Difficulty to use custom_target() to generate data files

I am trying to use custom_target to generate some data files via a python script. I am struggling on two points:

• I don’t want to have to manually list all the output file in the output argument, since they depend on the content of the input file. I may have found a workaround though: I output all the files in a fixed folder and then reference this folder in output. Not sure if it has any negative implications.
• I would like the script to be only run when the input file changes, and I couldn’t make this work: either the script never runs or it is run each time I build. I tried many combinations of install and build_by_default, and even tried to use the output files as dependencies with no success.

From what I understood of the manual, custom_target() should be the way to go here, but let me know if there is something more adapted for my use-case.

To Reproduce A simplified version of my meson file (in this case, script never runs):

prog_python = import('python').find_installation('python3')

custom_target('my_target',
    input: 'input.json',
    output: 'out', # folder where the script will write the output files
    output_dir: out_dir,
    command: [prog_python, 'my_script.py', '@INPUT@', out_dir / 'out'],
    install: true,
    install_dir: out_dir,
    build_by_default: false,
)

System parameters

• windows 10
• python: 3.8.2
• meson: 0.58.0
• ninja: 1.10.2

[eli-schwartz]

output: ['out']
command: [prog_python, in_dir / 'my_script.py', '@INPUT@', meson.current_build_dir() / 'out'],
install_dir: get_option('datadir') / 'myprog', # ends up in /usr/share/myprog/out/{foo.txt,bar.log}

If your output is a directory, ninja becomes a bit clueless about how it got generated, and meson is pretty fuzzy on how to install it so it simply copies over the entire directory. But it does only build when the input gets changed, which is the important bit.

You just need the out/ directory which the python program is told to use, to be in the current_build_dir() to correspond to the kwarg value of output.

In fact, you may just use '@OUTPUT@' directly.

[eli-schwartz]

out_dir = meson.current_source_dir()

With this context the problem becomes clear, when you originally tried passing output_dir: out_dir, you no doubt intended for meson to generate a build.ninja which generates rules targeting the source directory rather than the build directory.

meson does not support in tree builds, so trying to tell it to do so was futile… and since my_script.py never tried to generate the output which you specified (instead it created totally unrelated files in out_dir) the build rule notices on every single build that the output still hasn’t been created, so it tries to rerun the rules which should create it.

Your install_dir argument is also nonsense – if you fix the out_dir location, then run ninja install, it will install builddir/out/foo.txt to $PWD/builddir/out/foo.txt. It then fails, because installing a file will first remove the existing destination file, then install the source file to the destination.