generate make dependencies

[Dirk Jagdmann]

Hello Cap'n'Proto experts,

I would like to generate dependencies for Cap'n'Proto schema file imports in the syntax understood by (GNU) make. Similar to the gcc compiler's -M command line argument.

I'm happy to look into the Cap'n'Proto source code and see if I can implement this myself, but it would help if you can give me some guide which data structures in which files would have the information that I need (currently processed schema file, list of include directories, ...)

[Ian Denhardt]

I've vaguely wanted this in the past, glad someone is interested and
motivated enough to get their hands dirty :)

Assuming you're interested in doing this only for C++, you'd want to make
the changes in the capnpc-c++ code generator plugin. Maybe Kenton can
provide more specific guidance than this, since I'm not familiar with
that part of the codebase. But at a high level, the main schema compiler
is going to feed that program a CodeGeneratorRequest as defined in
schema.capnp. RequestedFile.imports should have the information you need
to generate dependencies.

A possible alternative wuold be to write a (separate) schema compiler
plugin that just dumps the input to a file (say foo.capnp.cgr) and also
generates Makefile stubs for that file; you could then feed in the
generated file to capnpc-c++ (or other plugins) manually, and add
rules like:

%.capnp.cgr: %.capnp
# ...
%.capnp.h %.capnp.c++: %.capnp.cgr
# ...

This would be more general, and might be a little easier.

Note that one thing that's a little sad about gcc's -M is that it will
fail if the file doesn't exist, so you can't use it to gather dependencies
for the *.capnp.{h,c++} files as easily as you can non-generated files.

-Ian

Quoting Dirk Jagdmann (2021-08-05 11:50:58)

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[Kenton Varda]

I think this could be implemented as a regular old code generator plugin that directly produces the deps files. No need to integrate with the C++ plugin nor output an intermediate representation.

Note that `capnp compile` supports passing multiple `-o` flags to run multiple plugins, so you can run both your C++ generator and your deps generator in one pass.

-Kenton

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