Testing curl_multi_socket_action

We’re introducing a brand new way to test the event-based socket_action API in libcurl! (available in curl since commit 6cf8413e3162)


Since 2006 we’ve had three major API families in libcurl for doing file transfers:

  1. the easy interface – a synchronous and yes, easy, interface for getting things done
  2. the multi interface – a non-blocking interface that allows multiple simultaneous transfers (in both directions) in the same thread
  3. the socket_action interface Рa brother of the multi interface but designed for  use with an event-based library/engine for high performance and large scale transfers

The curl command line tool uses the easy interface and our test suite for curl + libcurl consists of perhaps 80% curl tests, while the rest are libcurl-using programs testing both the easy interface and the multi interface.

Early this year we modified libcurl’s internals so that the functions driving the easy interface transfer would use the multi interface internally. Then all of a sudden all the curl-using tests using the easy interface also then by definition tested that the operation worked fine with the multi interface. Needless to say, this pushed several bugs up to the surface that we could fix.

So the multi and the easy interfaces are tested by many hundred test cases on a large number of various systems every day around the clock. Nice! But what about the third interface? The socket_action interface isn’t tested at all! Time to change this sorry state.

Event-based test challenges

The event-based API has its own set of challenges; like it needs to react on socket state changes (only) and allow smooth interactions with the user’s own choice of event library etc. This is our newest API family and also the least commonly used. One reason for this may very well be that event-based coding is generally harder to do than more traditional poll-based code. Event-based code forces the application into using state-machines all over to a much higher degree and the frequent use of callbacks easily makes the code hard to read and its logic hard to follow.

So, curl_multi_socket_action() acts in ways that aren’t done or even necessary when the regular select-oriented multi interface is used. Code that then needs to be tested to remain working!

Introducing an alternative curl_easy_perform

As I mentioned before, we made the general multi interface widely tested by making sure the easy interface code uses the multi interface under the hood. We can’t easily do the same operation again, but instead this time we introduce a separate implementation (for debug-enabled builds) called curl_easy_perform_ev that instead uses the event-based API internally to drive the transfer.

The curl_multi_socket_action() is meant to use an event library to work really well multi-platform, or something like epoll directly if Linux-only functionality is fine for you. curl and libcurl is quite likely among the most portable code you can find so after having fought with this agony a while (how to introduce event-based testing without narrowing the tested platforms too much) I settled on a simple but yet brilliant solution (I can call it brilliant since I didn’t come up with the idea on my own):

We write an internal “simulated” event-based library with functionality provided by the libcurl internal function Curl_poll() (the link unfortunately goes to a line number, you may need to move around in the file to find the function). It is in itself a wrapper function that can work with either poll() or select() and should therefor work on just about any operating system written since the 90s, and most of the ones since before that as well! Doing such an emulation code may not be the most clever action if the aim would be to write a high performance and low latency application, but since my objective now is to exercise the API and make an curl_easy_perform clone it was perfect!

It should be carefully noted that curl_easy_perform_ev is only for testing and will only exist in debug-enabled builds and is therefor not considered stable nor a part of the public API!

Running event-based tests

The fake event library works with the curl_multi_socket_action() family of functions and when curl is invoked with –test-event, it will call curl_easy_perform_ev instead of curl_easy_perform and the transfer should then work exactly as without –test-event.

The main test suite running script called ‘runtests.pl’ now features the option -e that will run all ~800 curl tests with –test-event. It will skip tests it can’t run event-based – basically all the tests that don’t use the curl tool.

Many sockets is slow if not done with events

This picture on the right shows some very old performance measurements done on libcurl in the year 2005, but the time spent growing exponentially when the amount of sockets grow is exactly why you want to use something event-based instead of something poll or select based.

See also my document discussing poll, select and event-based.

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