flexnbd: Move building the allocation map to before server socket bind()

Building the allocation map takes time, which scales with the size of the disc
being presented. By building that map in the space between bind() and accept(),
we leave the process in a useless state after the only good signal we have for
"we are ready" and the state where it is actually ready. This was breaking
migrations of large files.

tests/acceptance/flexnbd/fake_source.rb

@@ -8,16 +8,35 @@ module FlexNBD
   class FakeSource
 
     def initialize( addr, port, err_msg, source_addr=nil, source_port=0 )
-      timing_out( 2, err_msg ) do
-        @sock = if source_addr
-          TCPSocket.new( addr, port, source_addr, source_port )
-        else 
-          TCPSocket.new( addr, port )
-        end
+      timing_out( 10, err_msg ) {
+        @sock = wait_for_server_socket( addr, port, source_addr, source_port )
+      }
+    end
+
+
+    def wait_for_server_socket(addr, port, saddr = nil, sport = 0)
+      sock = nil
+
+      loop do
+        sock = try_get_server_socket( addr, port, saddr, sport )
+        break if sock
+        sleep 0.1
+      end
+
+      sock
+    end
+
+    def try_get_server_socket(addr, port, saddr = nil, sport = 0)
+      if saddr
+        TCPSocket.new( addr, port, saddr, sport ) rescue nil
+      else
+        TCPSocket.new( addr, port ) rescue nil
       end
     end
 
 
+
+
     def close
       @sock.close
     end
@@ -137,3 +156,4 @@ module FlexNBD
 
   end # class FakeSource
 end # module FlexNBD
+