Fix two bugs in mirroring.

First, Leaving off the source address caused a segfault in the
command-sending process because there was no NULL check on the ARGV
entry.

Second, while the migration thread sent a signal to the server to close
on successful completion, it didn't wait until the close actually
happened before releasing the IO lock.  This meant that any client
thread waiting on that IO lock could have a read or a write queued up
which could succeed despite the server shutdown.  This would have meant
dataloss as the guest would see a successful write to the wrong instance
of the file.  This patch adds a noddy serve_wait_for_close() function
which the mirror_runner calls to ensure that any clients will reject
operations they're waiting to complete.

This patch also adds a simple scenario test for migration, and fixes
TempFileWriter#read_original.

tests/test_file_writer.rb

@@ -27,20 +27,15 @@ class TestFileWriter
     self
   end
   
+ 
   # Returns what the data ought to be at the given offset and length
   #
-  def read_original(off, len)
-    r=""
-    current = 0
-    @pattern.split("").each do |block|
-      if off >= current && (off+len) < current + blocksize
-        current += data(block, current)[
-          current-off..(current+blocksize)-(off+len)
-        ]
-      end
-      current += @blocksize
-    end
-    r
+  def read_original( off, len )
+    patterns = @pattern.split( "" )
+    patterns.zip( (0...patterns.length).to_a ).
+      map { |blk, blk_off|
+      data(blk, blk_off)
+    }.join[off...(off+len)]
   end
   
   # Read what's actually in the file
@@ -51,7 +46,7 @@ class TestFileWriter
   end
   
   def untouched?(offset, len)
-    read(off, len) == read_original(off, len)
+    read(offset, len) == read_original(offset, len)
   end
   
   def close
@@ -81,3 +76,48 @@ class TestFileWriter
   
 end
 
+if __FILE__==$0
+  require 'tempfile'
+  require 'test/unit'
+
+  class TestFileWriterTest < Test::Unit::TestCase
+    def test_read_original_zeros
+      Tempfile.open("test_read_original_zeros") do |tempfile|
+        tempfile.close
+        file = TestFileWriter.new( tempfile.path, 4096 )
+        file.write( "0" )
+        assert_equal file.read( 0, 4096 ), file.read_original( 0, 4096 )
+        assert( file.untouched?(0,4096) , "Untouched file was touched." )
+      end
+    end
+
+    def test_read_original_offsets
+      Tempfile.open("test_read_original_offsets") do |tempfile|
+        tempfile.close
+        file = TestFileWriter.new( tempfile.path, 4096 )
+        file.write( "f" )
+        assert_equal file.read( 0, 4096 ), file.read_original( 0, 4096 )
+        assert( file.untouched?(0,4096) , "Untouched file was touched." )
+      end
+    end
+
+    def test_file_size
+      Tempfile.open("test_file_size") do |tempfile|
+        tempfile.close
+        file = TestFileWriter.new( tempfile.path, 4096 )
+        file.write( "f" )
+        assert_equal 4096, File.stat( tempfile.path ).size
+      end
+    end
+
+    def test_read_original_size
+      Tempfile.open("test_read_original_offsets") do |tempfile|
+        tempfile.close
+        file = TestFileWriter.new( tempfile.path, 4)
+        file.write( "f"*4 )
+        assert_equal 4, file.read_original(0, 4).length
+      end
+    end
+  end
+end
+