rtfeldman · lydell · Jul 19, 2026 · Jul 21, 2026
diff --git a/benchmark.js b/benchmark.js
diff --git a/elm/src/Test/Runner/Node.elm b/elm/src/Test/Runner/Node.elm
@@ -81,7 +81,7 @@ type Msg
 {-| The port names are prefixed to reduce the likelihood of the project
 having a port with the same name, which is a compile error.
 -}
-port elmTestPort__send : String -> Cmd msg
+port elmTestPort__send : Decode.Value -> Cmd msg
 
 
 port elmTestPort__receive : (Decode.Value -> msg) -> Sub msg
@@ -140,7 +140,6 @@ update msg ({ testReporter } as model) =
                                 , ( "exitCode", Encode.int exitCode )
                                 , ( "message", summary )
                                 ]
-                                |> Encode.encode 0
                                 |> elmTestPort__send
                     in
                     ( model, cmd )
@@ -152,7 +151,6 @@ update msg ({ testReporter } as model) =
                                 [ ( "type", Encode.string "ERROR" )
                                 , ( "message", Encode.string (Decode.errorToString err) )
                                 ]
-                                |> Encode.encode 0
                                 |> elmTestPort__send
                     in
                     ( model, cmd )
@@ -224,7 +222,6 @@ sendResults isFinished testReporter results =
                 |> Encode.object
           )
         ]
-        |> Encode.encode 0
         |> elmTestPort__send
 
 
@@ -245,7 +242,6 @@ sendBegin model =
                     []
     in
     Encode.object (baseFields ++ extraFields)
-        |> Encode.encode 0
         |> elmTestPort__send
 
 
@@ -336,7 +332,6 @@ failInit message report _ =
                 , ( "exitCode", Encode.int 1 )
                 , ( "message", Encode.string message )
                 ]
-                |> Encode.encode 0
                 |> elmTestPort__send
     in
     ( model, cmd )

diff --git a/lib/Generate.js b/lib/Generate.js
@@ -15,19 +15,17 @@ const after = fs.readFileSync(
 );
 
 /**
- * @param { string } pipeFilename
  * @param { string } dest
  * @returns { void }
  */
-function prepareCompiledJsFile(pipeFilename, dest) {
+function prepareCompiledJsFile(dest) {
   const content = fs.readFileSync(dest, 'utf8');
   const finalContent = `
 ${before}
 var Elm = (function(module) {
 ${addKernelTestChecking(content)}
 return this.Elm;
 })({});
-var pipeFilename = ${JSON.stringify(pipeFilename)};
 ${after}
   `.trim();
   fs.writeFileSync(dest, finalContent);

diff --git a/lib/RunTests.js b/lib/RunTests.js
@@ -10,26 +10,6 @@ const Project = require('./Project');
 const Report = require('./Report');
 const Supervisor = require('./Supervisor');
 
-/**
- * Incorporate the process PID into the socket name, so elm-test processes can
- * be run parallel without accidentally sharing each others' sockets.
- *
- * See https://github.com/rtfeldman/node-test-runner/pull/231
- * Also incorporate a salt number into it on Windows, to avoid EADDRINUSE -
- * see https://github.com/rtfeldman/node-test-runner/issues/275 - because the
- * alternative approach of deleting the file before creating a new one doesn't
- * work on Windows. We have to let Windows clean up the named pipe. This is
- * essentially a band-aid fix. The alternative is to rewrite a ton of stuff.
- *
- * @param { number } runsExecuted
- * @returns { string }
- */
-function getPipeFilename(runsExecuted) {
-  return process.platform === 'win32'
-    ? `\\\\.\\pipe\\elm_test-${process.pid}-${runsExecuted}`
-    : `/tmp/elm_test-${process.pid}.sock`;
-}
-
 /**
  * This lets you log something like `elm.json changed > Compiling > Starting tests`
  * where each segment appears over time.
@@ -166,8 +146,6 @@ function runTests(
   let watcher = undefined;
   /** @type { Array<string> } */
   let watchedPaths = [];
-  /** @type { number } */
-  let runsExecuted = 0;
   /** @type { Promise<void> | void } */
   let currentRun = undefined;
   /** @type { Queue } */
@@ -236,8 +214,6 @@ function runTests(
         watcher.unwatch(diff.removed);
       }
 
-      runsExecuted++;
-      const pipeFilename = getPipeFilename(runsExecuted);
       const testModules = FindTests.findTests(testFilePaths, project);
       const mainModule = Generate.getMainModule(project.generatedCodeDir);
       const dest = path.join(project.generatedCodeDir, 'elmTestOutput.js');
@@ -265,19 +241,12 @@ function runTests(
         report
       );
 
-      Generate.prepareCompiledJsFile(pipeFilename, dest);
+      Generate.prepareCompiledJsFile(dest);
 
       progressLogger.log('Starting tests');
       progressLogger.newLine();
 
-      return await Supervisor.run(
-        packageInfo.version,
-        pipeFilename,
-        report,
-        processes,
-        dest,
-        watch
-      );
+      return await Supervisor.run(packageInfo.version, report, processes, dest);
     } catch (err) {
       progressLogger.newLine();
       console.error(err.message);

diff --git a/lib/Supervisor.js b/lib/Supervisor.js
@@ -1,34 +1,25 @@
-const chalk = require('./chalk');
-const child_process = require('child_process');
-const fs = require('fs');
-const net = require('net');
-const split = require('split');
+const { Worker } = require('worker_threads');
 const Report = require('./Report');
 const XMLBuilder = require('./XMLBuilder');
 
 /**
  * @param { string } elmTestVersion
- * @param { string } pipeFilename
  * @param { import('./Report').Report } report
  * @param { number } processes
  * @param { string } dest
- * @param { boolean } watch
  * @returns { Promise<number> }
  */
-function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
+function run(elmTestVersion, report, processes, dest) {
   return new Promise(function (resolve) {
     /** @type { number | null } */
     var nextResultToPrint = null;
     var finishedWorkers = 0;
-    var closedWorkers = 0;
     var results = new Map();
     var failures = 0;
     /** @type { Array<{ labels: Array<string>, todo: string }> } */
     var todos = [];
     var testsToRun = -1;
     var startingTime = Date.now();
-    /** @type { Array<import('child_process').ChildProcess> } */
-    var workers = [];
 
     /**
      * @param { any } result This `any` became explicit instead of implicit when migrating from Flow to TypeScript.
@@ -95,13 +86,6 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
         }
       }
     }
-    function reportRuntimeException() {
-      console.error(
-        chalk.red(
-          '\n\nThere was an unexpected runtime exception while running tests\n\n'
-        )
-      );
-    }
 
     /**
      * @param { any } response This `any` became explicit instead of implicit when migrating from Flow to TypeScript.
@@ -155,44 +139,26 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
       flushResults();
     }
 
-    /**
-     * @param { import('net').Socket } socket
-     * @returns { void }
-     */
-    function initWorker(socket) {
-      socket.setEncoding('utf8');
-      socket.setNoDelay(true);
-
-      // See the long note near client.write() in worker.js for why we do this.
-      // It fixes a nasty bug!
-      var stream = socket.pipe(split());
-
-      stream.on('data', function (data) {
-        // In watch mode, the socket is drained which results in an extraneous
-        // message being sent. If we receive no data, ignore it.
-        if (!data) {
-          return;
-        }
-
-        var response = JSON.parse(data);
+    /** @type { Array<import('worker_threads').Worker> } */
+    var workers = Array.from({ length: processes }, (_, index) => {
+      var worker = new Worker(dest, { workerData: index });
 
+      worker.on('message', function (response) {
         switch (response.type) {
           case 'FINISHED':
             handleResults(response);
 
             // This worker found no tests remaining to run; it's finished!
             finishedWorkers++;
 
-            // If all the workers have finished, print the summmary.
+            // If all the workers have finished, print the summary.
             if (finishedWorkers === workers.length) {
-              socket.write(
-                JSON.stringify({
-                  type: 'SUMMARY',
-                  duration: Date.now() - startingTime,
-                  failures: failures,
-                  todos: todos,
-                })
-              );
+              worker.postMessage({
+                type: 'SUMMARY',
+                duration: Date.now() - startingTime,
+                failures: failures,
+                todos: todos,
+              });
             }
             break;
           case 'SUMMARY':
@@ -219,7 +185,7 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
 
             // Close all the workers.
             workers.forEach(function (worker) {
-              worker.kill();
+              worker.terminate();
             });
             resolve(response.exitCode);
             break;
@@ -253,58 +219,9 @@ function run(elmTestVersion, pipeFilename, report, processes, dest, watch) {
             );
         }
       });
-    }
 
-    var pendingException = false,
-      server = net.createServer(initWorker);
-
-    server.on('error', function (err) {
-      console.error(err.stack);
-      server.close();
+      return worker;
     });
-
-    server.on('listening', function () {
-      workers = Array.from({ length: processes }, (_, index) => {
-        var worker = child_process.fork(dest, [index.toString()]);
-
-        worker.on('close', function (code) {
-          // code can be null.
-          var hasNonZeroExitCode = typeof code === 'number' && code !== 0;
-
-          if (watch && !Report.isMachineReadable(report)) {
-            if (hasNonZeroExitCode) {
-              // Queue up complaining about an exception.
-              // Don't print it immediately, or else it might print N times
-              // where N is the number of cores.
-              pendingException = true;
-            }
-            closedWorkers++;
-            // If all the workers have closed, we're done! Continue watching.
-            if (closedWorkers === workers.length) {
-              if (pendingException) {
-                // If we had an exception pending, print it and clear pending flag.
-                reportRuntimeException();
-                pendingException = false;
-              }
-              resolve(1);
-            }
-          } else if (hasNonZeroExitCode) {
-            reportRuntimeException();
-            resolve(1);
-          }
-        });
-
-        return worker;
-      });
-    });
-
-    if (fs.existsSync(pipeFilename) && process.platform !== 'win32') {
-      // Never remove named pipes on Windows. The OS will clean them up when
-      // nothing has a handle to them anymore.
-      fs.unlinkSync(pipeFilename);
-    }
-
-    server.listen(pipeFilename);
   });
 }