Fix ESM node processes being unable to fork into other scripts

Currently, Node processes instantiated through the `--esm` flag result
in a child process being created so that the ESM loader can be
registered. This works fine and is reasonable.

The child process approach to register ESM hooks currently prevents
the NodeJS `fork` method from being used because the `execArgv`
propagated into forked processes causes `ts-node` (which is also
propagated as child exec script -- this is good because it allows nested
type resolution to work) to always execute the original entry-point,
causing potential infinite loops because the designated fork module
script is not executed as expected.

This commit fixes this by not encoding the entry-point information into
the state that is captured as part of the `execArgv`. Instead the
entry-point information is always retrieved from the parsed rest command
line arguments in the final stage (`phase4`).

Fixes #1812.

src/bin.ts

@@ -48,7 +48,7 @@ export function main(
   const state: BootstrapState = {
     shouldUseChildProcess: false,
     isInChildProcess: false,
-    entrypoint: __filename,
+    tsNodeScript: __filename,
     parseArgvResult: args,
   };
   return bootstrap(state);
@@ -62,7 +62,7 @@ export function main(
 export interface BootstrapState {
   isInChildProcess: boolean;
   shouldUseChildProcess: boolean;
-  entrypoint: string;
+  tsNodeScript: string;
   parseArgvResult: ReturnType<typeof parseArgv>;
   phase2Result?: ReturnType<typeof phase2>;
   phase3Result?: ReturnType<typeof phase3>;
@@ -319,28 +319,16 @@ Options:
     process.exit(0);
   }
 
-  // Figure out which we are executing: piped stdin, --eval, REPL, and/or entrypoint
-  // This is complicated because node's behavior is complicated
-  // `node -e code -i ./script.js` ignores -e
-  const executeEval = code != null && !(interactive && restArgs.length);
-  const executeEntrypoint = !executeEval && restArgs.length > 0;
-  const executeRepl =
-    !executeEntrypoint &&
-    (interactive || (process.stdin.isTTY && !executeEval));
-  const executeStdin = !executeEval && !executeRepl && !executeEntrypoint;
-
   const cwd = cwdArg || process.cwd();
-  /** Unresolved.  May point to a symlink, not realpath.  May be missing file extension */
-  const scriptPath = executeEntrypoint ? resolve(cwd, restArgs[0]) : undefined;
 
-  if (esm) payload.shouldUseChildProcess = true;
+  // If ESM is explicitly enabled through the flag, stage3 should be run in a child process
+  // with the ESM loaders configured.
+  if (esm) {
+    payload.shouldUseChildProcess = true;
+  }
+
   return {
-    executeEval,
-    executeEntrypoint,
-    executeRepl,
-    executeStdin,
     cwd,
-    scriptPath,
   };
 }
 
@@ -372,7 +360,18 @@ function phase3(payload: BootstrapState) {
     esm,
     experimentalSpecifierResolution,
   } = payload.parseArgvResult;
-  const { cwd, scriptPath } = payload.phase2Result!;
+  const { cwd } = payload.phase2Result!;
+
+  // NOTE: When we transition to a child process for ESM, the entry-point script determined
+  // here might not be the one used later in `phase4`. This can happen when we execute the
+  // original entry-point but then the process forks itself using e.g. `child_process.fork`.
+  // We will always use the original TS project in forked processes anyway, so it is
+  // expected and acceptable to retrieve the entry-point information here in `phase2`.
+  // See: https://github.com/TypeStrong/ts-node/issues/1812.
+  const { entryPointPath } = getEntryPointInfo(
+    payload.parseArgvResult!,
+    payload.phase2Result!
+  );
 
   const preloadedConfig = findAndReadConfig({
     cwd,
@@ -387,7 +386,12 @@ function phase3(payload: BootstrapState) {
     compilerHost,
     ignore,
     logError,
-    projectSearchDir: getProjectSearchDir(cwd, scriptMode, cwdMode, scriptPath),
+    projectSearchDir: getProjectSearchDir(
+      cwd,
+      scriptMode,
+      cwdMode,
+      entryPointPath
+    ),
     project,
     skipProject,
     skipIgnore,
@@ -403,23 +407,76 @@ function phase3(payload: BootstrapState) {
       experimentalSpecifierResolution as ExperimentalSpecifierResolution,
   });
 
-  if (preloadedConfig.options.esm) payload.shouldUseChildProcess = true;
+  // If ESM is enabled through the parsed tsconfig, stage4 should be run in a child
+  // process with the ESM loaders configured.
+  if (preloadedConfig.options.esm) {
+    payload.shouldUseChildProcess = true;
+  }
+
   return { preloadedConfig };
 }
 
+/**
+ * Determines the entry-point information from the argv and phase2 result. This
+ * method will be invoked in two places:
+ *
+ *   1. In phase 3 to be able to find a project from the potential entry-point script.
+ *   2. In phase 4 to determine the actual entry-point script.
+ *
+ * Note that we need to explicitly re-resolve the entry-point information in the final
+ * stage because the previous stage information could be modified when the bootstrap
+ * invocation transitioned into a child process for ESM.
+ *
+ * Stages before (phase 4) can and will be cached by the child process through the Brotli
+ * configuration and entry-point information is only reliable in the final phase. More
+ * details can be found in here: https://github.com/TypeStrong/ts-node/issues/1812.
+ */
+function getEntryPointInfo(
+  argvResult: NonNullable<BootstrapState['parseArgvResult']>,
+  phase2Result: NonNullable<BootstrapState['phase2Result']>
+) {
+  const { code, interactive, restArgs } = argvResult;
+  const { cwd } = phase2Result;
+
+  // Figure out which we are executing: piped stdin, --eval, REPL, and/or entrypoint
+  // This is complicated because node's behavior is complicated
+  // `node -e code -i ./script.js` ignores -e
+  const executeEval = code != null && !(interactive && restArgs.length);
+  const executeEntrypoint = !executeEval && restArgs.length > 0;
+  const executeRepl =
+    !executeEntrypoint &&
+    (interactive || (process.stdin.isTTY && !executeEval));
+  const executeStdin = !executeEval && !executeRepl && !executeEntrypoint;
+
+  /** Unresolved.  May point to a symlink, not realpath.  May be missing file extension */
+  const entryPointPath = executeEntrypoint
+    ? resolve(cwd, restArgs[0])
+    : undefined;
+
+  return {
+    executeEval,
+    executeEntrypoint,
+    executeRepl,
+    executeStdin,
+    entryPointPath,
+  };
+}
+
 function phase4(payload: BootstrapState) {
-  const { isInChildProcess, entrypoint } = payload;
+  const { isInChildProcess, tsNodeScript } = payload;
   const { version, showConfig, restArgs, code, print, argv } =
     payload.parseArgvResult;
+  const { cwd } = payload.phase2Result!;
+  const { preloadedConfig } = payload.phase3Result!;
+
   const {
+    entryPointPath,
+    executeEntrypoint,
     executeEval,
-    cwd,
-    executeStdin,
     executeRepl,
-    executeEntrypoint,
-    scriptPath,
-  } = payload.phase2Result!;
-  const { preloadedConfig } = payload.phase3Result!;
+    executeStdin,
+  } = getEntryPointInfo(payload.parseArgvResult!, payload.phase2Result!);
+
   /**
    * <repl>, [stdin], and [eval] are all essentially virtual files that do not exist on disc and are backed by a REPL
    * service to handle eval-ing of code.
@@ -566,12 +623,13 @@ function phase4(payload: BootstrapState) {
 
   // Prepend `ts-node` arguments to CLI for child processes.
   process.execArgv.push(
-    entrypoint,
+    tsNodeScript,
     ...argv.slice(2, argv.length - restArgs.length)
   );
+
   // TODO this comes from BoostrapState
   process.argv = [process.argv[1]]
-    .concat(executeEntrypoint ? ([scriptPath] as string[]) : [])
+    .concat(executeEntrypoint ? ([entryPointPath] as string[]) : [])
     .concat(restArgs.slice(executeEntrypoint ? 1 : 0));
 
   // Execute the main contents (either eval, script or piped).

src/child/child-entrypoint.ts

@@ -8,8 +8,9 @@ const base64Payload = base64ConfigArg.slice(argPrefix.length);
 const payload = JSON.parse(
   brotliDecompressSync(Buffer.from(base64Payload, 'base64')).toString()
 ) as BootstrapState;
+
 payload.isInChildProcess = true;
-payload.entrypoint = __filename;
+payload.tsNodeScript = __filename;
 payload.parseArgvResult.argv = process.argv;
 payload.parseArgvResult.restArgs = process.argv.slice(3);
 

src/test/esm-loader.spec.ts

@@ -358,6 +358,28 @@ test.suite('esm', (test) => {
         });
       }
 
+      test.suite('esm child process and forking', (test) => {
+        test('should be able to fork vanilla NodeJS script', async () => {
+          const { err, stdout, stderr } = await exec(
+            `${BIN_PATH} --esm ./esm-child-process/process-forking/index.mts`
+          );
+
+          expect(err).toBe(null);
+          expect(stdout.trim()).toBe('Passing: from main');
+          expect(stderr).toBe('');
+        });
+
+        test('should be able to fork into a nested TypeScript ESM script', async () => {
+          const { err, stdout, stderr } = await exec(
+            `${BIN_PATH} --esm ./esm-child-process/process-forking-nested-esm/index.mts`
+          );
+
+          expect(err).toBe(null);
+          expect(stdout.trim()).toBe('Passing: from main');
+          expect(stderr).toBe('');
+        });
+      });
+
       test.suite('parent passes signals to child', (test) => {
         test.runSerially();
 

tests/esm-child-process/process-forking-nested-esm/index.mts

@@ -0,0 +1,23 @@
+import { fork } from 'child_process';
+import { dirname, join } from 'path';
+import { fileURLToPath } from 'url';
+
+// Initially set the exit code to non-zero. We only set it to `0` when the
+// worker process finishes properly with the expected stdout message.
+process.exitCode = 1;
+
+const projectDir = dirname(fileURLToPath(import.meta.url));
+const workerProcess = fork(join(projectDir, 'worker.mts'), [], {
+  stdio: 'pipe',
+});
+
+let stdout = '';
+
+workerProcess.stdout.on('data', (chunk) => (stdout += chunk.toString('utf8')));
+workerProcess.on('error', () => (process.exitCode = 1));
+workerProcess.on('close', (status, signal) => {
+  if (status === 0 && signal === null && stdout.trim() === 'Works') {
+    console.log('Passing: from main');
+    process.exitCode = 0;
+  }
+});

tests/esm-child-process/process-forking-nested-esm/tsconfig.json

@@ -0,0 +1,5 @@
+{
+  "compilerOptions": {
+    "module": "Node16"
+  }
+}

tests/esm-child-process/process-forking-nested-esm/worker.mts

@@ -0,0 +1,3 @@
+const message: string = 'Works';
+
+console.log(message);

tests/esm-child-process/process-forking/index.mts

@@ -0,0 +1,23 @@
+import { fork } from 'child_process';
+import { dirname, join } from 'path';
+import { fileURLToPath } from 'url';
+
+// Initially set the exit code to non-zero. We only set it to `0` when the
+// worker process finishes properly with the expected stdout message.
+process.exitCode = 1;
+
+const projectDir = dirname(fileURLToPath(import.meta.url));
+const workerProcess = fork(join(projectDir, 'worker.mjs'), [], {
+  stdio: 'pipe',
+});
+
+let stdout = '';
+
+workerProcess.stdout.on('data', (chunk) => (stdout += chunk.toString('utf8')));
+workerProcess.on('error', () => (process.exitCode = 1));
+workerProcess.on('close', (status, signal) => {
+  if (status === 0 && signal === null && stdout.trim() === 'Works') {
+    console.log('Passing: from main');
+    process.exitCode = 0;
+  }
+});

tests/esm-child-process/process-forking/tsconfig.json

@@ -0,0 +1,5 @@
+{
+  "compilerOptions": {
+    "module": "Node16"
+  }
+}

tests/esm-child-process/process-forking/worker.mjs

@@ -0,0 +1 @@
+console.log('Works');