Re: Decouple C++ support in Meson's PGXS from LLVM enablement

On Wed Apr 16, 2025 at 8:57 PM CDT, Tristan Partin wrote:
> Howdy folks,
>
> While playing around with pg_duckdb[0], a Postgres extension written in 
> C++ which uses PGXS, I came across a strange build error:
>
> std=c++17  -Wno-sign-compare...
> /bin/sh: line 1: -Wno-sign-compare: command not found
>
> I was very confused by the error, but reading the command line, it made 
> sense. After talking to Jelte off-list, he told me to try a Postgres 
> installation that had been built with autotools. Today, I finally had 
> a chance to try that tip, and building pg_duckdb succeeded.
>
> I spent some time exploring the Meson build a bit, and I realized that 
> C++ support in PGXS is tied to LLVM enablement. Checking the autotools 
> build in the configure.ac script indicates that that is not the case for 
> it.
>
> On master, C++ support looks like:
>
> llvmopt = get_option('llvm')
> llvm = not_found_dep
> if add_languages('cpp', required: llvmopt, native: false)
>   llvm = dependency('llvm', version: '>=14', method: 'config-tool', required: llvmopt)
>     if llvm.found()
>
>       cdata.set('USE_LLVM', 1)
>
>       cpp = meson.get_compiler('cpp')
>
> By default, the `llvm` option is disabled, which Meson takes to mean, 
> "do not check for C++ support". Thusly, add_languages() returns false. 
> In addition, every check for adding to cxxflags, et. al. is gated on 
> llvm.found(), which is always false for the `not_found_dep`. All this 
> considered, the Makefile.global of a Postgres build roughly looked like:
>
> CXX =
> CXXFLAGS = 
> ...
>
> This then accounts for the original pg_duckdb command line looking the 
> way that it did.
>
> Attached is a patch which decouples C++ support in PGXS from LLVM for 
> a Meson-compiled Postgres.
>
> [0]: https://github.com/duckdb/pg_duckdb

With PGConf NYC around the corner, I thought I would rebase the original 
patch. Please find v2 attached, which applies on top of 
b0fb2c6aa5a485e28210e13ae5536c1231b1261f[0] :D.

GitHub branch: https://github.com/tristan957/postgres/tree/meson-cpp

[0]: https://github.com/tristan957/postgres/commit/b0fb2c6aa5a485e28210e13ae5536c1231b1261f

-- 
Tristan Partin
https://tristan.partin.io