[Bugfix] Ensure stability of filename cache-keys

`JSON.stringify(structure)` isn’t inherently stable as it relies on
various internal details of how `structure` was created.

As written, if a given babel configuration is create in an dynamic
manner, it is possible for babel-loader to have spurious cache misses.

To address this, we can use one of the many stable stringify
alternatives.

For this PR I have selected
[fast-stable-stringify](https://www.npmjs.com/package/fast-stable-stringify)
for that task, as it appears both popular and it’s benchmarks look
  promising.

This PR does not explicitly include tests, as testing this is both
tricky to test in this context, and the important tests are contained
within fast-stable-stringify itself.

package.json

@@ -10,6 +10,7 @@
  "node": ">= 8.9"
  },
  "dependencies": {
+ "fast-stable-stringify": "^1.0.0",
  "find-cache-dir": "^3.3.1",
  "loader-utils": "^1.4.0",
  "make-dir": "^3.1.0",

src/cache.js

@@ -24,6 +24,7 @@ const writeFile = promisify(fs.writeFile);
 const gunzip = promisify(zlib.gunzip);
 const gzip = promisify(zlib.gzip);
 const makeDir = require("make-dir");
+const stringify = require("fast-stable-stringify");
 
 /**
  * Read the contents from the compressed file.
@@ -65,9 +66,7 @@ const write = async function (filename, compress, result) {
 const filename = function (source, identifier, options) {
  const hash = crypto.createHash("md4");
 
- const contents = JSON.stringify({ source, options, identifier });
-
- hash.update(contents);
+ hash.update(stringify({ source, options, identifier }));
 
  return hash.digest("hex") + ".json";
 };

src/index.js

@@ -28,6 +28,7 @@ const schema = require("./schema");
 const { isAbsolute } = require("path");
 const loaderUtils = require("loader-utils");
 const validateOptions = require("schema-utils");
+const stringify = require("fast-stable-stringify");
 
 function subscribe(subscriber, metadata, context) {
  if (context[subscriber]) {
@@ -186,7 +187,7 @@ async function loader(source, inputSourceMap, overrides) {
 
  const {
  cacheDirectory = null,
- cacheIdentifier = JSON.stringify({
+ cacheIdentifier = stringify({
  options,
  "@babel/core": transform.version,
  "@babel/loader": version,

test/cache.test.js

@@ -376,7 +376,7 @@ test.cb("should allow to specify the .babelrc file", t => {
 
  fs.readdir(t.context.cacheDirectory, (err, files) => {
  t.is(err, null);
- t.true(files.length === 2);
+ t.true(files.length === 1);
  t.end();
  });
  });

yarn.lock

@@ -2140,6 +2140,7 @@ __metadata:
  eslint-config-prettier: ^6.3.0
  eslint-plugin-flowtype: ^5.2.0
  eslint-plugin-prettier: ^3.0.0
+ fast-stable-stringify: ^1.0.0
  find-cache-dir: ^3.3.1
  husky: ^4.3.0
  lint-staged: ^10.5.1
@@ -3732,6 +3733,13 @@ __metadata:
  languageName: node
  linkType: hard
 
+"fast-stable-stringify@npm:^1.0.0":
+ version: 1.0.0
+ resolution: "fast-stable-stringify@npm:1.0.0"
+ checksum: 972291ed8b8a1a1e13130c91062852dc9142dceed14dc1655c1d0aebc07adcf8a60678252dd1410ec0cceddd3842c87cfea2a937ee3043dfaa6068c3f578f515
+ languageName: node
+ linkType: hard
+
 "fastq@npm:^1.6.0":
  version: 1.9.0
  resolution: "fastq@npm:1.9.0"