Delimit messages using varints, as per sass/embedded-protocol#38

See sass/embedded-protocol#37

lib/src/util/length_delimited_transformer.dart

@@ -8,6 +8,7 @@ import 'dart:typed_data';
 
 import 'package:async/async.dart';
 import 'package:stream_channel/stream_channel.dart';
+import 'package:typed_data/typed_data.dart';
 
 /// A [StreamChannelTransformer] that converts a channel that sends and receives
 /// arbitrarily-chunked binary data to one that sends and receives packets of
@@ -21,17 +22,16 @@ final StreamChannelTransformer<Uint8List, List<int>> lengthDelimited =
 /// into a stream of packet contents.
 final lengthDelimitedDecoder =
     StreamTransformer<List<int>, Uint8List>.fromBind((stream) {
-  // The buffer into which the four-byte little-endian length of the next packet
-  // will be written.
-  var lengthBuffer = Uint8List(4);
+  // The number of bits we've consumed so far to fill out [nextMessageLength].
+  int nextMessageLengthBits = 0;
 
-  // The index of the next byte to write to [lengthBuffer]. Once this is equal
-  // to [lengthBuffer.length], the full length is available.
-  var lengthBufferIndex = 0;
-
-  // The length of the next message, in bytes, read from [lengthBuffer] once
-  // it's full.
-  int nextMessageLength;
+  // The length of the next message, in bytes.
+  //
+  // This is built up from a [varint]. Once it's fully consumed, [buffer] is
+  // initialized and this is reset to 0.
+  //
+  // [varint]: https://developers.google.com/protocol-buffers/docs/encoding#varints
+  int nextMessageLength = 0;
 
   // The buffer into which the packet data itself is written. Initialized once
   // [nextMessageLength] is known.
@@ -53,47 +53,52 @@ final lengthDelimitedDecoder =
     // multiple messages.
     var i = 0;
 
-    // Adds bytes from [chunk] to [destination] at [destinationIndex] without
-    // overflowing the bounds of [destination], and increments [i] for each byte
-    // written.
-    //
-    // Returns the number of bytes written.
-    int writeFromChunk(Uint8List destination, int destinationIndex) {
-      var bytesToWrite =
-          math.min(destination.length - destinationIndex, chunk.length - i);
-      destination.setRange(
-          destinationIndex, destinationIndex + bytesToWrite, chunk, i);
-      i += bytesToWrite;
-      return bytesToWrite;
-    }
-
     while (i < chunk.length) {
       // We can be in one of two states here:
       //
-      // * Both [nextMessageLength] and [buffer] are `null`, in which case we're
-      //   waiting until we have four bytes in [lengthBuffer] to know how big of
-      //   a buffer to allocate.
+      // * [buffer] is `null`, in which case we're adding data to
+      //   [nextMessageLength] until we reach a byte with its most significant
+      //   bit set to 0.
       //
-      // * Neither [nextMessageLength] nor [buffer] are `null`, in which case
-      //   we're waiting for [buffer] to have [nextMessageLength] in it before
-      //   we send it to [queue.local.sink] and start waiting for the next
-      //   message.
-      if (nextMessageLength == null) {
-        lengthBufferIndex += writeFromChunk(lengthBuffer, lengthBufferIndex);
-        if (lengthBufferIndex < 4) return;
-
-        nextMessageLength =
-            ByteData.view(lengthBuffer.buffer).getUint32(0, Endian.little);
+      // * [buffer] is not `null`, in which case we're waiting for [buffer] to
+      //   have [nextMessageLength] bytes in it before we send it to
+      //   [queue.local.sink] and start waiting for the next message.
+      if (buffer == null) {
+        var byte = chunk[i];
+
+        // Varints encode data in the 7 lower bits of each byte, which we access
+        // by masking with 0x7f = 0b01111111.
+        nextMessageLength += (byte & 0x7f) << nextMessageLengthBits;
+        nextMessageLengthBits += 7;
+        i++;
+
+        // If the byte is higher than 0x7f = 0b01111111, that means its high bit
+        // is set which and so there are more bytes to consume before we know
+        // the full message length.
+        if (byte > 0x7f) continue;
+
+        // Otherwise, [nextMessageLength] is now finalized and we can allocate
+        // the data buffer.
         buffer = Uint8List(nextMessageLength);
         bufferIndex = 0;
       }
 
-      bufferIndex += writeFromChunk(buffer, bufferIndex);
+      // Copy as many bytes as we can from [chunk] to [buffer], making sure not
+      // to try to copy more than the buffer can hold (if the chunk has another
+      // message after the current one) or more than the chunk has available (if
+      // the current message is split across multiple chunks).
+      var bytesToWrite =
+          math.min(buffer.length - bufferIndex, chunk.length - i);
+      buffer.setRange(
+          bufferIndex, bufferIndex + bytesToWrite, chunk, i);
+      i += bytesToWrite;
+      bufferIndex += bytesToWrite;
       if (bufferIndex < nextMessageLength) return;
 
-      sink.add(Uint8List.view(buffer.buffer, 0, nextMessageLength));
-      lengthBufferIndex = 0;
-      nextMessageLength = null;
+      // Once we've filled the buffer, emit it and reset our state.
+      sink.add(buffer);
+      nextMessageLength = 0;
+      nextMessageLengthBits = 0;
       buffer = null;
       bufferIndex = null;
     }
@@ -106,9 +111,22 @@ final lengthDelimitedDecoder =
 final lengthDelimitedEncoder =
     StreamTransformer<Uint8List, List<int>>.fromHandlers(
         handleData: (message, sink) {
-  var messageLength = Uint8List(4);
-  ByteData.view(messageLength.buffer)
-      .setUint32(0, message.length, Endian.little);
-  sink.add(messageLength);
+  var length = message.length;
+  if (length == 0) {
+    sink.add([0]);
+    return;
+  }
+
+  // Write the length in varint format, 7 bits at a time from least to most
+  // significant.
+  var lengthBuffer = Uint8Buffer();
+  while (length > 0) {
+    // The highest-order bit indicates whether more bytes are necessary to fully
+    // express the number. The lower 7 bits indicate the number's value.
+    lengthBuffer.add((length > 0x7f ? 0x80 : 0)|(length & 0x7f));
+    length >>= 7;
+  }
+
+  sink.add(Uint8List.view(lengthBuffer.buffer, 0, lengthBuffer.length));
   sink.add(message);
 });

pubspec.yaml

@@ -1,5 +1,5 @@
 name: sass_embedded
-version: 1.0.0-beta.5
+version: 1.0.0-dev
 description: An implementation of the Sass embedded protocol using Dart Sass.
 author: Sass Team
 homepage: https://github.com/sass/dart-sass-embedded
@@ -19,6 +19,7 @@ dependencies:
   source_span: ^1.1.0
   stack_trace: ^1.6.0
   stream_channel: ">=1.6.0 <3.0.0"
+  typed_data: ^1.1.0
 
 dev_dependencies:
   cli_pkg: ^1.0.0-beta.8

test/length_delimited_test.dart

@@ -25,20 +25,20 @@ void main() {
     test("encodes an empty message", () {
       sink.add([]);
       sink.close();
-      expect(collectBytes(stream), completion(equals([0, 0, 0, 0])));
+      expect(collectBytes(stream), completion(equals([0])));
     });
 
     test("encodes a message of length 1", () {
       sink.add([123]);
       sink.close();
-      expect(collectBytes(stream), completion(equals([1, 0, 0, 0, 123])));
+      expect(collectBytes(stream), completion(equals([1, 123])));
     });
 
     test("encodes a message of length greater than 256", () {
       sink.add(List.filled(300, 1));
       sink.close();
       expect(collectBytes(stream),
-          completion(equals([44, 1, 0, 0, ...List.filled(300, 1)])));
+          completion(equals([172, 2, ...List.filled(300, 1)])));
     });
 
     test("encodes multiple messages", () {
@@ -49,7 +49,7 @@ void main() {
       expect(
           collectBytes(stream),
           completion(equals(
-              [1, 0, 0, 0, 10, 2, 0, 0, 0, 20, 30, 3, 0, 0, 0, 40, 50, 60])));
+              [1, 10, 2, 20, 30, 3, 40, 50, 60])));
     });
   });
 
@@ -64,75 +64,59 @@ void main() {
 
     group("decodes an empty message", () {
       test("from a single chunk", () {
-        sink.add([0, 0, 0, 0]);
-        expect(queue, emits(isEmpty));
-      });
-
-      test("from multiple chunks", () {
-        sink.add([0, 0]);
-        sink.add([0, 0]);
-        expect(queue, emits(isEmpty));
-      });
-
-      test("from one chunk per byte", () {
-        sink..add([0])..add([0])..add([0])..add([0]);
+        sink.add([0]);
         expect(queue, emits(isEmpty));
       });
 
       test("from a chunk that contains more data", () {
-        sink.add([0, 0, 0, 0, 1, 0, 0, 0, 100]);
+        sink.add([0, 1, 100]);
         expect(queue, emits(isEmpty));
       });
     });
 
     group("decodes a longer message", () {
       test("from a single chunk", () {
-        sink.add([4, 0, 0, 0, 1, 2, 3, 4]);
-        expect(queue, emits([1, 2, 3, 4]));
+        sink.add([172, 2, ...List.filled(300, 1)]);
+        expect(queue, emits(List.filled(300, 1)));
       });
 
       test("from multiple chunks", () {
-        sink..add([4, 0])..add([0, 0, 1, 2])..add([3, 4]);
-        expect(queue, emits([1, 2, 3, 4]));
+        sink..add([172])..add([2, 1])..add(List.filled(299, 1));
+        expect(queue, emits(List.filled(300, 1)));
       });
 
       test("from one chunk per byte", () {
-        for (var byte in [4, 0, 0, 0, 1, 2, 3, 4]) {
+        for (var byte in [172, 2, ...List.filled(300, 1)]) {
           sink.add([byte]);
         }
-        expect(queue, emits([1, 2, 3, 4]));
+        expect(queue, emits(List.filled(300, 1)));
       });
 
       test("from a chunk that contains more data", () {
-        sink.add([4, 0, 0, 0, 1, 2, 3, 4, 1, 0, 0, 0]);
-        expect(queue, emits([1, 2, 3, 4]));
-      });
-
-      test("of length greater than 256", () {
-        sink.add([44, 1, 0, 0, ...List.filled(300, 1)]);
+        sink.add([172, 2, ...List.filled(300, 1), 1, 10]);
         expect(queue, emits(List.filled(300, 1)));
       });
     });
 
     group("decodes multiple messages", () {
       test("from single chunk", () {
-        sink.add([4, 0, 0, 0, 1, 2, 3, 4, 2, 0, 0, 0, 101, 102]);
+        sink.add([4, 1, 2, 3, 4, 2, 101, 102]);
         expect(queue, emits([1, 2, 3, 4]));
         expect(queue, emits([101, 102]));
       });
 
       test("from multiple chunks", () {
-        sink..add([4, 0])..add([0, 0, 1, 2, 3, 4, 2, 0])..add([0, 0, 101, 102]);
+        sink..add([4])..add([1, 2, 3, 4, 172])..add([2, ...List.filled(300, 1)]);
         expect(queue, emits([1, 2, 3, 4]));
-        expect(queue, emits([101, 102]));
+        expect(queue, emits(List.filled(300, 1)));
       });
 
       test("from one chunk per byte", () {
-        for (var byte in [4, 0, 0, 0, 1, 2, 3, 4, 2, 0, 0, 0, 101, 102]) {
+        for (var byte in [4, 1, 2, 3, 4, 172, 2, ...List.filled(300, 1)]) {
           sink.add([byte]);
         }
         expect(queue, emits([1, 2, 3, 4]));
-        expect(queue, emits([101, 102]));
+        expect(queue, emits(List.filled(300, 1)));
       });
     });
   });

test/protocol_test.dart

@@ -26,7 +26,7 @@ void main() {
     });
 
     test("caused by an invalid message", () async {
-      process.stdin.add([1, 0, 0, 0, 0]);
+      process.stdin.add([1, 0]);
       await expectParseError(
           process, "Protocol message contained an invalid tag (zero).");
       expect(await process.exitCode, 76);