trimja

Ninja is a fast build system commonly used to build large C/C++ projects. It does this by looking at a Ninja build file, which contains the build commands and describes dependencies between files.

trimja is a command line utility to trim down Ninja build files to only those commands that are dependent or necessary for a subset of input files.

This can be used to speed up CI to create only those build artifacts that are affected by the current pull request. For example, a pull request updating README.md shouldn't need to build and test your entire software!

For a ready-to-use Github Action see trimja-action.

Help

The following instruction on how to use trimja can be found by running trimja --help or trimja -h.

trimja is a tool to create a smaller ninja build file containing only those
build commands that relate to a specified set of files. This is commonly used
to improve CI performance for pull requests.

trimja requires both the '.ninja_log' and '.ninja_deps' file from a succesful
run of the input ninja build file in order to correctly remove build commands.
Note that with simple ninja input files it is possible for ninja to not
generate either '.ninja_log' or '.ninja_deps', and in this case trimja will
work as expected.

Usage:
$ trimja --version
    Print out the version of trimja

$ trimja --help
    Print out this help dialog

$ trimja --builddir [-f FILE]
    Print out the $builddir path in the ninja build file relative to the cwd

$ trimja [-f FILE] [--write | -o OUT] [--affected PATH | -] [--explain]
    Trim down the ninja build file to only required outputs and inputs

Options:
  -f FILE, --file=FILE      path to input ninja build file [default=build.ninja]
  -a PATH, --affected=PATH  path to file containing affected file paths
  -                         read affected file paths from stdin
  -o OUT, --output=OUT      output file path [default=stdout]
  -w, --write               overwrite input ninja build file
  --explain                 print why each part of the build file was kept
  --builddir                print the $builddir variable relative to the cwd
  --memory=N                print memory stats and top N allocating functions
  -h, --help                print help
  -v, --version             print trimja version

Examples:

Build only those commands that relate to fibonacci.cpp,
  $ echo "fibonacci.cpp" > changed.txt
  $ trimja --file build.ninja --affected changed.txt --output small.ninja
  $ ninja -f small.ninja

Build only those commands that relate to files that differ from the 'main' git
branch, note the lone '-' argument to specify we are reading from stdin,
  $ git diff main --name-only | trimja - --write
  $ ninja

For more information visit the homepage https://github.com/elliotgoodrich/trimja

CI Design

Integrating trimja into a CI pipeline requires an external cache where the .ninja_log and .ninja_deps files can be stored for successful builds. The .ninja_log file contains a list of all files built with the hash of the build command and the .ninja_deps file contains dynamic dependencies generated during the build - such as includes header files when compiling C/C++ source files.

1. On requiring to build a new commit (e.g. on a PR or on merging to main)
2. Configure your build and generate the ninja.build file
3. Attempt to find the cache entry for the most recent ancestor commit of HEAD
4. If a cache entry is found, move to the next step, otherwise skip to step 7
5. Note the commit hash that generated this cache hit (call this EXISTING) and download the cache to the correct location
6. Run git diff EXISTING..HEAD --name-only | trimja - --write to replace the build.ninja file
7. Run ninja
8. If the build is successful upload .ninja_log and .ninja_deps to the cache