Flush bad write data from the client socket.

If the client makes a write that's out of range, by the time we get to
validate the message at the server end the client has already stuffed
the socket with data we can't use, so we have to flush it.

This patch also fixes a potential problem in the acceptance tests where
the error field was being returned as an array rather than a value.

tests/acceptance/fakes/source/write_out_of_range.rb

@@ -4,6 +4,9 @@
 # Connect, read the hello then make a write request with an impossible
 # (from,len) pair.  We expect an error response, and not to be
 # disconnected.
+#
+# We then expect to be able to issue a successful write: the destination
+# has to flush the data in the socket.
 
 require 'flexnbd/fake_source'
 include FlexNBD
@@ -11,12 +14,19 @@ include FlexNBD
 addr, port = *ARGV
 
 client = FakeSource.new( addr, port, "Timed out connecting" )
-client.read_hello
-client.write_write_request( 1 << 31, 1 << 31, "myhandle" )
+hello = client.read_hello
+client.write_write_request( hello[:size]+1, 32, "myhandle" )
+client.write_data("1"*32)
 response = client.read_response
 
 fail "Not an error" if response[:error] == 0
 fail "Wrong handle" unless "myhandle" == response[:handle]
 
+client.write_write_request( 0, 32 )
+client.write_data( "2"*32 )
+success_response = client.read_response
+
+fail "Second write failed" unless success_response[:error] == 0
+
 client.close
 exit(0)

tests/acceptance/flexnbd/fake_source.rb

@@ -26,9 +26,17 @@ module FlexNBD
     def read_hello()
       timing_out( FlexNBD::MS_HELLO_TIME_SECS,
                   "Timed out waiting for hello." ) do
-          fail "No hello." unless (hello = @sock.read( 152 )) &&
-            hello.length==152
-        hello
+        fail "No hello." unless (hello = @sock.read( 152 )) &&
+          hello.length==152
+
+        magic_s = hello[0..7]
+        ignore_s= hello[8..15]
+        size_s  = hello[16..23]
+
+        size_h, size_l = size_s.unpack("NN")
+        size = (size_h << 32) + size_l
+
+        return { :magic => magic_s, :size => size }
       end
     end
 
@@ -99,7 +107,7 @@ module FlexNBD
 
       {
         :magic => magic,
-        :error => error_s.unpack("N"),
+        :error => error_s.unpack("N").first,
         :handle => handle
       }
     end