Add support for pub install.

utils/pub/app_cache.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * The application-specific cache of installed packages.
+ *
+ * This cache contains symlinks to all packages used by an app. These links
+ * point either to the [SystemCache] or to some other location on the local
+ * filesystem.
+ *
+ * This corresponds to the application's packages directory.

[Bob Nystrom, 2012/05/03 00:15:49]

Put "packages" in quotes for clarity?

[nweiz, 2012/05/04 01:03:43]

Done.

+ */
+class AppCache {
+  /**
+   * The root directory of the application containing this cache.

[Bob Nystrom, 2012/05/03 00:15:49]

Instead of root directory, can this just be the package that represents the app
itself?

[nweiz, 2012/05/04 01:03:43]

Done.

+   */
+  final String appDir;
+
+  /**
+   * The system-wide cache which caches packages that need to be fetched over
+   * the network.
+   */
+  final SystemCache systemCache;
+
+  /**
+   * Packages which have already been loaded into memory.
+   */
+  final Map<PackageId, Package> _loadedPackages;
+
+  /**
+   * Packages which are currently being asynchronously installed to the cache.
+   */
+  final Map<PackageId, Future<Package>> _pendingInstalls;
+
+  /**
+   * Creates a new package cache for the given application directory. It's
+   * backed by the packages subdirectory of [appDir].
+   */
+  AppCache(this.appDir, this.systemCache)
+  : _loadedPackages  = new Map<PackageId, Package>(),
+    _pendingInstalls = new Map<PackageId, Future<Package>>();
+
+  /**
+   * Returns the directory containing the packages.
+   */
+  // TODO(rnystrom): Make this path configurable.

[Bob Nystrom, 2012/05/03 00:15:49]

TODO(nweiz)

I believe the goog convention is to not add TODOs that aren't your name. :)

[nweiz, 2012/05/04 01:03:43]

This was a pre-existing comment on code I moved from command_update.dart.

+  String get packagesDir() => join(this.appDir, 'packages');
+
+  /**
+   * Ensures that the package identified by [id] is installed to the cache,
+   * loads it, and returns it.
+   *
+   * If this completes successfully, the package is guaranteed to be importable
+   * using the `package:` scheme.
+   *
+   * This will automatically install the package to the system-wide cache as
+   * well if it requires network access to retrieve (specifically, if
+   * `id.source.shouldCache` is true).
+   *
+   * See also [installTransitively].
+   */
+  Future<Package> install(PackageId id) {
+    var package = _loadedPackages[id];
+    if (package != null) return package;

[Bob Nystrom, 2012/05/03 00:15:49]

new Future.immediate(package);

[nweiz, 2012/05/04 01:03:43]

Done.

+
+    var pending = _pendingInstalls[id];
+    if (pending != null) return pending;
+
+    var packageDir = join(packagesDir, id.name);
+    var future = ensureNestedDir(dirname(packageDir)).chain((_) {
+      return exists(packageDir);
+    }).chain((exists) {
+      // If the package already exists in the cache, no need to re-install.
+      if (exists) return new Future.immediate(null);
+
+      if (id.source.shouldCache) {
+        return systemCache.install(id).
+          chain((pkg) => createSymlink(pkg.dir, packageDir));

[Bob Nystrom, 2012/05/03 00:15:49]

Style nit, consider moving chain( to the previous line?

[nweiz, 2012/05/04 01:03:43]

Done.

+      } else {
+        return id.source.install(id, packageDir).transform((found) {
+          if (found) return null;
+          throw 'Package ${id.name} not found in source "${id.source.name}".';

[Bob Nystrom, 2012/05/03 00:15:49]

We should eventually have a more robust error-handling system, logging, etc.
Leave a TODO here to do something more sophisticated. At the very least it
should eventually throw a real exception type.

[nweiz, 2012/05/04 01:03:43]

Done.

+        });
+      }
+    }).chain((_) => Package.load(packageDir));
+
+    future.then((pkg) => _loadedPackages[id] = pkg);
+    always(future, () => _pendingInstalls.remove(id));
+    _pendingInstalls[id] = future;
+
+    return future;
+  }
+
+  /**
+   * Installs the package identified by [id] and all its transitive
+   * dependencies.
+   */
+  Future<Package> installTransitively(PackageId id) {
+    var seen = new Set<PackageId>();
+    Future<Package> helper(id) {
+      if (seen.contains(id)) return new Future.immediate(null);
+      seen.add(id);
+
+      return install(id).chain((package) {
+        return Futures.wait(package.dependencies.map(helper)).
+          transform((_) => package);
+      });
+    }
+
+    return helper(id);
+  }
+}