should gprbuild's Makefile (re-)create the knowledge base, or expect it to exist already?

[Ptival]

I am in the process of building libadalang for Nix, which includes building gprbuild.

Nix packages are a bit special: they are built in what you could essentially call a sandbox (no network, no files but the ones specified as inputs, no environment but the one specified), and must put their output in a directory generated on the fly by the build system, after which the build output is copied to a readonly store with a unique path.

To build gprbuild, I first built the "bootstrap" version with the bootstrap script. It all went well, and the generated gprbuild worked just fine. This one lived at, say, /nix/store/0123-gprbuild-bootstrap/ (0123 standing for some unique hash).

I then attempted to use it to build gprbuild again, this time compiled by itself. The build worked, and this one lives at, say, /nix/store/4567-gprbuild/. But I got a bit of a surprise when trying to run it: it would keep failing, telling me it knew no compilers. After a bit of search, I found out it meant that gprbuild could not find the "knowledge base". Indeed, thinking back, while I passed it with the --with-kb argument to the bootstrap version, I have never declared it as a dependency to the second build.

I also figured out you could run gprbuild -v to know where the compiler is looking for that knowledge base. Indeed it was looking at the non-existent /nix/store/4567-gprbuild/share/gprconfig directory.
On the other hand, the bootstrap script had correctly created a /nix/store/1234-gprbuild-bootstrap/share/gprconfig directory, with all the XML files there, which explains why it succeeded while this version had trouble.
It is easy to circumvent the problem (for the curious: https://github.com/Ptival/nur-packages/blob/860cb7bf7981e3ea8dd919ff5343dfa1ad30d36c/pkgs/adaPackages/gprbuild/default.nix#L31-L32 )

My understanding is that this issue does not occur when all packages are writing to a shared location like /usr/share, since then the bootstrap version would populate /usr/share/gprconfig, and the the second build would stumble upon this pre-existing directory.

My question is:

• do you intend for the Makefile-based build of gprbuild to be depending on a pre-installed gprconfig knowledge base?
• or is there some way of pointing the Makefile to the knowledge base in such a way that it'd install it if missing?

[Ptival]

Actually, a convenient third option would be having a way to specify a folder where the knowledge base exists to the Makefile. Is this already feasible?

[t-14]

At the moment Makefile assumes that gprconfig kb has been pre-installed. I agree that supporting a case of external kb would add flexibility. Do you feel like proposing a pull request for that?