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/flexnbd.rb

@@ -21,7 +21,7 @@ class FlexNBD
   end
 
   def debug?
-    !@debug.empty?
+    !@debug.empty? || ENV['DEBUG']
   end
 
   def debug( msg )
@@ -60,6 +60,14 @@ class FlexNBD
   end
 
 
+  def mirror_cmd(dest_ip, dest_port)
+    "#{@bin} mirror "\
+      "--addr #{dest_ip} "\
+      "--port #{dest_port} "\
+      "--sock #{ctrl} "\
+      "#{@debug} "
+  end
+
   def serve(file, *acl)
     File.unlink(ctrl) if File.exists?(ctrl)
     cmd =serve_cmd( file, acl )
@@ -79,7 +87,10 @@ class FlexNBD
   def start_wait_thread( pid )
     Thread.start do
       Process.waitpid2( pid )
-      unless @kill
+      if @kill
+        fail "flexnbd quit with a bad status #{$?.exitstatus}" unless 
+          $?.exitstatus == @kill
+      else
         $stderr.puts "flexnbd quit"
         fail "flexnbd quit early"
       end
@@ -87,9 +98,18 @@ class FlexNBD
   end
 
 
+  def can_die(status=0)
+    @kill = status
+  end
+
   def kill
-    @kill = true
-    Process.kill("INT", @pid)
+    can_die()
+    begin
+      Process.kill("INT", @pid)
+    rescue Errno::ESRCH => e
+      # already dead.  Presumably this means it went away after a
+      # can_die() call.
+    end
   end
 
   def read(offset, length)
@@ -114,8 +134,12 @@ class FlexNBD
     nil
   end
 
-  def mirror(bandwidth=nil, action=nil)
-    control_command("mirror", ip, port, ip, bandwidth, action)
+  def mirror(dest_ip, dest_port, bandwidth=nil, action=nil)
+    cmd = mirror_cmd( dest_ip, dest_port)
+    debug( cmd )
+    system cmd
+    raise IOError.new( "Migrate command failed") unless $?.success?
+    nil
   end
 
   def acl(*acl)