Include stdout on a failed gn desc call, and test for it.

tools/engine_tool/lib/src/gn.dart

@@ -67,7 +67,7 @@ interface class Gn {
       result = JsonObject.parse(process.stdout);
     } on FormatException catch (e) {
       _environment.logger.fatal(
-        'Failed to parse JSON output from `gn desc`:\n$e',
+        'Failed to parse JSON output from `gn desc`:\n$e\n${process.stdout}',
       );
     }
 
@@ -136,6 +136,10 @@ sealed class BuildTarget {
   @mustBeOverridden
   @override
   int get hashCode;
+
+  @mustBeOverridden
+  @override
+  String toString();
 }
 
 /// A build target that produces a [shared library][] or [static library][].
@@ -158,6 +162,9 @@ final class LibraryBuildTarget extends BuildTarget {
 
   @override
   int get hashCode => Object.hash(label, testOnly);
+
+  @override
+  String toString() => 'LibraryBuildTarget($label, testOnly=$testOnly)';
 }
 
 /// A build target that produces an [executable][] program.
@@ -184,4 +191,7 @@ final class ExecutableBuildTarget extends BuildTarget {
 
   @override
   int get hashCode => Object.hash(label, testOnly, executable);
+
+  @override
+  String toString() => 'ExecutableBuildTarget($label, testOnly=$testOnly, executable=$executable)';
 }

tools/engine_tool/test/gn_test.dart

@@ -0,0 +1,112 @@
+// Copyright 2013 The Flutter Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+import 'package:engine_tool/src/gn.dart';
+import 'package:engine_tool/src/label.dart';
+import 'package:litetest/litetest.dart';
+
+import 'utils.dart';
+
+void main() {
+  test('gn.desc handles a non-zero exit code', () async {
+    final TestEnvironment testEnv = TestEnvironment.withTestEngine(
+      cannedProcesses: <CannedProcess>[
+        CannedProcess(
+          (List<String> command) => command.contains('desc'),
+          exitCode: 1,
+          stdout: 'stdout',
+          stderr: 'stderr',
+        ),
+      ],
+    );
+    try {
+      final Gn gn = Gn.fromEnvironment(testEnv.environment);
+      await gn.desc('out/Release', TargetPattern('//foo', 'bar'));
+      fail('Expected an exception');
+    } catch (e) {
+      final String message = '$e';
+      expect(message, contains('Failed to run'));
+      expect(message, contains('exit code 1'));
+      expect(message, contains('STDOUT:\nstdout'));
+      expect(message, contains('STDERR:\nstderr'));
+    } finally {
+      testEnv.cleanup();
+    }
+  });
+
+  test('gn.desc handles unparseable stdout', () async {
+    final TestEnvironment testEnv = TestEnvironment.withTestEngine(
+      cannedProcesses: <CannedProcess>[
+        CannedProcess(
+          (List<String> command) => command.contains('desc'),
+          stdout: 'not json',
+        ),
+      ],
+    );
+    try {
+      final Gn gn = Gn.fromEnvironment(testEnv.environment);
+      await gn.desc('out/Release', TargetPattern('//foo', 'bar'));
+      fail('Expected an exception');
+    } catch (e) {
+      final String message = '$e';
+      expect(message, contains('Failed to parse JSON'));
+      expect(message, contains('not json'));
+    } finally {
+      testEnv.cleanup();
+    }
+  });
+
+  test('gn.desc parses build targets', () async {
+    final TestEnvironment testEnv = TestEnvironment.withTestEngine(
+      cannedProcesses: <CannedProcess>[
+        CannedProcess(
+          (List<String> command) => command.contains('desc'),
+          stdout: '''
+            {
+              "//foo/bar:baz_test": {
+                "outputs": ["//out/host_debug/foo/bar/baz_test"],
+                "testonly": true,
+                "type": "executable"
+              },
+              "//foo/bar:baz_shared_library": {
+                "testonly": false,
+                "type": "shared_library"
+              },
+              "//foo/bar:baz_static_library": {
+                "testonly": false,
+                "type": "static_library"
+              }
+            }
+          ''',
+        ),
+      ],
+    );
+    try {
+      final Gn gn = Gn.fromEnvironment(testEnv.environment);
+      final List<BuildTarget> targets = await gn.desc('out/Release', TargetPattern('//foo', 'bar'));
+      expect(targets, hasLength(3));
+
+      // There should be exactly one binary test target and two library targets.
+      final ExecutableBuildTarget testTarget = targets.whereType<ExecutableBuildTarget>().single;
+      expect(testTarget, ExecutableBuildTarget(
+        label: Label('//foo/bar', 'baz_test'),
+        testOnly: true,
+        executable: 'out/host_debug/foo/bar/baz_test',
+      ));
+
+      final List<LibraryBuildTarget> libraryTargets = targets.whereType<LibraryBuildTarget>().toList();
+      expect(libraryTargets, hasLength(2));
+      expect(libraryTargets.contains(LibraryBuildTarget(
+        label: Label('//foo/bar', 'baz_shared_library'),
+        testOnly: false,
+      )), isTrue);
+      expect(libraryTargets.contains(LibraryBuildTarget(
+        label: Label('//foo/bar', 'baz_static_library'),
+        testOnly: false,
+      )), isTrue);
+    } finally {
+      testEnv.cleanup();
+    }
+  });
+}