Incremental builds for Travis CI

Travis CI is one of those fantastic "how is this free?" tools (for other examples, see GitHub, Coursera, and 30+ years of *nix development). However, there is one issue which causes a major inconvenience for me and perhaps some others on the edgiest of edge cases: there are hardcoded limits on the maximum runtime of a job.

The idea of these timeouts is to prevent hung jobs from consuming resources forever, but they also mean that innocent jobs which simply take a really long time to run never complete. This can be a dealbreaker for projects containing very large dependencies, but for those cases, we can workaround the issue using build caches.

One of the neat features of Travis CI is the ability to cache arbitrary directories. The idea is that if your project contains a large dependency, you can compile the dependency once and then simply cache the compiled result. However, this alone is not enough to solve the problem of building really big dependencies, since if a job times out, it is marked as failed, and if a job fails, the caches are not updated.

As an example, I have been working on a big project which depends on (amongst other things) compiling a full LLVM stack locally. Compiling LLVM takes around 2 hours, far exceeding the Travis CI timeouts. After a few frustrating days of fiddling I managed to develop a config file which enables the build to be split into two phases: a "cache cold" job which incrementally compiles the big dependency, and a "cache hot" job which is for actually running the tests.

Let's say for the sake of example that your project has a big dependency big_dep/exe, which is compiled from source and takes longer than the Travis CI timeouts. To perform a two phase build, define an explicit make big_dep/exe build target and put this in your Travis config:

  - big_dep

# cache-cold build: (restart job as required)
  - timeout 1800 make big_dep/exe || true
  - touch NOTEST

# cache-hot build:
# install:
#  - make all

  - test -f NOTEST || ./run_tests

This config forces the big dependency to compile in chunks of 30 minutes, and cache the intermediate results. Manually restart the job as many times as is required to complete the build. Once complete uncomment the "cache-hot" rule and comment out the "cache-cold" rule to run your tests as normal.

If your project has big dependencies and is suffering from repeated timeouts, give this a try.