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
--
Tristan Partin
Neon (https://neon.tech)
From aba365ed36a82f41e8bd6a7f61b90c91f1556d08 Mon Sep 17 00:00:00 2001
From: Tristan Partin <[email protected]>
Date: Wed, 16 Apr 2025 20:25:21 -0500
Subject: [PATCH] Decouple C++ support in Meson's PGXS from LLVM enablement
This is important for Postgres extensions that are written in C++, such
as pg_duckdb, which uses PGXS as the build system currently. In the
autotools build, C++ is not coupled to LLVM. If the autotools build is
configured without --with-llvm, the C++ compiler and the various flags
get persisted into the Makefile.global.
Signed-off-by: Tristan Partin <[email protected]>
---
meson.build | 27 +++++++++++++++++----------
src/include/meson.build | 2 +-
src/makefiles/meson.build | 9 ++++++---
3 files changed, 24 insertions(+), 14 deletions(-)
diff --git a/meson.build b/meson.build
index a1516e54529..f63769f8b67 100644
--- a/meson.build
+++ b/meson.build
@@ -40,6 +40,10 @@ build_system = build_machine.system()
host_cpu = host_machine.cpu_family()
cc = meson.get_compiler('c')
+have_cpp = add_languages('cpp', required: false, native: false)
+if have_cpp
+ cpp = meson.get_compiler('cpp')
+endif
not_found_dep = dependency('', required: false)
thread_dep = dependency('threads')
@@ -798,15 +802,13 @@ endif
llvmopt = get_option('llvm')
llvm = not_found_dep
-if add_languages('cpp', required: llvmopt, native: false)
+if have_cpp and not llvmopt.disabled()
llvm = dependency('llvm', version: '>=14', method: 'config-tool', required: llvmopt)
if llvm.found()
cdata.set('USE_LLVM', 1)
- cpp = meson.get_compiler('cpp')
-
llvm_binpath = llvm.get_variable(configtool: 'bindir')
ccache = find_program('ccache', native: true, required: false)
@@ -815,8 +817,13 @@ if add_languages('cpp', required: llvmopt, native: false)
# find via PATH, too.
clang = find_program(llvm_binpath / 'clang', 'clang', required: true)
endif
-elif llvmopt.auto()
- message('llvm requires a C++ compiler')
+else
+ msg = 'llvm requires a C++ compiler'
+ if llvmopt.auto()
+ message(msg)
+ elif llvmopt.enabled()
+ error(msg)
+ endif
endif
@@ -2053,7 +2060,7 @@ common_functional_flags = [
]
cflags += cc.get_supported_arguments(common_functional_flags)
-if llvm.found()
+if have_cpp
cxxflags += cpp.get_supported_arguments(common_functional_flags)
endif
@@ -2077,7 +2084,7 @@ common_warning_flags = [
]
cflags_warn += cc.get_supported_arguments(common_warning_flags)
-if llvm.found()
+if have_cpp
cxxflags_warn += cpp.get_supported_arguments(common_warning_flags)
endif
@@ -2131,7 +2138,7 @@ foreach w : negative_warning_flags
if cc.has_argument('-W' + w)
cflags_warn += '-Wno-' + w
endif
- if llvm.found() and cpp.has_argument('-W' + w)
+ if have_cpp and cpp.has_argument('-W' + w)
cxxflags_warn += '-Wno-' + w
endif
endforeach
@@ -2192,7 +2199,7 @@ elif optimization == 's'
endif
cflags_builtin = cc.get_supported_arguments(common_builtin_flags)
-if llvm.found()
+if have_cpp
cxxflags_builtin = cpp.get_supported_arguments(common_builtin_flags)
endif
@@ -3920,7 +3927,7 @@ if meson.version().version_compare('>=0.57')
section: 'Compiler Flags',
)
- if llvm.found()
+ if have_cpp
summary(
{
'C++ compiler': '@0@ @1@'.format(cpp.get_id(), cpp.version()),
diff --git a/src/include/meson.build b/src/include/meson.build
index 2e4b7aa529e..c2aef484fcc 100644
--- a/src/include/meson.build
+++ b/src/include/meson.build
@@ -36,7 +36,7 @@ config_paths_data.set_quoted('MANDIR', dir_prefix / dir_man)
var_cc = ' '.join(cc.cmd_array())
var_cpp = ' '.join(cc.cmd_array() + ['-E'])
var_cflags = ' '.join(cflags + cflags_builtin + cflags_warn + get_option('c_args'))
-if llvm.found()
+if have_cpp
var_cxxflags = ' '.join(cxxflags + cxxflags_builtin + cxxflags_warn + get_option('cpp_args'))
else
var_cxxflags = ''
diff --git a/src/makefiles/meson.build b/src/makefiles/meson.build
index 55da678ec27..bee83a66e81 100644
--- a/src/makefiles/meson.build
+++ b/src/makefiles/meson.build
@@ -31,7 +31,6 @@ if not working_strip
strip_shared_cmd = [':']
endif
-
pgxs_kv = {
'PACKAGE_URL': pg_url,
'PACKAGE_VERSION': pg_version,
@@ -121,16 +120,20 @@ pgxs_kv = {
'LIBS': var_libs,
}
+if have_cpp
+ pgxs_kv += {
+ 'CXX': ' '.join(cpp.cmd_array()),
+ }
+endif
+
if llvm.found()
pgxs_kv += {
'CLANG': clang.path(),
- 'CXX': ' '.join(cpp.cmd_array()),
'LLVM_BINPATH': llvm_binpath,
}
else
pgxs_kv += {
'CLANG': '',
- 'CXX': '',
'LLVM_BINPATH': '',
}
endif
--
Tristan Partin
https://tristan.partin.io
