Don’t erase the `bin/` folder in CI (#317)

Summary:
Previously, our CI script would run `yarn backend`, which has the
side-effect of erasing the `bin/` directory. By itself, this is not
great, but not awful. However, this frequently triggers a race condition
in Prettier, causing the `check-pretty` step of the build to fail. (More
details: https://github.com/prettier/prettier/issues/4468.)

This patch changes the CI script to build the backend scripts into a
temporary directory.

Test Plan:
Before applying this patch: `yarn backend` and then `yarn travis`. If
this consistently causes a Travis failure due to `check-pretty`, then
your machine can reproduce the race condition that we‛re trying to
eliminate. (Otherwise, you can try creating a bunch more Git history…
I’m not really sure what to say. It is a race condition, after all.)
Then, apply this patch, and repeat the above steps; note that the error
no longer occurs, and that the build output is to a temporary directory.

wchargin-branch: ci-preserve-bin

config/travis.js

@@ -59,7 +59,7 @@ function makeTasks(mode /*: "BASIC" | "FULL" */) {
     },
     {
       id: "backend",
-      cmd: ["npm", "run", "--silent", "backend"],
+      cmd: ["npm", "run", "--silent", "backend", "--", "--dry-run"],
       deps: [],
     },
   ];

config/webpack.config.backend.js

@@ -7,7 +7,7 @@ const nodeExternals = require("webpack-node-externals");
 
 // This is the backend configuration. It builds applications that target
 // Node and will not run in a browser.
-module.exports = {
+module.exports = (outputPath) => ({
   // Don't attempt to continue if there are any errors.
   bail: true,
   // Target Node instead of the browser.
@@ -15,7 +15,7 @@ module.exports = {
   entry: paths.backendEntryPoints,
   externals: [nodeExternals()],
   output: {
-    path: paths.backendBuild,
+    path: outputPath,
     // 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.
@@ -77,4 +77,4 @@ module.exports = {
       ),
     }),
   ],
-};
+});

scripts/backend.js

@@ -19,6 +19,7 @@ require("../config/env");
 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");
@@ -40,13 +41,17 @@ if (!checkRequiredFiles(Object.values(paths.backendEntryPoints))) {
   process.exit(1);
 }
 
+const outputPath = process.argv.some((s) => s === "--dry-run" || s === "-n")
+  ? tmp.dirSync({unsafeCleanup: true, prefix: "sourcecred-"}).name
+  : paths.backendBuild;
+
 // First, read the current file sizes in build directory.
 // This lets us display how much they changed later.
-measureFileSizesBeforeBuild(paths.backendBuild)
+measureFileSizesBeforeBuild(outputPath)
   .then((previousFileSizes) => {
     // Remove all content but keep the directory so that
     // if you're in it, you don't end up in Trash
-    fs.emptyDirSync(paths.backendBuild);
+    fs.emptyDirSync(outputPath);
     // Start the webpack build
     return build(previousFileSizes);
   })
@@ -73,13 +78,13 @@ measureFileSizesBeforeBuild(paths.backendBuild)
       printFileSizesAfterBuild(
         stats,
         previousFileSizes,
-        paths.backendBuild,
+        outputPath,
         WARN_AFTER_BUNDLE_GZIP_SIZE,
         WARN_AFTER_CHUNK_GZIP_SIZE
       );
       console.log();
 
-      const buildFolder = path.relative(process.cwd(), paths.backendBuild);
+      const buildFolder = path.relative(process.cwd(), outputPath);
       console.log(`Build completed; results in '${buildFolder}'.`);
     },
     (err) => {
@@ -93,7 +98,7 @@ measureFileSizesBeforeBuild(paths.backendBuild)
 function build(previousFileSizes) {
   console.log("Building backend applications...");
 
-  let compiler = webpack(config);
+  let compiler = webpack(config(outputPath));
   return new Promise((resolve, reject) => {
     compiler.run((err, stats) => {
       if (err) {