truncated send always callback on writeable

Suggested by a Windows log where leaf.jpg meets EAGAIN and after
issuing the truncated send buffer, never sends any more

https://github.com/warmcat/libwebsockets/issues/111#issuecomment-39873129

Added note in README.coding about WRITEABLE callbacks able to
be generated by lws.

Signed-off-by: Andy Green <andy.green@linaro.org>

README.coding

@@ -67,6 +67,20 @@ in the ...WRITEABLE callback.
 See the test server code for an example of how to do this.
 
 
+Do not rely on only your own WRITEABLE requests appearing
+---------------------------------------------------------
+
+Libwebsockets may generate additional LWS_CALLBACK_CLIENT_WRITEABLE events
+if it met network conditions where it had to buffer your send data internally.
+
+So your code for LWS_CALLBACK_CLIENT_WRITEABLE needs to own the decision
+about what to send, it can't assume that just because the writeable callback
+came it really is time to send something.
+
+It's quite possible you get an 'extra' writeable callback at any time and
+just need to return 0 and wait for the expected callback later.
+
+
 Closing connections from the user side
 --------------------------------------
 

lib/output.c

@@ -148,8 +148,9 @@ handle_truncated_send:
 			lwsl_info("***** %x partial send completed\n", wsi);
 			/* done with it, but don't free it */
 			n = real_len;
-		} else
-			libwebsocket_callback_on_writable(
+		}
+		/* always callback on writeable */
+		libwebsocket_callback_on_writable(
 					     wsi->protocol->owning_server, wsi);
 
 		return n;