Implement sass --embedded in pure JS mode

lib/src/compiler-path.ts

@@ -53,8 +53,10 @@ export const compilerCommand = (() => {
         `sass-embedded-${platform}-${arch}/dart-sass/src/sass.snapshot`
       ),
     ];
-  } catch (ignored) {
-    // ignored
+  } catch (e) {
+    if (!(isErrnoException(e) && e.code === 'MODULE_NOT_FOUND')) {
+      throw e;
+    }
   }
 
   try {
@@ -70,10 +72,21 @@ export const compilerCommand = (() => {
     }
   }
 
+  try {
+    return [
+      process.execPath,
+      p.join(p.dirname(require.resolve('sass')), 'sass.js'),
+    ];
+  } catch (e: unknown) {
+    if (!(isErrnoException(e) && e.code === 'MODULE_NOT_FOUND')) {
+      throw e;
+    }
+  }
+
   throw new Error(
     "Embedded Dart Sass couldn't find the embedded compiler executable. " +
       'Please make sure the optional dependency ' +
-      `sass-embedded-${platform}-${arch} is installed in ` +
+      `sass-embedded-${platform}-${arch} or sass is installed in ` +
       'node_modules.'
   );
 })();

lib/src/embedded/index.mjs

@@ -0,0 +1,7 @@
+// Copyright 2024 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import * as embedded from './index.js';
+
+export const main = embedded.main;

lib/src/embedded/index.ts

@@ -0,0 +1,46 @@
+// Copyright 2024 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import {MessagePort, isMainThread, workerData} from 'worker_threads';
+import {toJson} from '@bufbuild/protobuf';
+
+import {SyncMessagePort} from '../sync-process/sync-message-port';
+import {WorkerDispatcher} from './worker_dispatcher';
+import * as proto from '../vendor/embedded_sass_pb';
+
+export function main(
+  spawnCompilationDispatcher: (
+    mailbox: SyncMessagePort,
+    sendPort: MessagePort
+  ) => void
+): void {
+  if (isMainThread) {
+    if (process.argv.length > 3) {
+      if (process.argv[3] === '--version') {
+        console.log(
+          toJson(
+            proto.OutboundMessage_VersionResponseSchema,
+            WorkerDispatcher.versionResponse()
+          )
+        );
+      } else {
+        console.error(
+          'sass --embedded is not intended to be executed with additional arguments.\n' +
+            'See https://github.com/sass/dart-sass#embedded-dart-sass for details.'
+        );
+        process.exitCode = 64;
+      }
+      return;
+    }
+
+    new WorkerDispatcher().listen();
+  } else {
+    const port = workerData.port as MessagePort;
+    spawnCompilationDispatcher(new SyncMessagePort(port), {
+      postMessage(buffer: Uint8Array): void {
+        port.postMessage(buffer, [buffer.buffer]);
+      },
+    } as MessagePort);
+  }
+}

lib/src/embedded/reusable_worker.ts

@@ -0,0 +1,60 @@
+// Copyright 2024 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import {MessagePort, Worker} from 'worker_threads';
+
+import {SyncMessagePort} from '../sync-process/sync-message-port';
+
+export class ReusableWorker {
+  private readonly worker: Worker;
+
+  private readonly receivePort: MessagePort;
+
+  private readonly sendPort: SyncMessagePort;
+
+  private onMessage = this.defaultOnMessage;
+
+  constructor(path: string) {
+    const {port1, port2} = SyncMessagePort.createChannel();
+    this.worker = new Worker(path, {
+      workerData: {port: port2},
+      transferList: [port2],
+      argv: process.argv.slice(2),
+    });
+    this.receivePort = port1;
+    this.sendPort = new SyncMessagePort(port1);
+
+    this.receivePort.on('message', value => this.onMessage(value));
+  }
+
+  borrow(listener: (value: Uint8Array) => void): void {
+    if (this.onMessage !== this.defaultOnMessage) {
+      throw new Error('ReusableWorker has already been borrowed.');
+    }
+    this.onMessage = listener;
+  }
+
+  release(): void {
+    if (this.onMessage === this.defaultOnMessage) {
+      throw new Error('ReusableWorker has not been borrowed.');
+    }
+    this.onMessage = this.defaultOnMessage;
+  }
+
+  send(value: Uint8Array): void {
+    this.sendPort.postMessage(value, [value.buffer]);
+  }
+
+  terminate(): void {
+    this.sendPort.close();
+    this.worker.terminate();
+    this.receivePort.close();
+  }
+
+  private defaultOnMessage(value: Uint8Array): void {
+    throw new Error(
+      `Shouldn't receive a message before being borrowed: ${value}.`
+    );
+  }
+}

lib/src/embedded/utils.ts

@@ -0,0 +1,50 @@
+// Copyright 2024 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import {create} from '@bufbuild/protobuf';
+
+import * as proto from '../vendor/embedded_sass_pb';
+
+export const errorId = 0xffffffff;
+
+export function paramsError(message: string): proto.ProtocolError {
+  return create(proto.ProtocolErrorSchema, {
+    id: errorId,
+    type: proto.ProtocolErrorType.PARAMS,
+    message: message,
+  });
+}
+
+export function parseError(message: string): proto.ProtocolError {
+  return create(proto.ProtocolErrorSchema, {
+    type: proto.ProtocolErrorType.PARSE,
+    message: message,
+  });
+}
+
+export function handleError(
+  error: Error | proto.ProtocolError,
+  {messageId}: {messageId?: number} = {}
+): proto.ProtocolError {
+  if (error instanceof Error) {
+    const errorMessage = `${error.message}\n${error.stack}`;
+    process.stderr.write(`Internal compiler error: ${errorMessage}`);
+    process.exitCode = 70; // EX_SOFTWARE
+    return create(proto.ProtocolErrorSchema, {
+      id: messageId ?? errorId,
+      type: proto.ProtocolErrorType.INTERNAL,
+      message: errorMessage,
+    });
+  } else {
+    error.id = messageId ?? errorId;
+    process.stderr.write(
+      `Host caused ${proto.ProtocolErrorType[error.type].toLowerCase()} error`
+    );
+    if (error.id !== errorId) process.stderr.write(` with request ${error.id}`);
+    process.stderr.write(`: ${error.message}\n`);
+    // PROTOCOL error from https://bit.ly/2poTt90
+    process.exitCode = 76; // EX_PROTOCOL
+    return error;
+  }
+}

lib/src/embedded/worker_dispatcher.ts

@@ -0,0 +1,175 @@
+// Copyright 2024 Google LLC. Use of this source code is governed by an
+// MIT-style license that can be found in the LICENSE file or at
+// https://opensource.org/licenses/MIT.
+
+import {Observable} from 'rxjs';
+import {takeUntil} from 'rxjs/operators';
+import {create, fromBinary, toBinary} from '@bufbuild/protobuf';
+import * as varint from 'varint';
+
+import * as pkg from '../../../package.json';
+import {PacketTransformer} from '../packet-transformer';
+import {ReusableWorker} from './reusable_worker';
+import {errorId, handleError, paramsError, parseError} from './utils';
+import * as proto from '../vendor/embedded_sass_pb';
+
+export class WorkerDispatcher {
+  private readonly allWorkers: ReusableWorker[] = [];
+
+  private readonly inactiveWorkers: ReusableWorker[] = [];
+
+  private readonly activeWorkers = new Map<number, ReusableWorker>();
+
+  private readonly stdin$ = new Observable<Buffer>(observer => {
+    process.stdin.on('data', buffer => observer.next(buffer));
+  }).pipe(
+    takeUntil(
+      new Promise(resolve => {
+        process.stdin.on('close', () => resolve(undefined));
+      })
+    )
+  );
+
+  private readonly packetTransformer = new PacketTransformer(
+    this.stdin$,
+    buffer => process.stdout.write(buffer)
+  );
+
+  listen(): void {
+    this.packetTransformer.protobufs$.subscribe({
+      next: (buffer: Uint8Array) => {
+        let compilationId: number;
+        try {
+          compilationId = varint.decode(buffer);
+        } catch (error) {
+          throw parseError(`Invalid compilation ID varint: ${error}`);
+        }
+
+        try {
+          if (compilationId !== 0) {
+            if (this.activeWorkers.has(compilationId)) {
+              const worker = this.activeWorkers.get(compilationId)!;
+              worker.send(buffer);
+            } else {
+              const worker = this.getWorker(compilationId);
+              this.activeWorkers.set(compilationId, worker);
+              worker.send(buffer);
+            }
+            return;
+          }
+
+          let message;
+          try {
+            message = fromBinary(
+              proto.InboundMessageSchema,
+              new Uint8Array(buffer.buffer, varint.decode.bytes)
+            );
+          } catch (error) {
+            throw parseError(`Invalid protobuf: ${error}`);
+          }
+
+          if (message.message.case !== 'versionRequest') {
+            throw paramsError(
+              `Only VersionRequest may have wire ID 0, was ${message.message.case}.`
+            );
+          }
+          const request = message.message.value;
+          const response = WorkerDispatcher.versionResponse();
+          response.id = request.id;
+          this.send(
+            0,
+            create(proto.OutboundMessageSchema, {
+              message: {
+                case: 'versionResponse',
+                value: response,
+              },
+            })
+          );
+        } catch (error) {
+          this.handleError(error);
+        }
+      },
+      complete: () => {
+        this.allWorkers.forEach(worker => worker.terminate());
+      },
+      error: error => {
+        this.handleError(parseError(error.message));
+      },
+    });
+  }
+
+  private getWorker(compilationId: number): ReusableWorker {
+    let worker: ReusableWorker;
+    if (this.inactiveWorkers.length > 0) {
+      worker = this.inactiveWorkers.pop()!;
+    } else {
+      worker = new ReusableWorker(process.argv[1]);
+      this.allWorkers.push(worker);
+    }
+
+    worker.borrow(buffer => {
+      const category = buffer.at(0);
+      const packet = Buffer.from(buffer.buffer, 1);
+
+      switch (category) {
+        case 0:
+          this.packetTransformer.writeProtobuf(packet);
+          break;
+        case 1:
+          this.activeWorkers.delete(compilationId);
+          worker.release();
+          this.inactiveWorkers.push(worker);
+          this.packetTransformer.writeProtobuf(packet);
+          break;
+        case 2: {
+          this.packetTransformer.writeProtobuf(packet);
+          /* eslint-disable-next-line n/no-process-exit */
+          process.exit();
+        }
+      }
+    });
+
+    return worker;
+  }
+
+  private handleError(
+    error: Error | proto.ProtocolError,
+    {
+      compilationId,
+      messageId,
+    }: {compilationId?: number; messageId?: number} = {}
+  ): void {
+    this.sendError(compilationId ?? errorId, handleError(error, {messageId}));
+    process.stdin.destroy();
+  }
+
+  private send(compilationId: number, message: proto.OutboundMessage): void {
+    const compilationIdLength = varint.encodingLength(compilationId);
+    const encodedMessage = toBinary(proto.OutboundMessageSchema, message);
+    const buffer = new Uint8Array(compilationIdLength + encodedMessage.length);
+    varint.encode(compilationId, buffer);
+    buffer.set(encodedMessage, compilationIdLength);
+    this.packetTransformer.writeProtobuf(buffer);
+  }
+
+  private sendError(compilationId: number, error: proto.ProtocolError): void {
+    this.send(
+      compilationId,
+      create(proto.OutboundMessageSchema, {
+        message: {
+          case: 'error',
+          value: error,
+        },
+      })
+    );
+  }
+
+  static versionResponse(): proto.OutboundMessage_VersionResponse {
+    return create(proto.OutboundMessage_VersionResponseSchema, {
+      protocolVersion: pkg['protocol-version'],
+      compilerVersion: pkg['compiler-version'],
+      implementationVersion: pkg['version'],
+      implementationName: 'dart-sass',
+    });
+  }
+}