Make the Git source install to the system cache.

utils/pub/git_source.dart

Index: utils/pub/git_source.dart
diff --git a/utils/pub/git_source.dart b/utils/pub/git_source.dart
index ae7f53b1a2489d61a1cf4f9c7fefc5ba08121597..90f619bb500e26dd166af3e997f6cd4b4898bc2d 100644
--- a/utils/pub/git_source.dart
+++ b/utils/pub/git_source.dart
@@ -7,6 +7,7 @@
 #import('io.dart');
 #import('package.dart');
 #import('source.dart');
+#import('source_registry.dart');
 #import('utils.dart');
 
 /**
@@ -15,32 +16,44 @@
 class GitSource extends Source {
   final String name = "git";
 
-  // TODO(rnystrom): Git packages could in theory be cached, but that adds a
-  // lot of complexity. When you install a git package, you are installing
-  // and pinning to a specific commit. That means different installs of the
-  // same git path but at different commits need to be disambiguated in the
-  // system cache. It may also lead to a lot of garbage in the system cache.
-  // For now, we are punting and simply not caching them.
-  final bool shouldCache = false;
+  final bool shouldCache = true;
 
   GitSource();
 
   /**
    * Clones a Git repo to the local filesystem.
+   *
+   * The Git cache directory is a little idiosyncratic. At the top level, it
+   * contains a directory for each commit of each repository, named `<package
+   * name>-<commit hash>`. These are the canonical package directories that are
+   * linked to from the `packages/` directory.
+   *
+   * In addition, the Git system cache contains a subdirectory named `cache/`
+   * which contains a directory for each separate repository URL, named
+   * `<package name>-<url hash>`. These are used to check out the repository
+   * itself; each of the commit-specific directories are clones of a directory
+   * in `cache/`.
    */
-  Future<bool> install(PackageId id, String destPath) {
+  Future<Package> installToSystemCache(PackageId id) {
+    var revisionCachePath;
+
     return isGitInstalled.chain((installed) {
-      if (installed) {
-        return runProcess("git",
-            ["clone", "--progress", id.description, destPath],
-            pipeStdout: true, pipeStderr: true).
-          transform((result) => result.success);
-      } else {
+      if (!installed) {
         throw new Exception(
             "Cannot install '${id.name}' from Git (${id.description}).\n"
             "Please ensure Git is correctly installed.");
       }
-    });
+
+      return ensureDir(join(systemCacheRoot, 'cache'));
+    }).chain((_) => _ensureRepoCache(id))
+      .chain((_) => _revisionCachePath(id, "HEAD"))
+      .chain((path) {
+      revisionCachePath = path;
+      return exists(revisionCachePath);
+    }).chain((exists) {
+      if (exists) return new Future.immediate(null);
+      return _clone(_repoCachePath(id), revisionCachePath);
+    }).chain((_) => Package.load(revisionCachePath, systemCache.sources));
   }
 
   /**
@@ -58,4 +71,66 @@ class GitSource extends Source {
       throw new FormatException("The description must be a git URL.");
     }
   }
+
+  /**
+   * Ensure that the canonical clone of the repository referred to by [id] (the
+   * one in `<system cache>/git/cache`) exists and is up-to-date. Returns a
+   * future that completes once this is finished and throws an exception if it
+   * fails.
+   */
+  Future _ensureRepoCache(PackageId id) {
+    var path = _repoCachePath(id);
+    return exists(path).chain((exists) {
+      if (!exists) return _clone(id.description, path);
+
+      return runProcess("git", ["pull", "--force"], workingDir: path,
+          pipeStdout: true, pipeStderr: true).transform((result) {
+        if (!result.success) throw 'Git failed.';
+        return null;
+      });
+    });
+  }
+
+  /**
+   * Returns a future that completes to the revision hash of the repository for
+   * [id] at [ref], which can be any Git ref.
+   */
+  Future<String> _revisionAt(PackageId id, String ref) {
+    return runProcess("git", ["rev-parse", ref],
+        workingDir: _repoCachePath(id), pipeStderr: true).transform((result) {
+      if (!result.success) throw 'Git failed.';
+      return result.stdout[0];
+    });
+  }
+
+  /**
+   * Returns the path to the revision-specific cache of [id] at [ref], which can
+   * be any Git ref.
+   */
+  Future<String> _revisionCachePath(PackageId id, String ref) {
+    return _revisionAt(id, ref).transform((rev) {
+      var revisionCacheName = '${id.name}-$rev';
+      return join(systemCacheRoot, revisionCacheName);
+    });
+  }
+
+  /**
+   * Clones the repo at the URI [from] to the path [to] on the local filesystem.
+   */
+  Future _clone(String from, String to) {
+    return runProcess("git", ["clone", from, to], pipeStdout: true,
+        pipeStderr: true).transform((result) {
+      if (!result.success) throw 'Git failed.';
+      return null;
+    });
+  }
+
+  /**
+   * Returns the path to the canonical clone of the repository referred to by
+   * [id] (the one in `<system cache>/git/cache`).
+   */
+  String _repoCachePath(PackageId id) {
+    var repoCacheName = '${id.name}-${sha1(id.description)}';
+    return join(systemCacheRoot, 'cache', repoCacheName);
+  }
 }