PCH step doesn't depend on generated headers

[nicolas17]

If a header to be precompiled includes a header that is generated during the build, meson doesn't create a proper dependency between them. This leads to ninja trying to precompile the pch before the headers it includes are generated.

Full example:

main.c:

#include <stdio.h>
#include "bigheader.h"

int main() {
    printf("The answer is %d\n", x);
    return 0;
}

generator.sh:

#!/bin/sh
sleep 2
echo "#pragma once" > $1
echo "const int x = 42;" >> $1

pch/pch.h:

#include "bigheader.h"

meson.build:

project('bugtest', 'c')

# pretend this generates a giant file
gen = custom_target('header',
    output: ['bigheader.h'],
    command: ['sh', files('generator.sh'), '@OUTPUT@']
)

executable('bugtest', ['main.c', gen], c_pch: 'pch/pch.h')

The resulting build.ninja has an order-only dependency from main.c.o to pch.h.gch bigheader.h which ensures the compilation waits for bigheader.h to be generated, but there is nothing making bigheader.h a dependency of the PCH step.

On a ninja -j1 this might work, but in a parallel build this leads to pch.h being precompiled before bigheader.h is generated.

One way to solve this is to have meson put all the target's generated headers as order-only dependencies of the precompiled header. Although this might cause too-strict dependencies for the common case where the pch only includes external libraries, in which case I guess this needs an extra keyword argument to be explicit...

[nioncode]

Another way would be to have PCH as a separate pseudo-target or just a method that can take source + header arguments, as well as a list of dependencies. Then, we could add arbitrary dependencies before generating the PCH without adding ALL generators as dependencies.

[jpakkane]

The dependencies of pch compilation should be identical to regular compilation. It is not because of historical reasons.

[nioncode]

Fixed by #4860, please close.