Generating depfiles with custom tools

[lhearachel]

I'm interested in building some tooling which will generate its own dependencies from an input file. As a toy kitchen example, I have the following meson.build setup:

project('meson-depfile')

worker = find_program('worker.py', native: true)

custom_target(
  'output',
  output: 'output.txt',
  input: files('manifest.txt'),
  command: [
    worker,
    '--emit-deps', '@DEPFILE@',
    '--dir', meson.current_source_dir(),
    '@INPUT@',
    '@OUTPUT@',
  ],
  depfile: '@[email protected]',
)

The worker is some straightforward Python code (heavily simplified for brevity)

#!/usr/bin/env python3

import re
import sys

def lower_snake(s):
    # snip

def upper_snake(s):
    # snip

[progname, emitdeps, depfile, dir, dir_arg, manifest, output] = sys.argv

deps = set([])
outlines = []

with open(manifest, "r", encoding="utf-8") as manifest_f:
    # accept lines from manifest to apply a transformation to each line of a file

with open(depfile, "w", encoding="utf-8") as depfile_f:
    depfile_f.write(f"{output}: {' '.join(deps)}")

with open(output, "w", encoding="utf-8") as output_f:
    output_f.write("\n".join(outlines))

And my manifest.txt looks like this:

dep.txt:upper_snake

While dep.txt looks like this:

this is some text
that will be manipulated

However, ninja does not seem to detect changes to dep.txt and trigger a rebuild. Running my worker manually (outside of ninja) generates the following dependency output:

output.txt: dep.txt

Is there an element of the syntax that I am missing here? In this toy example, I could easily declare dep.txt as an explicit dependency, but I would like to avoid that when building the real implementation.

[dcbaker]

You can use ninja -d keepdepfile to see what is being written into the depfile, and thus why ninja is ignoring it. Usually when I get these wrong it's because I'm missing a subdir and so ninja is looking for $srcdir/depfile.d instead of $srdir/subdir/depfile.d.