From 63809a7258d2b14e0abd82859b90db02649061bf Mon Sep 17 00:00:00 2001
From: Nick Gauthier <nick@smartlogicsolutions.com>
Date: Tue, 26 Jan 2010 13:50:44 -0500
Subject: [PATCH] added pipe class. docced and tested

---
 Rakefile           |   2 +-
 lib/hydra.rb       |   1 +
 lib/hydra/pipe.rb  | 109 +++++++++++++++++++++++++++++++++++++++++++++
 test/test_hydra.rb |   7 ---
 test/test_pipe.rb  |  38 ++++++++++++++++
 5 files changed, 149 insertions(+), 8 deletions(-)
 create mode 100644 lib/hydra/pipe.rb
 delete mode 100644 test/test_hydra.rb
 create mode 100644 test/test_pipe.rb

diff --git a/Rakefile b/Rakefile
index dc0faeb..af857fa 100644
--- a/Rakefile
+++ b/Rakefile
@@ -10,7 +10,7 @@ begin
     gem.email = "nick@smartlogicsolutions.com"
     gem.homepage = "http://github.com/ngauthier/hydra"
     gem.authors = ["Nick Gauthier"]
-    gem.add_development_dependency "thoughtbot-shoulda", ">= 0"
+    gem.add_development_dependency "shoulda", "= 2.10.3"
     # gem is a Gem::Specification... see http://www.rubygems.org/read/chapter/20 for additional settings
   end
   Jeweler::GemcutterTasks.new
diff --git a/lib/hydra.rb b/lib/hydra.rb
index e69de29..7f4ad76 100644
--- a/lib/hydra.rb
+++ b/lib/hydra.rb
@@ -0,0 +1 @@
+require 'hydra/pipe'
diff --git a/lib/hydra/pipe.rb b/lib/hydra/pipe.rb
new file mode 100644
index 0000000..f372894
--- /dev/null
+++ b/lib/hydra/pipe.rb
@@ -0,0 +1,109 @@
+module Hydra #:nodoc:
+  # Read and write between two processes via pipes. For example:
+  #   @pipe = Hydra::Pipe.new
+  #   Process.fork do
+  #     @pipe.identify_as_child
+  #     sleep(1)
+  #     puts "A message from my parent:\n#{@pipe.gets}"
+  #     @pipe.close
+  #   end
+  #   @pipe.identify_as_parent
+  #   @pipe.write "Hello, Child!"
+  #   @pipe.close
+  # When the process forks, the pipe is copied. When a pipe is
+  # identified as a parent or child, it is choosing which ends
+  # of the pipe to use.
+  #
+  # A pipe is actually two pipes:
+  #
+  # Parent === Pipe 1 ==> Child
+  # Parent <== Pipe 2 === Child
+  class Pipe
+    # Creates a new uninitialized pipe pair.
+    def initialize
+      @child_read, @parent_write = IO.pipe
+      @parent_read, @child_write = IO.pipe
+      [@parent_write, @child_write].each{|io| io.sync = true}
+    end
+
+    # Read a line from a pipe. It will have a trailing newline.
+    def gets
+      force_identification
+      @reader.gets
+    end
+
+    # Write a line to a pipe. It must have a trailing newline.
+    def write(str)
+      force_identification
+      begin
+        @writer.write(str)
+        return str
+      rescue Errno::EPIPE
+        raise Hydra::PipeError::Broken
+      end
+    end
+
+    # Returns true if there is nothing to read (right now). However it is
+    # not exactly eof, if the other side writes, this will return false.
+    #
+    # It's a good way to tell if there is anything to process right now,
+    # otherwise, you can sleep.
+    def eof?
+      @reader.eof?
+    end
+
+    # Identify this side of the pipe as the child.
+    def identify_as_child
+      @parent_write.close
+      @parent_read.close
+      @reader = @child_read
+      @writer = @child_write
+    end
+
+    # Identify this side of the pipe as the parent
+    def identify_as_parent
+      @child_write.close
+      @child_read.close
+      @reader = @parent_read
+      @writer = @parent_write
+    end
+
+    # closes the pipes. Once a pipe is closed on one end, the other
+    # end will get a PipeError::Broken if it tries to write.
+    def close
+      done_reading
+      done_writing
+    end
+
+    private
+    def done_writing #:nodoc:
+      @writer.close unless @writer.closed?
+    end
+
+    def done_reading #:nodoc:
+      @reader.close unless @reader.closed?
+    end
+
+    def force_identification #:nodoc:
+      raise PipeError::Unidentified if @reader.nil? or @writer.nil?
+    end
+  end
+
+  module PipeError #:nodoc:
+    # Raised if you try to read or write to a pipe when it is unidentified.
+    # Use identify_as_parent and identify_as_child to identify a pipe.
+    class Unidentified < RuntimeError
+      def message #:nodoc:
+        "Must identify as child or parent"
+      end
+    end
+    # Raised when a pipe has been broken between two processes.
+    # This happens when a process exits, and is a signal that
+    # there is no more data to communicate.
+    class Broken < RuntimeError
+      def message #:nodoc:
+        "Other side closed the connection"
+      end
+    end
+  end
+end
diff --git a/test/test_hydra.rb b/test/test_hydra.rb
deleted file mode 100644
index 0bff8b9..0000000
--- a/test/test_hydra.rb
+++ /dev/null
@@ -1,7 +0,0 @@
-require 'helper'
-
-class TestHydra < Test::Unit::TestCase
-  should "probably rename this file and start testing for real" do
-    flunk "hey buddy, you should probably rename this file and start testing for real"
-  end
-end
diff --git a/test/test_pipe.rb b/test/test_pipe.rb
new file mode 100644
index 0000000..52932e9
--- /dev/null
+++ b/test/test_pipe.rb
@@ -0,0 +1,38 @@
+require 'helper'
+
+class TestPipeDream < Test::Unit::TestCase
+  context "a pipe" do
+    setup do
+      @pipe = Hydra::Pipe.new
+    end
+    should "be able to write messages" do
+      Process.fork do
+        @pipe.identify_as_child
+        assert_equal "Test Message\n", @pipe.gets
+        @pipe.write "Message Received\n"
+        @pipe.write "Second Message\n"
+        @pipe.close
+      end
+      @pipe.identify_as_parent
+      @pipe.write "Test Message\n"
+      assert_equal "Message Received\n", @pipe.gets
+      assert !@pipe.eof?
+      assert_equal "Second Message\n", @pipe.gets
+      assert @pipe.eof?
+      assert_raise Hydra::PipeError::Broken do
+        @pipe.write "anybody home?"
+      end
+      @pipe.close
+    end
+    should "not allow writing if unidentified" do
+      assert_raise Hydra::PipeError::Unidentified do
+        @pipe.write "hey\n"
+      end
+    end
+    should "not allow reading if unidentified" do
+      assert_raise Hydra::PipeError::Unidentified do
+        @pipe.gets
+      end
+    end
+  end
+end