Meson Live at Head

This note describes how to use a branch of a git repository in the Meson build system.

The rationale behind this note is to describe how to enable projects to “live at head”, or use the most recent commit from a dependency. Several Google libraries such as Abseil and Googletest follow this philosophy.

Meson dependencies: the wrap file

Meson includes a strong dependency management system via the use of wrap files. A wrap file describes how to include a dependency in a project, which usually involves fetching source code from a URL and describing how to build it. Here is an example wrap file for Googletest:

[wrap-git]
directory = googletest
url = https://github.com/google/googletest.git
revision = master

If the contents of this file are placed in subprojects/gtest-git.wrap, then Meson will download the master branch of the git repository at the following URL and place the contents in a new directory subprojects/googletest. This is a great way to get the latest stable commit of a repository included in a Meson project!

How to build non Meson git repositories

Meson also includes the concept of a patch, which allows for overlaying the downloaded git repository with custom files. The primary use case is to include a separate project which contains meson.build files that teach Meson how to build the repository.

This patch can be a single file, a local directory, or a remote URL of an archive. For a remote archive, take a look at the examples in Meson’s WrapDB. Here is an example patch using a local directory:

[wrap-git]
directory = googletest
url = https://github.com/google/googletest.git
revision = master
patch_directory = googletest_patch

Meson will apply the contents of subprojects/packagefiles/googletest_patch as an overlay of subprojects/googletest. For example, subprojects/packagefiles/googletest_patch/googlemock/meson.build will be copied to subprojects/googletest/googlemock/meson.build, overwriting an existing file (if there is one).

With the overlay directory, it’s possible to use dependencies following the “live at head” philosophy.

Updating dependencies

Meson provides a handy command to update to the latest commits of dependencies:

meson subprojects update

However, this does not apply any patch files again. As a workaround, I currently delete the subproject folder (subprojects/googletest) to force Meson to redownload and reapply the patch. I’ve filed a GitHub issue to resolve this behavior.