Test suite basics
The curl test suite fires up a whole bunch of test servers for the various supported protocols, and then command lines using curl or libcurl-using dedicated test apps are run against those servers to make sure curl is acting exactly as it is supposed to.
The curl test suite was introduced back in the year 2000 and has of course grown and been developed substantially since then.
Each test server is invoked and handles a specific protocol or set of protocols, so to make sure curl’s 24 transfer protocols are tested, a lot of test server instances are spun up. The test servers normally don’t shut down after each individual test but instead keep running in case there are more tests for that protocol for speedier operations. When all tests are completed, all the test servers are shut down again.
The protocols curl communicates with are all done over TCP or UDP, and therefore each test server needs to listen to a dedicated port so that the tests can be invoked and curl can use the protocols correctly.
We started the test suite once by using port number 8990 as a default “base port”, and then each subsequent server it invokes gets the next port number. A full round can use up to 30 or so ports.
The script needed to know what port number to use so that it could invoke the tests to use the correct port number. But port numbers are both a scarce resource and a resource that can only be used by one server at a time, so if you would simultaneously invoke the curl test suite a second time on the same machine, it would cause havoc since it would attempt to use the same port numbers again.
Not to mention that when various users run the test suite on their machines, on servers or in CI jobs, just assuming or hoping that we could use a range of 30 fixed port numbers is error-prone and it would occasionally cause us trouble.
Dynamic port numbers
A few months ago, in April 2020, I started the work on changing how the curl test suite uses port numbers for its test servers. Each used test server would instead get fired up on a totally random port number, and then feed back that number to the test script that then would use the actually used port number to run the tests against.
It took some rearranging of internal logic to make sure things still worked and that the comparison of the generated protocol etc still would work. Then it was “just” a matter of converting over all test servers to this new dynamic concept and made sure that there was no old-style assumptions lingering around in the test cases.
Some of the more tricky changes for this new paradigm was the protocol parts that use the port number in data where curl base64 encodes the chunk and sends it to the server.
Reaching the end of that journey
The final fixed port number in the curl test suite was removed when we merged PR 5783 on August 7 2020. Now, every test in curl that needs a port number uses a dynamic port number.
Now we can run “make test” from multiple shells on the same machine in parallel without problems.
It can allow us to improve the test suite further and maybe for example run multiple tests in parallel in order to reduce the total execution time. It will avoid future port collisions. A small, but very good step forward.