fix(worker): workaround webpack4 fs write caching

Ref #126

webpack/private/webpack_worker.js

@@ -88,10 +88,43 @@ class WebpackWorker extends WebpackCLI {
       this.compiler = await super.createCompiler(options, cb)
       this.options = options
 
-      // The output directory will be cleaned between runs, webpack assumes the output directory will not be modified:
-      // Webpack5: https://github.com/webpack/webpack/blob/v5.36.2/lib/Compiler.js#L783-L788
+      // The output directory will be cleaned between runs, however webpack assumes the
+      // output directory will not be modified and caches the file system state.
+
+      // Webpack5: clear the "previously emitted" cache
+      // https://github.com/webpack/webpack/blob/v5.36.2/lib/Compiler.js#L783-L788
       if (this.compiler._assetEmittingPreviousFiles) {
-        this.compiler.hooks.afterEmit.tap("rules_webpack", () => this.compiler._assetEmittingPreviousFiles.clear())
+        this.compiler.hooks.afterEmit.tap("rules_webpack", this.compiler._assetEmittingPreviousFiles.clear());
+      } else {
+        // Webpack4: patch the compiler to ensure all assets are re-written on each completion
+        const fsCache = new Map()
+        const fsWrites = new Set()
+
+        // Clear the "writes" before compilation.
+        this.compiler.hooks.emit.tap('rules_webpack', () => fsWrites.clear())
+
+        // Cache all file write operations throughout compilation
+        const originalWriteFile = this.compiler.outputFileSystem.writeFile
+        this.compiler.outputFileSystem.writeFile = function workerWriteFile(p, data, cb) {
+          fsCache.set(p, data)
+          fsWrites.add(p)
+          return originalWriteFile.apply(this, arguments)
+        }
+
+        // Ensure all past write operations are rewritten.
+        this.compiler.hooks.afterEmit.tap('rules_webpack', () => {
+          for (const [p, data] of fsCache.entries()) {
+            if (!fsWrites.has(p)) {
+              // TODO: wait for async write?
+              originalWriteFile.call(this.compiler.outputFileSystem, p, data, () => {})
+            }
+          }
+
+          // Clear the writes-done for the next compilation.
+          fsWrites.clear()
+        })
+
+        // TODO: clear entries when no longer outputted by webpack
       }
     }
   }