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@@ -5,6 +5,9 @@
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 module Piston
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   module Git
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     class Commit < Piston::Revision
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+      class InvalidCommit < RuntimeError; end
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+      class Gone < InvalidCommit; end
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+
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       alias_method :commit, :revision
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       def client
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@@ -13,6 +16,19 @@
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       def git(*args)
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         client.git(*args)
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+      end
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+
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+      def recalled_commit_id
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+        recalled_values[Piston::Git::COMMIT]
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+      end
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+
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+      def validate!
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+        begin
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+          data = git(:ls_remote, @repository.url)
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+          self
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+        rescue Piston::Git::Client::CommandError
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+          raise Piston::Git::Commit::Gone, "Repository at #{@repository.url} does not exist anymore"
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+        end
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       end
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       def name

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@@ -1 +1,34 @@
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+require File.dirname(__FILE__) + "/../../../test_helper"
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+
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+class TestGitCommitValidation < Test::Unit::TestCase
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+  def setup
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+    @repository = mock("repository")
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+    @repository.stubs(:url).returns("git://my-git-repos/my-project.git")
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+  end
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+
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+  def test_is_invalid_if_cannot_ls_remote_repository
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+    commit = new_commit("HEAD")
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+    commit.expects(:git).with(:ls_remote, @repository.url).raises(Piston::Git::Client::CommandError)
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+    assert_raise Piston::Git::Commit::Gone do
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+      commit.validate!
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+    end
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+  end
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+
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+  def test_is_valid_when_ls_remote_succeeds
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+    commit = new_commit("HEAD")
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+    commit.expects(:git).with(:ls_remote, @repository.url).returns(INFO)
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+    assert_nothing_raised do
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+      commit.validate!
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+    end
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+  end
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+
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+  protected
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+  def new_commit(commit, recalled_values={})
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+    Piston::Git::Commit.new(@repository, commit, recalled_values)
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+  end
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+  
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+  INFO = <<EOF
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+a7c46c702243f145a4089b0cb33d189870f1ae53	HEAD
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+EOF
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+end