Getting geth client mining to work in Travis CI builds

2 minute read

Yesterday I finally got geth-private automated builds working on Travis CI. There were two main problems in getting the builds working:

  • The geth sub-processes launched by during tests weren’t always existing even though Node.js was telling me that they were.

  • The tests which relied on mining to happen were always failing.

The second problem was occurring due to the fact that in order to mine, Geth first needs to build a DAG for the Ethash algorithm to work. This is a one-time thing and takes a number of minutes to do - only after this is done can mining begin.

In my .travis.yml I added the following:

  ... # other stuff here
  - mkdir -p ~/.ethash
  - geth makedag 0 ~/.ethash

This got the mining-related tests passing. But this now raised another problem. Creating the DAG was taking upwards of 6 minutes, which meant that every build would take a minimum of this much time even before it got to running the tests. I needed to be able to cache the built DAG between builds.

Luckily Travis supports directory caching:

    - ~/.ethash

With this set the first build would build the DAG and place it into cache and then subsequent builds would simply re-use the contents from cache - actually, every build runs the makedag call but this call doesn’t need to do any work if it finds that the DAG is already there.

A note on the first problem - geth sometimes failing to exit properly.

This actually took longer to solve. I tried using SIGKILL, tried using the terminate module, and tried a whole bunch of other tricks. In the end what fixed it for me was changing how I was launching the geth child process in the first place.

Node’s child_process.exec() launches a new process from a shell command by first creating a shell to launch it from, whereas child_process.spawn() directly launches the command. On my dev machine it made no difference but on Travis CI it seemed to make a big difference when it came to killing the child process.

Actually I was using the shelljs module to launch the process because it provides a simpler API. Switching to child_process.spawn() and managing the I/O streams more directly resulted in more robust child process management.

I’ll be careful to use child_process.spawn() in future when I want to be sure that I can manage my child processes effectively.

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