Add assets digests handler (#1709)

This entry point will be used for hot reloading, in order to initially
retrieve digests of all assets, as build_runner might not know list of
all of them, unlike client.

Author: samogot

build_runner/lib/src/server/server.dart

@@ -21,6 +21,7 @@ import 'path_to_asset_id.dart';
 
 const _performancePath = r'$perf';
 final _graphPath = r'$graph';
+final _assetsDigestPath = r'$assetDigests';
 final _buildUpdatesProtocol = r'$livereload';
 final _buildUpdatesMessage = 'update';
 final entrypointExtensionMarker = '/* ENTRYPOINT_EXTENTION_MARKER */';
@@ -79,6 +80,9 @@ class ServeHandler implements BuildState {
       if (request.url.path == _performancePath) {
         return _performanceHandler(request);
       }
+      if (request.url.path == _assetsDigestPath) {
+        return _assetsDigestHandler(request, rootDir);
+      }
       if (request.url.path.startsWith(_graphPath)) {
         var graphHandler = await _assetGraphHandler;
         return await graphHandler.handle(
@@ -112,6 +116,24 @@ class ServeHandler implements BuildState {
         headers: {HttpHeaders.contentTypeHeader: 'text/html'});
   }
 
+  Future<shelf.Response> _assetsDigestHandler(
+      shelf.Request request, String rootDir) async {
+    var assertPathList = jsonDecode(await request.readAsString()) as List;
+    var rootPackage = _state.packageGraph.root.name;
+    var results = <String, String>{};
+    for (String path in assertPathList) {
+      try {
+        var assetId = pathToAssetId(rootPackage, rootDir, p.url.split(path));
+        var digest = await _state.reader.digest(assetId);
+        results[path] = digest.toString();
+      } on AssetNotFoundException {
+        results.remove(path);
+      }
+    }
+    return new shelf.Response.ok(jsonEncode(results),
+        headers: {HttpHeaders.contentTypeHeader: 'application/json'});
+  }
+
   void _warnForEmptyDirectory(String rootDir) {
     if (!_state.assetGraph
         .packageNodes(_rootPackage)

build_runner/test/server/serve_handler_test.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
+import 'dart:convert';
 import 'dart:io';
 
 import 'package:logging/logging.dart';
@@ -165,6 +166,23 @@ void main() {
     });
   });
 
+  test('serve asset digests', () async {
+    _addSource('a|web/index.html', 'content1');
+    _addSource('a|lib/some.dart.js', 'content2');
+    _addSource('a|lib/another.dart.js', 'content3');
+    var response = await serveHandler.handlerFor('web')(new Request(
+        'GET', Uri.parse('http://server.com/\$assetDigests'),
+        body: jsonEncode([
+          'index.html',
+          'packages/a/some.dart.js',
+          'packages/a/absent.dart.js'
+        ])));
+    expect(jsonDecode(await response.readAsString()), {
+      'index.html': '7e55db001d319a94b0b713529a756623',
+      'packages/a/some.dart.js': 'eea670f4ac941df71a3b5f268ebe3eac',
+    });
+  });
+
   group('build updates', () {
     test('injects client code if enabled', () async {
       _addSource('a|web/some.js', entrypointExtensionMarker + '\nalert(1)');