negative build dependencies

This post is part 2/4 of a series about build systems. The next post is I want a better action graph serialization.

This post is about a limitation of the dependencies you can express in a build file. It uses Ninja just because it's simple and I'm very familiar with it, but the problem exists in most build systems.

Say you have a C project with two different include paths: dependency/include and src:

.
├── build.ninja
├── dependency
│   └── include
│       └── interface.h
└── src
    └── main.c

and the following build.ninja¹ :

rule cc
  command = cc -I dependency/include -I src $in -o $out

build src/main: cc src/main.c | dependency/include/interface.h

and some small implementations, just so we can see that it actually works:

// src/main.c
#include <stdio.h>
#include "interface.h"

int main(void) {
        printf("%d\n", interface_version);
}

// dependency/include/interface.h
const static short interface_version = 1;

This works ok on our first build, and when dependency/include/interface.h changes:

$ ninja
[1/1] cc -I dependency/include -I src src/main.c -o src/main

But say now that we add our own src/interface.h file:

$ cat src/interface.h
const static char *version = "1.0";
$ ninja
ninja: no work to do.

Now we have a problem. If we remove our build artifacts, ninja does rerun, and shows us what that problem is:

[1/1] cc -I dependency/include -I src src/main.c -o src/main
FAILED: [code=1] src/main
cc -I dependency/include -I src src/main.c -o src/main
src/main.c: In function ‘main’:
src/main.c:5:24: error: ‘interface_version’ undeclared (first use in this function)
    5 |         printf("%d\n", interface_version);
      |                        ^~~~~~~~~~~~~~~~~
src/main.c:5:24: note: each undeclared identifier is reported only once for each function it appears in
ninja: build stopped: subcommand failed.

We switched out what interface.h resolved to, but ninja didn't know about the changed dependency.

What we want is a way to tell it to rebuild if the file src/interface.h is created. To my knowledge, ninja has no way of expressing this kind of negative dependency edge.

Thanks to David Chisnall and Ben Boeckel (@mathstuf) for making me aware of this issue.