Dependency not resolved correctly when Product (CodeGenerator) used in Rule ([asd] -> [cpp]) - qbs

Question

when using a Rule inside a Module with a Dependency to a Product, a FileTagger breaks the resolution of dependencies in qbs.
We have a CodeGenerator in our project which is build by the project itself.
This CodeGenerator generates C++-Classes from *.asd-Files.
A Product called "Core" uses that CodeGenerator to generate Classes from Core.asd Files.
I am not sure if this is a bug in qbs, but since qbs 1.8 this part of our project does not work anymore.
I created a small test project that illustrates that problem:
RuleUsesProduct.qbs
import qbs
Project {
minimumQbsVersion: "1.8.0"
references: [
"Core/Core.qbs",
"CodeGenerator/CodeGenerator.qbs"
]
qbsSearchPaths: "QBS"
}
Core.qbs
import qbs
CppApplication {
Depends { name: "Qt.core" }
cpp.cxxLanguageVersion: "c++11"
cpp.defines: [
]
consoleApplication: true
files: [
"main.cpp",
"core.asd"
]
Depends{ name:"CodeGenerator"}
Depends{ name:"CodeGeneration"}
Group { // Properties for the produced executable
fileTagsFilter: product.type
qbs.install: true
}
}
Db2cppModule.qbs
This is indirectly included via the qbsSearchPath from RuleUsesProductTest.qbs
import qbs 1.0
import qbs.Environment
import qbs.FileInfo
import qbs.TextFile
import qbs.Process
import qbs.File
Module {
FileTagger {
patterns: ["*.asd"]
fileTags: ["asd"]
}
Rule {
id: dbRule
inputs: ["asd"]
inputsFromDependencies: ["application"]
multiplex: true
outputFileTags: ["cpp", "hpp"]
outputArtifacts: {
// dummy code that should call the CodeGenerator.exe with some parameters...
var process = new Process();
console.warn("# " + inputs["application"][0].filePath)
var cmdExp = "" + inputs["application"][0].filePath;
process.exec(cmdExp, [], false);
return []
}
prepare: {
console.warn("*" + inputs["application"][0].filePath)
var cmdExp = "" + inputs["application"][0].filePath;
var cmd = new Command(cmdExp, []);
return cmd
}
}
}
CodeGenerator.qbs
import qbs
CppApplication {
Depends { name: "Qt.core" }
cpp.cxxLanguageVersion: "c++11"
cpp.defines: [
]
consoleApplication: true
files: [
"codegenerator.cpp"
]
Group { // Properties for the produced executable
fileTagsFilter: product.type
qbs.install: true
}
}
Any help is much appreciated!

Answer 1

Qbs knows two types of rules: Multiplex and non-multiplex ones. Details can be found here: https://doc.qt.io/qbs/rule-item.html.
The important point for you is that non-multiplex rules invoke their prepare script once for every input. This means that in your example, the prepare script runs twice: Once for the asd input and once for the application input. The respective other inputs are not visible. This does not fit your use case, because you want to see both inputs at the same time. Therefore, you need to make your rule a multiplex rule:
multiplex: true
In the prepare script, you need to create one command for every input of type asd (unless your generator can take several inputs at once).
Rule of thumb (no pun intended): If your rule declares more than one input tag, it probably should be a multiplex rule.