlClient: MysqlModule): Promise<MysqlConnection> {
+  return new Promise((resolve, reject) => {
+    let connection = mysqlClient.createConnection(mysqlConfig)
+
+    connection.connect((err: MaybeMysqlError) => {
+      if (err) reject(err)
+      resolve(connection)
+    })
+  })
+}
+
+function dummyQueryPromise(connection: MysqlConnection): Promise<any> {
+  return new Promise((resolve, reject) => {
+    connection.query(
+      "SELECT 1 + 1 AS solution",
+      (err: MaybeMysqlError, rows: any, _fields: any) => {
+        if (err) reject(err)
+        try {
+          resolve(rows[0].solution)
+        } catch (err) {
+          reject(err)
+        }
+      }
+    )
+  })
+}
+
+function endConnectionPromise(connection: MysqlConnection): Promise<void> {
+  return new Promise<void>((resolve, reject) => {
+    connection.end((err: MaybeMysqlError) => {
+      if (err) reject(err)
+      resolve()
+    })
+  })
+}
+
+async function dummyQuery(mysqlClient: MysqlModule): Promise<any> {
+  let connection = await connectionPromise(mysqlClient)
+  let result = await dummyQueryPromise(connection)
+  await endConnectionPromise(connection)
+
+  return result
+}
+
+interface MysqlModule {
+  createConnection(config: typeof mysqlConfig): MysqlConnection
+}
+
+const mysqlModule: MysqlModule = mysql
+const mysql2Module: MysqlModule = mysql2
+
 const app = new Koa()
 const router = new Router()
 const port = process.env.PORT
@@ -26,32 +88,17 @@ router.get("/error", async (_ctx: any) => {
 })
 
 router.get("/mysql-query", async (ctx: any) => {
-  await dummyQuery(mysql)
+  await dummyQuery(mysqlModule)
 
   ctx.body = "MySQL query received!"
 })
 
 router.get("/mysql2-query", async (ctx: any) => {
-  await dummyQuery(mysql2)
+  await dummyQuery(mysql2Module)
 
   ctx.body = "MySQL2 query received!"
 })
 
-function dummyQuery(mysqlClient: any) {
-  let connection: Connection
-
-  return new Promise((resolve, reject) => {
-    connection = mysqlClient.createConnection(mysqlConfig)
-
-    connection.connect()
-
-    connection.query("SELECT 1 + 1 AS solution", (err, rows, _fields) => {
-      if (err) reject(err)
-      resolve(rows[0].solution)
-    })
-  }).finally(() => connection.end())
-}
-
 app.use(router.routes()).use(router.allowedMethods())
 
 app.listen(port)