backend: export a normal Webpack config object (#779)

Summary:
Previously, our `webpack.config.backend.js` file actually exported a
function that could be used to make a Webpack configuration object.
(This is not to be confused with the late `makeWebpackConfig.js`, which
actually exported a configuration object!)

In addition to being confusing nomenclature, this was a sneaky trap for
CLI users. Invoking `webpack --config config/webpack.config.backend.js`
would actually work, but do the wrong thing: Webpack _allows_ your
configuration object to be a function, but with different semantics. In
particular, the result was that Webpack would emit the build output into
your current directory instead of into `bin/`.

This commit fixes that by making `webpack.config.backend.js` export the
Webpack configuration object for the backend JavaScript applications.
The logic to change the path is now handled by the caller, by
overwriting `config.output.path`; this is exactly [the same approach
that the Webpack CLI takes when given an `--output-path`][1], so it’s
okay with me.

[1]: 368e2640e6/bin/convert-argv.js (L406-L409)

Test Plan:
Run `yarn backend` and `yarn backend --dry-run`. Note that each runs
with appropriate output (both emitted files and console logs).

wchargin-branch: backend-webpack-config-object

config/webpack.config.backend.js

@@ -11,7 +11,7 @@ const env = getClientEnvironment();
 
 // This is the backend configuration. It builds applications that target
 // Node and will not run in a browser.
-module.exports = (outputPath) => ({
+module.exports = {
   // Don't attempt to continue if there are any errors.
   bail: true,
   // Target Node instead of the browser.
@@ -19,7 +19,7 @@ module.exports = (outputPath) => ({
   entry: paths.backendEntryPoints,
   externals: [nodeExternals()],
   output: {
-    path: outputPath,
+    path: paths.backendBuild,
     // Generated JS file names (with nested folders).
     // There will be one main bundle, and one file per asynchronous chunk.
     // We don't currently advertise code splitting but Webpack supports it.
@@ -62,4 +62,4 @@ module.exports = (outputPath) => ({
     new RemoveBuildDirectoryPlugin(),
     new webpack.DefinePlugin(env.individuallyStringified),
   ],
-});
+};

scripts/backend.js

@@ -11,12 +11,12 @@ require("../src/tools/entry");
 // Ensure environment variables are read.
 require("../config/env");
 
+const cloneDeep = require("lodash.clonedeep");
 const path = require("path");
 const chalk = require("chalk");
 const fs = require("fs-extra");
 const tmp = require("tmp");
 const webpack = require("webpack");
-const config = require("../config/webpack.config.backend");
 const paths = require("../config/paths");
 const checkRequiredFiles = require("react-dev-utils/checkRequiredFiles");
 const formatWebpackMessages = require("react-dev-utils/formatWebpackMessages");
@@ -27,12 +27,8 @@ const printBuildError = require("react-dev-utils/printBuildError");
 const WARN_AFTER_BUNDLE_GZIP_SIZE = 512 * 1024;
 const WARN_AFTER_CHUNK_GZIP_SIZE = 1024 * 1024;
 
-const outputPath = process.argv.some((s) => s === "--dry-run" || s === "-n")
-  ? tmp.dirSync({unsafeCleanup: true, prefix: "sourcecred-"}).name
-  : paths.backendBuild;
-
 build().then(
-  ({stats, warnings}) => {
+  ({stats, outputPath, warnings}) => {
     if (warnings.length) {
       console.log(chalk.yellow("Compiled with warnings.\n"));
       console.log(warnings.join("\n\n"));
@@ -64,7 +60,16 @@ build().then(
 function build() {
   console.log("Building backend applications...");
 
-  let compiler = webpack(config(outputPath));
+  const config = cloneDeep(require("../config/webpack.config.backend"));
+  if (process.argv.some((s) => s === "--dry-run" || s === "-n")) {
+    config.output.path = tmp.dirSync({
+      unsafeCleanup: true,
+      prefix: "sourcecred-",
+    }).name;
+  }
+  const outputPath = config.output.path;
+
+  let compiler = webpack(config);
   return new Promise((resolve, reject) => {
     compiler.run((err, stats) => {
       if (err) {
@@ -95,6 +100,7 @@ function build() {
       }
       return resolve({
         stats,
+        outputPath,
         warnings: messages.warnings,
       });
     });