// Copyright 2014 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 'dart:async';

import 'package:async/async.dart';
import 'package:convert/convert.dart';
import 'package:crypto/crypto.dart';
import 'package:meta/meta.dart';
import 'package:pool/pool.dart';
import 'package:process/process.dart';

import '../artifacts.dart';
import '../base/file_system.dart';
import '../base/logger.dart';
import '../base/platform.dart';
import '../base/utils.dart';
import '../cache.dart';
import '../convert.dart';
import 'exceptions.dart';
import 'file_store.dart';
import 'source.dart';

export 'source.dart';

/// A reasonable amount of files to open at the same time.
///
/// This number is somewhat arbitrary - it is difficult to detect whether
/// or not we'll run out of file descriptors when using async dart:io
/// APIs.
const int kMaxOpenFiles = 64;

/// Configuration for the build system itself.
class BuildSystemConfig {
  /// Create a new [BuildSystemConfig].
  const BuildSystemConfig({this.resourcePoolSize});

  /// The maximum number of concurrent tasks the build system will run.
  ///
  /// If not provided, defaults to [platform.numberOfProcessors].
  final int resourcePoolSize;
}

/// A Target describes a single step during a flutter build.
///
/// The target inputs are required to be files discoverable via a combination
/// of at least one of the environment values and zero or more local values.
///
/// To determine if the action for a target needs to be executed, the
/// [BuildSystem] computes a key of the file contents for both inputs and
/// outputs. This is tracked separately in the [FileStore]. The key may
/// be either an md5 hash of the file contents or a timestamp.
///
/// A Target has both implicit and explicit inputs and outputs. Only the
/// later are safe to evaluate before invoking the [buildAction]. For example,
/// a wildcard output pattern requires the outputs to exist before it can
/// glob files correctly.
///
/// - All listed inputs are considered explicit inputs.
/// - Outputs which are provided as [Source.pattern].
///   without wildcards are considered explicit.
/// - The remaining outputs are considered implicit.
///
/// For each target, executing its action creates a corresponding stamp file
/// which records both the input and output files. This file is read by
/// subsequent builds to determine which file hashes need to be checked. If the
/// stamp file is missing, the target's action is always rerun.
///
///  file: `example_target.stamp`
///
/// {
///   "inputs": [
///      "absolute/path/foo",
///      "absolute/path/bar",
///      ...
///    ],
///    "outputs": [
///      "absolute/path/fizz"
///    ]
/// }
///
/// ## Code review
///
/// ### Targets should only depend on files that are provided as inputs
///
/// Example: gen_snapshot must be provided as an input to the aot_elf
/// build steps, even though it isn't a source file. This ensures that changes
/// to the gen_snapshot binary (during a local engine build) correctly
/// trigger a corresponding build update.
///
/// Example: aot_elf has a dependency on the dill and packages file
/// produced by the kernel_snapshot step.
///
/// ### Targets should declare all outputs produced
///
/// If a target produces an output it should be listed, even if it is not
/// intended to be consumed by another target.
///
/// ## Unit testing
///
/// Most targets will invoke an external binary which makes unit testing
/// trickier. It is recommend that for unit testing that a Fake is used and
/// provided via the dependency injection system. a [Testbed] may be used to
/// set up the environment before the test is run. Unit tests should fully
/// exercise the rule, ensuring that the existing input and output verification
/// logic can run, as well as verifying it correctly handles provided defines
/// and meets any additional contracts present in the target.
abstract class Target {
  const Target();
  /// The user-readable name of the target.
  ///
  /// This information is surfaced in the assemble commands and used as an
  /// argument to build a particular target.
  String get name;

  /// A name that measurements can be categorized under for this [Target].
  ///
  /// Unlike [name], this is not expected to be unique, so multiple targets
  /// that are conceptually the same can share an analytics name.
  ///
  /// If not provided, defaults to [name]
  String get analyticsName => name;

  /// The dependencies of this target.
  List<Target> get dependencies;

  /// The input [Source]s which are diffed to determine if a target should run.
  List<Source> get inputs;

  /// The output [Source]s which we attempt to verify are correctly produced.
  List<Source> get outputs;

  /// A list of zero or more depfiles, located directly under {BUILD_DIR}.
  List<String> get depfiles => const <String>[];

  /// Whether this target can be executed with the given [environment].
  ///
  /// Returning `true` will cause [build] to be skipped. This is equivalent
  /// to a build that produces no outputs.
  bool canSkip(Environment environment) => false;

  /// The action which performs this build step.
  Future<void> build(Environment environment);

  /// Create a [Node] with resolved inputs and outputs.
  Node _toNode(Environment environment) {
    final ResolvedFiles inputsFiles = resolveInputs(environment);
    final ResolvedFiles outputFiles = resolveOutputs(environment);
    return Node(
      this,
      inputsFiles.sources,
      outputFiles.sources,
      <Node>[
        for (final Target target in dependencies) target._toNode(environment),
      ],
      environment,
      inputsFiles.containsNewDepfile,
    );
  }

  /// Invoke to remove the stamp file if the [buildAction] threw an exception.
  void clearStamp(Environment environment) {
    final File stamp = _findStampFile(environment);
    if (stamp.existsSync()) {
      stamp.deleteSync();
    }
  }

  void _writeStamp(
    List<File> inputs,
    List<File> outputs,
    Environment environment,
  ) {
    final File stamp = _findStampFile(environment);
    final List<String> inputPaths = <String>[];
    for (final File input in inputs) {
      inputPaths.add(input.path);
    }
    final List<String> outputPaths = <String>[];
    for (final File output in outputs) {
      outputPaths.add(output.path);
    }
    final Map<String, Object> result = <String, Object>{
      'inputs': inputPaths,
      'outputs': outputPaths,
    };
    if (!stamp.existsSync()) {
      stamp.createSync();
    }
    stamp.writeAsStringSync(json.encode(result));
  }

  /// Resolve the set of input patterns and functions into a concrete list of
  /// files.
  ResolvedFiles resolveInputs(Environment environment) {
    return _resolveConfiguration(inputs, depfiles, environment, implicit: true, inputs: true);
  }

  /// Find the current set of declared outputs, including wildcard directories.
  ///
  /// The [implicit] flag controls whether it is safe to evaluate [Source]s
  /// which uses functions, behaviors, or patterns.
  ResolvedFiles resolveOutputs(Environment environment) {
    return _resolveConfiguration(outputs, depfiles, environment, inputs: false);
  }

  /// Performs a fold across this target and its dependencies.
  T fold<T>(T initialValue, T combine(T previousValue, Target target)) {
    final T dependencyResult = dependencies.fold(
        initialValue, (T prev, Target t) => t.fold(prev, combine));
    return combine(dependencyResult, this);
  }

  /// Convert the target to a JSON structure appropriate for consumption by
  /// external systems.
  ///
  /// This requires constants from the [Environment] to resolve the paths of
  /// inputs and the output stamp.
  Map<String, Object> toJson(Environment environment) {
    return <String, Object>{
      'name': name,
      'dependencies': <String>[
        for (final Target target in dependencies) target.name,
      ],
      'inputs': <String>[
        for (final File file in resolveInputs(environment).sources) file.path,
      ],
      'outputs': <String>[
        for (final File file in resolveOutputs(environment).sources) file.path,
      ],
      'stamp': _findStampFile(environment).absolute.path,
    };
  }

  /// Locate the stamp file for a particular target name and environment.
  File _findStampFile(Environment environment) {
    final String fileName = '$name.stamp';
    return environment.buildDir.childFile(fileName);
  }

  static ResolvedFiles _resolveConfiguration(List<Source> config,
    List<String> depfiles, Environment environment, { bool implicit = true, bool inputs = true,
  }) {
    final SourceVisitor collector = SourceVisitor(environment, inputs);
    for (final Source source in config) {
      source.accept(collector);
    }
    depfiles.forEach(collector.visitDepfile);
    return collector;
  }
}

/// The [Environment] defines several constants for use during the build.
///
/// The environment contains configuration and file paths that are safe to
/// depend on and reference during the build.
///
/// Example (Good):
///
/// Use the environment to determine where to write an output file.
///
/// ```dart
///    environment.buildDir.childFile('output')
///      ..createSync()
///      ..writeAsStringSync('output data');
/// ```
///
/// Example (Bad):
///
/// Use a hard-coded path or directory relative to the current working
/// directory to write an output file.
///
/// ```dart
///   globals.fs.file('build/linux/out')
///     ..createSync()
///     ..writeAsStringSync('output data');
/// ```
///
/// Example (Good):
///
/// Using the build mode to produce different output. Note that the action
/// is still responsible for outputting a different file, as defined by the
/// corresponding output [Source].
///
/// ```dart
///    final BuildMode buildMode = getBuildModeFromDefines(environment.defines);
///    if (buildMode == BuildMode.debug) {
///      environment.buildDir.childFile('debug.output')
///        ..createSync()
///        ..writeAsStringSync('debug');
///    } else {
///      environment.buildDir.childFile('non_debug.output')
///        ..createSync()
///        ..writeAsStringSync('non_debug');
///    }
/// ```
class Environment {
  /// Create a new [Environment] object.
  ///
  /// [engineVersion] should be set to null for local engine builds.
  factory Environment({
    @required Directory projectDir,
    @required Directory outputDir,
    @required Directory cacheDir,
    @required Directory flutterRootDir,
    @required FileSystem fileSystem,
    @required Logger logger,
    @required Artifacts artifacts,
    @required ProcessManager processManager,
    @required String engineVersion,
    Directory buildDir,
    Map<String, String> defines = const <String, String>{},
    Map<String, String> inputs = const <String, String>{},
  }) {
    // Compute a unique hash of this build's particular environment.
    // Sort the keys by key so that the result is stable. We always
    // include the engine and dart versions.
    String buildPrefix;
    final List<String> keys = defines.keys.toList()..sort();
    final StringBuffer buffer = StringBuffer();
    // The engine revision is `null` for local or custom engines.
    if (engineVersion != null) {
      buffer.write(engineVersion);
    }
    for (final String key in keys) {
      buffer.write(key);
      buffer.write(defines[key]);
    }
    buffer.write(outputDir.path);
    final String output = buffer.toString();
    final Digest digest = md5.convert(utf8.encode(output));
    buildPrefix = hex.encode(digest.bytes);

    final Directory rootBuildDir = buildDir ?? projectDir.childDirectory('build');
    final Directory buildDirectory = rootBuildDir.childDirectory(buildPrefix);
    return Environment._(
      outputDir: outputDir,
      projectDir: projectDir,
      buildDir: buildDirectory,
      rootBuildDir: rootBuildDir,
      cacheDir: cacheDir,
      defines: defines,
      flutterRootDir: flutterRootDir,
      fileSystem: fileSystem,
      logger: logger,
      artifacts: artifacts,
      processManager: processManager,
      engineVersion: engineVersion,
      inputs: inputs,
    );
  }

  /// Create a new [Environment] object for unit testing.
  ///
  /// Any directories not provided will fallback to a [testDirectory]
  @visibleForTesting
  factory Environment.test(Directory testDirectory, {
    Directory projectDir,
    Directory outputDir,
    Directory cacheDir,
    Directory flutterRootDir,
    Directory buildDir,
    Map<String, String> defines = const <String, String>{},
    Map<String, String> inputs = const <String, String>{},
    String engineVersion,
    @required FileSystem fileSystem,
    @required Logger logger,
    @required Artifacts artifacts,
    @required ProcessManager processManager,
  }) {
    return Environment(
      projectDir: projectDir ?? testDirectory,
      outputDir: outputDir ?? testDirectory,
      cacheDir: cacheDir ?? testDirectory,
      flutterRootDir: flutterRootDir ?? testDirectory,
      buildDir: buildDir,
      defines: defines,
      inputs: inputs,
      fileSystem: fileSystem,
      logger: logger,
      artifacts: artifacts,
      processManager: processManager,
      engineVersion: engineVersion,
    );
  }

  Environment._({
    @required this.outputDir,
    @required this.projectDir,
    @required this.buildDir,
    @required this.rootBuildDir,
    @required this.cacheDir,
    @required this.defines,
    @required this.flutterRootDir,
    @required this.processManager,
    @required this.logger,
    @required this.fileSystem,
    @required this.artifacts,
    @required this.engineVersion,
    @required this.inputs,
  });

  /// The [Source] value which is substituted with the path to [projectDir].
  static const String kProjectDirectory = '{PROJECT_DIR}';

  /// The [Source] value which is substituted with the path to [buildDir].
  static const String kBuildDirectory = '{BUILD_DIR}';

  /// The [Source] value which is substituted with the path to [cacheDir].
  static const String kCacheDirectory = '{CACHE_DIR}';

  /// The [Source] value which is substituted with a path to the flutter root.
  static const String kFlutterRootDirectory = '{FLUTTER_ROOT}';

  /// The [Source] value which is substituted with a path to [outputDir].
  static const String kOutputDirectory = '{OUTPUT_DIR}';

  /// The `PROJECT_DIR` environment variable.
  ///
  /// This should be root of the flutter project where a pubspec and dart files
  /// can be located.
  final Directory projectDir;

  /// The `BUILD_DIR` environment variable.
  ///
  /// The root of the output directory where build step intermediates and
  /// outputs are written. Current usages of assemble configure ths to be
  /// a unique directory under `.dart_tool/flutter_build`, though it can
  /// be placed anywhere. The uniqueness is only enforced by callers, and
  /// is currently done by hashing the build configuration.
  final Directory buildDir;

  /// The `CACHE_DIR` environment variable.
  ///
  /// Defaults to `{FLUTTER_ROOT}/bin/cache`. The root of the artifact cache for
  /// the flutter tool.
  final Directory cacheDir;

  /// The `FLUTTER_ROOT` environment variable.
  ///
  /// Defaults to the value of [Cache.flutterRoot].
  final Directory flutterRootDir;

  /// The `OUTPUT_DIR` environment variable.
  ///
  /// Must be provided to configure the output location for the final artifacts.
  final Directory outputDir;

  /// Additional configuration passed to the build targets.
  ///
  /// Setting values here forces a unique build directory to be chosen
  /// which prevents the config from leaking into different builds.
  final Map<String, String> defines;

  /// Additional input files passed to the build targets.
  ///
  /// Unlike [defines], values set here do not force a new build configuration.
  /// This is useful for passing file inputs that may have changing paths
  /// without running builds from scratch.
  ///
  /// It is the responsibility of the [Target] to declare that an input was
  /// used in an output depfile.
  final Map<String, String> inputs;

  /// The root build directory shared by all builds.
  final Directory rootBuildDir;

  final ProcessManager processManager;

  final Logger logger;

  final Artifacts artifacts;

  final FileSystem fileSystem;

  /// The version of the current engine, or `null` if built with a local engine.
  final String engineVersion;
}

/// The result information from the build system.
class BuildResult {
  BuildResult({
    @required this.success,
    this.exceptions = const <String, ExceptionMeasurement>{},
    this.performance = const <String, PerformanceMeasurement>{},
    this.inputFiles = const <File>[],
    this.outputFiles = const <File>[],
  });

  final bool success;
  final Map<String, ExceptionMeasurement> exceptions;
  final Map<String, PerformanceMeasurement> performance;
  final List<File> inputFiles;
  final List<File> outputFiles;

  bool get hasException => exceptions.isNotEmpty;
}

/// The build system is responsible for invoking and ordering [Target]s.
abstract class BuildSystem {
  /// Const constructor to allow subclasses to be const.
  const BuildSystem();

  /// Build [target] and all of its dependencies.
  Future<BuildResult> build(
    Target target,
    Environment environment, {
    BuildSystemConfig buildSystemConfig = const BuildSystemConfig(),
  });

  /// Perform an incremental build of [target] and all of its dependencies.
  ///
  /// If [previousBuild] is not provided, a new incremental build is
  /// initialized.
  Future<BuildResult> buildIncremental(
    Target target,
    Environment environment,
    BuildResult previousBuild,
  );
}

class FlutterBuildSystem extends BuildSystem {
  const FlutterBuildSystem({
    @required FileSystem fileSystem,
    @required Platform platform,
    @required Logger logger,
  }) : _fileSystem = fileSystem,
       _platform = platform,
       _logger = logger;

  final FileSystem _fileSystem;
  final Platform _platform;
  final Logger _logger;

  @override
  Future<BuildResult> build(
    Target target,
    Environment environment, {
    BuildSystemConfig buildSystemConfig = const BuildSystemConfig(),
  }) async {
    environment.buildDir.createSync(recursive: true);
    environment.outputDir.createSync(recursive: true);

    // Load file store from previous builds.
    final File cacheFile = environment.buildDir.childFile(FileStore.kFileCache);
    final FileStore fileCache = FileStore(
      cacheFile: cacheFile,
      logger: _logger,
    )..initialize();

    // Perform sanity checks on build.
    checkCycles(target);

    final Node node = target._toNode(environment);
    final _BuildInstance buildInstance = _BuildInstance(
      environment: environment,
      fileCache: fileCache,
      buildSystemConfig: buildSystemConfig,
      logger: _logger,
      fileSystem: _fileSystem,
      platform: _platform,
    );
    bool passed = true;
    try {
      passed = await buildInstance.invokeTarget(node);
    } finally {
      // Always persist the file cache to disk.
      fileCache.persist();
    }
    // TODO(jonahwilliams): this is a bit of a hack, due to various parts of
    // the flutter tool writing these files unconditionally. Since Xcode uses
    // timestamps to track files, this leads to unnecessary rebuilds if they
    // are included. Once all the places that write these files have been
    // tracked down and moved into assemble, these checks should be removable.
    // We also remove files under .dart_tool, since these are intermediaries
    // and don't need to be tracked by external systems.
    {
      buildInstance.inputFiles.removeWhere((String path, File file) {
        return path.contains('.flutter-plugins') ||
                       path.contains('xcconfig') ||
                     path.contains('.dart_tool');
      });
      buildInstance.outputFiles.removeWhere((String path, File file) {
        return path.contains('.flutter-plugins') ||
                       path.contains('xcconfig') ||
                     path.contains('.dart_tool');
      });
    }
    trackSharedBuildDirectory(
      environment, _fileSystem, buildInstance.outputFiles,
    );
    environment.buildDir.childFile('outputs.json')
      .writeAsStringSync(json.encode(buildInstance.outputFiles.keys.toList()));

    return BuildResult(
      success: passed,
      exceptions: buildInstance.exceptionMeasurements,
      performance: buildInstance.stepTimings,
      inputFiles: buildInstance.inputFiles.values.toList()
          ..sort((File a, File b) => a.path.compareTo(b.path)),
      outputFiles: buildInstance.outputFiles.values.toList()
          ..sort((File a, File b) => a.path.compareTo(b.path)),
    );
  }

  static final Expando<FileStore> _incrementalFileStore = Expando<FileStore>();

  @override
  Future<BuildResult> buildIncremental(
    Target target,
    Environment environment,
    BuildResult previousBuild,
  ) async {
    environment.buildDir.createSync(recursive: true);
    environment.outputDir.createSync(recursive: true);

    FileStore fileCache;
    if (previousBuild == null || _incrementalFileStore[previousBuild] == null) {
      final File cacheFile = environment.buildDir.childFile(FileStore.kFileCache);
      fileCache = FileStore(
        cacheFile: cacheFile,
        logger: _logger,
        strategy: FileStoreStrategy.timestamp,
      )..initialize();
    } else {
      fileCache = _incrementalFileStore[previousBuild];
    }
    final Node node = target._toNode(environment);
    final _BuildInstance buildInstance = _BuildInstance(
      environment: environment,
      fileCache: fileCache,
      buildSystemConfig: const BuildSystemConfig(),
      logger: _logger,
      fileSystem: _fileSystem,
      platform: _platform,
    );
    bool passed = true;
    try {
      passed = await buildInstance.invokeTarget(node);
    } finally {
      fileCache.persistIncremental();
    }
    final BuildResult result = BuildResult(
      success: passed,
      exceptions: buildInstance.exceptionMeasurements,
      performance: buildInstance.stepTimings,
    );
    _incrementalFileStore[result] = fileCache;
    return result;
  }

  /// Write the identifier of the last build into the output directory and
  /// remove the previous build's output.
  ///
  /// The build identifier is the basename of the build directory where
  /// outputs and intermediaries are written, under `.dart_tool/flutter_build`.
  /// This is computed from a hash of the build's configuration.
  ///
  /// This identifier is used to perform a targeted cleanup of the last output
  /// files, if these were not already covered by the built-in cleanup. This
  /// cleanup is only necessary when multiple different build configurations
  /// output to the same directory.
  @visibleForTesting
  void trackSharedBuildDirectory(
    Environment environment,
    FileSystem fileSystem,
    Map<String, File> currentOutputs,
  ) {
    final String currentBuildId = fileSystem.path.basename(environment.buildDir.path);
    final File lastBuildIdFile = environment.outputDir.childFile('.last_build_id');
    if (!lastBuildIdFile.existsSync()) {
      lastBuildIdFile.writeAsStringSync(currentBuildId);
      // No config file, either output was cleaned or this is the first build.
      return;
    }
    final String lastBuildId = lastBuildIdFile.readAsStringSync().trim();
    if (lastBuildId == currentBuildId) {
      // The last build was the same configuration as the current build
      return;
    }
    // Update the output dir with the latest config.
    lastBuildIdFile
      ..createSync()
      ..writeAsStringSync(currentBuildId);
    final File outputsFile = environment.buildDir
      .parent
      .childDirectory(lastBuildId)
      .childFile('outputs.json');

    if (!outputsFile.existsSync()) {
      // There is no output list. This could happen if the user manually
      // edited .last_config or deleted .dart_tool.
      return;
    }
    final List<String> lastOutputs = (json.decode(outputsFile.readAsStringSync()) as List<Object>)
      .cast<String>();
    for (final String lastOutput in lastOutputs) {
      if (!currentOutputs.containsKey(lastOutput)) {
        final File lastOutputFile = fileSystem.file(lastOutput);
        if (lastOutputFile.existsSync()) {
          lastOutputFile.deleteSync();
        }
      }
    }
  }
}


/// An active instance of a build.
class _BuildInstance {
  _BuildInstance({
    this.environment,
    this.fileCache,
    this.buildSystemConfig,
    this.logger,
    this.fileSystem,
    Platform platform,
  })
    : resourcePool = Pool(buildSystemConfig.resourcePoolSize ?? platform?.numberOfProcessors ?? 1);

  final Logger logger;
  final FileSystem fileSystem;
  final BuildSystemConfig buildSystemConfig;
  final Pool resourcePool;
  final Map<String, AsyncMemoizer<bool>> pending = <String, AsyncMemoizer<bool>>{};
  final Environment environment;
  final FileStore fileCache;
  final Map<String, File> inputFiles = <String, File>{};
  final Map<String, File> outputFiles = <String, File>{};

  // Timings collected during target invocation.
  final Map<String, PerformanceMeasurement> stepTimings = <String, PerformanceMeasurement>{};

  // Exceptions caught during the build process.
  final Map<String, ExceptionMeasurement> exceptionMeasurements = <String, ExceptionMeasurement>{};

  Future<bool> invokeTarget(Node node) async {
    final List<bool> results = await Future.wait(node.dependencies.map(invokeTarget));
    if (results.any((bool result) => !result)) {
      return false;
    }
    final AsyncMemoizer<bool> memoizer = pending[node.target.name] ??= AsyncMemoizer<bool>();
    return memoizer.runOnce(() => _invokeInternal(node));
  }

  Future<bool> _invokeInternal(Node node) async {
    final PoolResource resource = await resourcePool.request();
    final Stopwatch stopwatch = Stopwatch()..start();
    bool succeeded = true;
    bool skipped = false;

    // The build system produces a list of aggregate input and output
    // files for the overall build. This list is provided to a hosting build
    // system, such as Xcode, to configure logic for when to skip the
    // rule/phase which contains the flutter build.
    //
    // When looking at the inputs and outputs for the individual rules, we need
    // to be careful to remove inputs that were actually output from previous
    // build steps. This indicates that the file is an intermediary. If
    // these files are included as both inputs and outputs then it isn't
    // possible to construct a DAG describing the build.
    void updateGraph() {
      for (final File output in node.outputs) {
        outputFiles[output.path] = output;
      }
      for (final File input in node.inputs) {
        final String resolvedPath = input.absolute.path;
        if (outputFiles.containsKey(resolvedPath)) {
          continue;
        }
        inputFiles[resolvedPath] = input;
      }
    }

    try {
      // If we're missing a depfile, wait until after evaluating the target to
      // compute changes.
      final bool canSkip = !node.missingDepfile &&
        await node.computeChanges(environment, fileCache, fileSystem, logger);

      if (canSkip) {
        skipped = true;
        logger.printTrace('Skipping target: ${node.target.name}');
        updateGraph();
        return succeeded;
      }
      // Clear old inputs. These will be replaced with new inputs/outputs
      // after the target is run. In the case of a runtime skip, each list
      // must be empty to ensure the previous outputs are purged.
      node.inputs.clear();
      node.outputs.clear();

      // Check if we can skip via runtime dependencies.
      final bool runtimeSkip = node.target.canSkip(environment);
      if (runtimeSkip) {
        logger.printTrace('Skipping target: ${node.target.name}');
        skipped = true;
      } else {
        logger.printTrace('${node.target.name}: Starting due to ${node.invalidatedReasons}');
        await node.target.build(environment);
        logger.printTrace('${node.target.name}: Complete');
        node.inputs.addAll(node.target.resolveInputs(environment).sources);
        node.outputs.addAll(node.target.resolveOutputs(environment).sources);
      }

      // If we were missing the depfile, resolve input files after executing the
      // target so that all file hashes are up to date on the next run.
      if (node.missingDepfile) {
        await fileCache.diffFileList(node.inputs);
      }

      // Always update hashes for output files.
      await fileCache.diffFileList(node.outputs);
      node.target._writeStamp(node.inputs, node.outputs, environment);
      updateGraph();

      // Delete outputs from previous stages that are no longer a part of the
      // build.
      for (final String previousOutput in node.previousOutputs) {
        if (outputFiles.containsKey(previousOutput)) {
          continue;
        }
        final File previousFile = fileSystem.file(previousOutput);
        if (previousFile.existsSync()) {
          previousFile.deleteSync();
        }
      }
    } on Exception catch (exception, stackTrace) {
      // TODO(jonahwilliams): throw specific exception for expected errors to mark
      // as non-fatal. All others should be fatal.
      node.target.clearStamp(environment);
      succeeded = false;
      skipped = false;
      exceptionMeasurements[node.target.name] = ExceptionMeasurement(
          node.target.name, exception, stackTrace);
    } finally {
      resource.release();
      stopwatch.stop();
      stepTimings[node.target.name] = PerformanceMeasurement(
        target: node.target.name,
        elapsedMilliseconds: stopwatch.elapsedMilliseconds,
        skipped: skipped,
        succeeded: succeeded,
        analyicsName: node.target.analyticsName,
      );
    }
    return succeeded;
  }
}

/// Helper class to collect exceptions.
class ExceptionMeasurement {
  ExceptionMeasurement(this.target, this.exception, this.stackTrace, {this.fatal = false});

  final String target;
  final dynamic exception;
  final StackTrace stackTrace;

  /// Whether this exception was a fatal build system error.
  final bool fatal;

  @override
  String toString() => 'target: $target\nexception:$exception\n$stackTrace';
}

/// Helper class to collect measurement data.
class PerformanceMeasurement {
  PerformanceMeasurement({
    @required this.target,
    @required this.elapsedMilliseconds,
    @required this.skipped,
    @required this.succeeded,
    @required this.analyicsName,
  });

  final int elapsedMilliseconds;
  final String target;
  final bool skipped;
  final bool succeeded;
  final String analyicsName;
}

/// Check if there are any dependency cycles in the target.
///
/// Throws a [CycleException] if one is encountered.
void checkCycles(Target initial) {
  void checkInternal(Target target, Set<Target> visited, Set<Target> stack) {
    if (stack.contains(target)) {
      throw CycleException(stack..add(target));
    }
    if (visited.contains(target)) {
      return;
    }
    visited.add(target);
    stack.add(target);
    for (final Target dependency in target.dependencies) {
      checkInternal(dependency, visited, stack);
    }
    stack.remove(target);
  }
  checkInternal(initial, <Target>{}, <Target>{});
}

/// Verifies that all files exist and are in a subdirectory of [Environment.buildDir].
void verifyOutputDirectories(List<File> outputs, Environment environment, Target target) {
  final String buildDirectory = environment.buildDir.resolveSymbolicLinksSync();
  final String projectDirectory = environment.projectDir.resolveSymbolicLinksSync();
  final List<File> missingOutputs = <File>[];
  for (final File sourceFile in outputs) {
    if (!sourceFile.existsSync()) {
      missingOutputs.add(sourceFile);
      continue;
    }
    final String path = sourceFile.path;
    if (!path.startsWith(buildDirectory) && !path.startsWith(projectDirectory)) {
      throw MisplacedOutputException(path, target.name);
    }
  }
  if (missingOutputs.isNotEmpty) {
    throw MissingOutputException(missingOutputs, target.name);
  }
}

/// A node in the build graph.
class Node {
  Node(
    this.target,
    this.inputs,
    this.outputs,
    this.dependencies,
    Environment environment,
    this.missingDepfile,
  ) {
    final File stamp = target._findStampFile(environment);

    // If the stamp file doesn't exist, we haven't run this step before and
    // all inputs were added.
    if (!stamp.existsSync()) {
      // No stamp file, not safe to skip.
      _dirty = true;
      return;
    }
    final String content = stamp.readAsStringSync();
    // Something went wrong writing the stamp file.
    if (content == null || content.isEmpty) {
      stamp.deleteSync();
      // Malformed stamp file, not safe to skip.
      _dirty = true;
      return;
    }
    Map<String, Object> values;
    try {
      values = castStringKeyedMap(json.decode(content));
    } on FormatException {
      // The json is malformed in some way.
      _dirty = true;
      return;
    }
    final Object inputs = values['inputs'];
    final Object outputs = values['outputs'];
    if (inputs is List<Object> && outputs is List<Object>) {
      inputs?.cast<String>()?.forEach(previousInputs.add);
      outputs?.cast<String>()?.forEach(previousOutputs.add);
    } else {
      // The json is malformed in some way.
      _dirty = true;
    }
  }

  /// The resolved input files.
  ///
  /// These files may not yet exist if they are produced by previous steps.
  final List<File> inputs;

  /// The resolved output files.
  ///
  /// These files may not yet exist if the target hasn't run yet.
  final List<File> outputs;

  /// Whether this node is missing a depfile.
  ///
  /// This requires an additional pass of source resolution after the target
  /// has been executed.
  final bool missingDepfile;

  /// The target definition which contains the build action to invoke.
  final Target target;

  /// All of the nodes that this one depends on.
  final List<Node> dependencies;

  /// Output file paths from the previous invocation of this build node.
  final Set<String> previousOutputs = <String>{};

  /// Input file paths from the previous invocation of this build node.
  final Set<String> previousInputs = <String>{};

  /// One or more reasons why a task was invalidated.
  ///
  /// May be empty if the task was skipped.
  final Set<InvalidatedReason> invalidatedReasons = <InvalidatedReason>{};

  /// Whether this node needs an action performed.
  bool get dirty => _dirty;
  bool _dirty = false;

  /// Collect hashes for all inputs to determine if any have changed.
  ///
  /// Returns whether this target can be skipped.
  Future<bool> computeChanges(
    Environment environment,
    FileStore fileStore,
    FileSystem fileSystem,
    Logger logger,
  ) async {
    final Set<String> currentOutputPaths = <String>{
      for (final File file in outputs) file.path,
    };
    // For each input, first determine if we've already computed the key
    // for it. Then collect it to be sent off for diffing as a group.
    final List<File> sourcesToDiff = <File>[];
    final List<File> missingInputs = <File>[];
    for (final File file in inputs) {
      if (!file.existsSync()) {
        missingInputs.add(file);
        continue;
      }

      final String absolutePath = file.path;
      final String previousAssetKey = fileStore.previousAssetKeys[absolutePath];
      if (fileStore.currentAssetKeys.containsKey(absolutePath)) {
        final String currentHash = fileStore.currentAssetKeys[absolutePath];
        if (currentHash != previousAssetKey) {
          invalidatedReasons.add(InvalidatedReason.inputChanged);
          _dirty = true;
        }
      } else {
        sourcesToDiff.add(file);
      }
    }

    // For each output, first determine if we've already computed the key
    // for it. Then collect it to be sent off for hashing as a group.
    for (final String previousOutput in previousOutputs) {
      // output paths changed.
      if (!currentOutputPaths.contains(previousOutput)) {
        _dirty = true;
        invalidatedReasons.add(InvalidatedReason.outputSetChanged);
        // if this isn't a current output file there is no reason to compute the key.
        continue;
      }
      final File file = fileSystem.file(previousOutput);
      if (!file.existsSync()) {
        invalidatedReasons.add(InvalidatedReason.outputMissing);
        _dirty = true;
        continue;
      }
      final String absolutePath = file.path;
      final String previousHash = fileStore.previousAssetKeys[absolutePath];
      if (fileStore.currentAssetKeys.containsKey(absolutePath)) {
        final String currentHash = fileStore.currentAssetKeys[absolutePath];
        if (currentHash != previousHash) {
          invalidatedReasons.add(InvalidatedReason.outputChanged);
          _dirty = true;
        }
      } else {
        sourcesToDiff.add(file);
      }
    }

    // If we depend on a file that doesnt exist on disk, mark the build as
    // dirty. if the rule is not correctly specified, this will result in it
    // always being rerun.
    if (missingInputs.isNotEmpty) {
      _dirty = true;
      final String missingMessage = missingInputs.map((File file) => file.path).join(', ');
      logger.printTrace('invalidated build due to missing files: $missingMessage');
      invalidatedReasons.add(InvalidatedReason.inputMissing);
    }

    // If we have files to diff, compute them asynchronously and then
    // update the result.
    if (sourcesToDiff.isNotEmpty) {
      final List<File> dirty = await fileStore.diffFileList(sourcesToDiff);
      if (dirty.isNotEmpty) {
        invalidatedReasons.add(InvalidatedReason.inputChanged);
        _dirty = true;
      }
    }
    return !_dirty;
  }
}

/// A description of why a target was rerun.
enum InvalidatedReason {
  /// An input file that was expected is missing. This can occur when using
  /// depfile dependencies, or if a target is incorrectly specified.
  inputMissing,

  /// An input file has an updated key.
  inputChanged,

  /// An output file has an updated key.
  outputChanged,

  /// An output file that is expected is missing.
  outputMissing,

  /// The set of expected output files changed.
  outputSetChanged,
}