Configure sourcemap file

`webpack` will by default emit a sourcemap that maps to a `main.js`
file. However, the worker runtime will create the script with a
`worker.js` name. In some cases (like devtools) it doesn't matter
because it resolves the source infos based on the sourcemap `sources`
key. In Sentry for instance, that would fail.

Fixes #681

tests/build.rs

@@ -3,6 +3,7 @@ extern crate lazy_static;
 
 pub mod fixture;
 
+use std::fs;
 use std::process::Command;
 use std::str;
 
@@ -313,14 +314,55 @@ fn it_builds_with_webpack_target_webworker() {
     build_creates_assets(&fixture, vec!["script.js"]);
 }
 
-fn build_creates_assets(fixture: &Fixture, script_names: Vec<&str>) {
+#[test]
+fn it_builds_with_webpack_name_output() {
+    let fixture = Fixture::new();
+    fixture.scaffold_webpack();
+
+    fixture.create_file(
+        "webpack.config.js",
+        r#"
+        module.exports = {
+            "entry": "./index.js",
+            "devtool": "cheap-module-source-map"
+        }
+    "#,
+    );
+    fixture.create_file("index.js", "");
+
+    let wrangler_toml = WranglerToml::webpack_std_config("test-build-webpack-name-output");
+    fixture.create_wrangler_toml(wrangler_toml);
+
+    let (_, stderr) = build_creates_assets(&fixture, vec!["script.js"]);
+
+    let out = fs::read_to_string(fixture.get_output_path().join("script.js")).unwrap();
+    let sourcemap = out.split("\n").last().unwrap();
+    assert_eq!(
+        sourcemap,
+        r#"//# sourceMappingURL=worker.js.map{"version":3,"file":"worker.js","sources":["webpack:///worker.js"],"mappings":"AAAA","sourceRoot":""}"#
+    );
+
+    println!("stderr : {}", stderr);
+    assert_eq!(stderr, "foo");
+}
+
+fn build_creates_assets(fixture: &Fixture, script_names: Vec<&str>) -> (String, String) {
     let _lock = fixture.lock();
     let mut build = Command::cargo_bin(env!("CARGO_PKG_NAME")).unwrap();
     build.current_dir(fixture.get_path());
-    build.arg("build").assert().success();
+    build.arg("build");
+
+    let output = build.output().expect("failed to execute process");
+    assert!(output.status.success());
+
     for script_name in script_names {
         assert!(fixture.get_output_path().join(script_name).exists());
     }
+
+    (
+        str::from_utf8(&output.stdout).unwrap().to_string(),
+        str::from_utf8(&output.stderr).unwrap().to_string(),
+    )
 }
 
 fn build_fails_with(fixture: &Fixture, expected_message: &str) {

wranglerjs/index.js

@@ -3,11 +3,17 @@ const { join } = require("path");
 const fs = require("fs");
 const WasmMainTemplatePlugin = require("webpack/lib/wasm/WasmMainTemplatePlugin");
 
+const WEBPACK_OUTPUT_FILENAME = "worker.js";
+const WEBPACK_OUTPUT_SOURCEMAPFILENAME = WEBPACK_OUTPUT_FILENAME + ".map";
+
 function error(msg) {
   console.error("Error: " + msg);
   process.exit(1);
   return new Error("error");
 }
+function warn(...msg) {
+  console.warn("Warning: " + msg.join(" "));
+}
 
 function filterByExtension(ext) {
   return v => v.indexOf("." + ext) !== -1;
@@ -54,6 +60,38 @@ function filterByExtension(ext) {
   }
   config.target = "webworker";
 
+  // The worker runtime will set the name of the script to `worker.js`,
+  // regardless of what's specified in the sourcemap.
+  // We can tell webpack to name the generated worker by configuring the output.
+  // It's also safe to force that configuration because it mirrors what the
+  // runtime does.
+  // https://github.com/cloudflare/wrangler/issues/681
+  if (config.output === undefined) {
+    config.output = {};
+  }
+  if (
+    config.output.filename !== undefined &&
+    config.output.filename !== WEBPACK_OUTPUT_FILENAME
+  ) {
+    warn(
+      "webpack's output filename is being renamed to",
+      WEBPACK_OUTPUT_FILENAME,
+      "because of requirements from the Workers runtime"
+    );
+  }
+  if (
+    config.output.sourceMapFilename !== undefined &&
+    config.output.sourceMapFilename !== WEBPACK_OUTPUT_SOURCEMAPFILENAME
+  ) {
+    warn(
+      "webpack's output sourcemap filename is being renamed to",
+      WEBPACK_OUTPUT_SOURCEMAPFILENAME,
+      "because of requirements from the Workers runtime"
+    );
+  }
+  config.output.filename = WEBPACK_OUTPUT_FILENAME;
+  config.output.sourceMapFilename = WEBPACK_OUTPUT_SOURCEMAPFILENAME;
+
   const compiler = webpack(config);
   const fullConfig = compiler.options;