GeezerVison

Found when trying to test what configure options to add for my buildbots and I saw the _decimal module wasn't actually building on centos stream 9 and 10, because they ship mpdecimal 2.5.1, hence no .pc files.

I followed the convention of other libraries.

Could you please add a news entry?

CC @zware, @vstinner

How can I test this change? I need a system with libmpdec library and header files installed, but where pkg-config 'libmpdec >= 2.5.0' --libs fails? Can I replace libmpdec >= 2.5.0 with libmpdec >= 55.0 to test the change for example? What is the expected behavior?

How can I test this change?

You can install libmpdec from sources for versions < 4.0.0. (./configure && make all install)

I also checked Debian and derivatives, nobody ships *.pc files as a patch :(.

I applied the following change on configure.ac and I ran make regen-configure.

diff --git a/configure.ac b/configure.ac
index c8dce3fd49b..b9020368664 100644
--- a/configure.ac
+++ b/configure.ac
@@ -4376,7 +4376,7 @@ if test "$ac_cv_ffi_complex_double_supported" = "yes"; then
 fi
 
 dnl Check for libmpdec >= 2.5.0
-PKG_CHECK_MODULES([LIBMPDEC], [libmpdec >= 2.5.0], [have_mpdec=yes], [
+PKG_CHECK_MODULES([LIBMPDEC], [libmpdec >= 55.0], [have_mpdec=yes], [
   WITH_SAVE_ENV([
     CPPFLAGS="$CPPFLAGS $LIBMPDEC_CFLAGS"
     LIBS="$LIBS $LIBMPDEC_LIBS"

./configure fails to locate libmpdecimal:

checking for libmpdec >= 55.0... no
...
checking for stdlib extension module _decimal... missing

I downloaded this PR, made a similar ./configure.ac change, and ran make regen-configure. This time, it succeeded:

checking for libmpdec >= 55.0... no
checking for mpdecimal.h... (cached) yes
checking for mpd_version in -lmpdec... (cached) yes
...
checking for stdlib extension module _decimal... yes

I get these flags in Makefile:

$ grep ^MODULE__DECIMAL Makefile
MODULE__DECIMAL_STATE=yes
MODULE__DECIMAL_CFLAGS= -DTEST_COVERAGE
MODULE__DECIMAL_LDFLAGS=-lmpdec -lm

Python builds successfully and I can import the decimal module.

Merged, thanks for the fix.

The 3.15 branch has different code in configure.ac, it doesn't seem to be affected.

The 3.15 branch has different code in configure.ac, it doesn't seem to be affected.

Old releases are affected. But in such cases the fallback is using bundled libmpdec, instead of undetected from the system. I think it's less severe issue, probably not worth to be fixed.

The 3.15 branch has different code in configure.ac, it doesn't seem to be affected.

Old releases are affected. But in such cases the fallback is using bundled libmpdec, instead of undetected from the system. I think it's less severe issue, probably not worth to be fixed.

I don't see any issues via testing on previous branches, some stale makefile rules but if the system libmpdec option is used then it will properly link to that one without building the bundled one.

if the system libmpdec option is used then it will properly link to that one without building the bundled one.

I don't understand how this could be. See code e.g. in the v3.14:

if the system libmpdec option is used then it will properly link to that one without building the bundled one.

I don't understand how this could be. See code e.g. in the v3.14:

cpython/configure.ac

Lines 4218 to 4232 in 7756f59

	AC_DEFUN([USE_BUNDLED_LIBMPDEC],
	[LIBMPDEC_CFLAGS="-I\$(srcdir)/Modules/_decimal/libmpdec"
	LIBMPDEC_LIBS="-lm \$(LIBMPDEC_A)"
	LIBMPDEC_INTERNAL="\$(LIBMPDEC_HEADERS) \$(LIBMPDEC_A)"
	have_mpdec=yes
	with_system_libmpdec=no])
	AS_VAR_IF(
	[with_system_libmpdec], [yes],
	[PKG_CHECK_MODULES(
	[LIBMPDEC], [libmpdec >= 2.5.0], [],
	[LIBMPDEC_CFLAGS=${LIBMPDEC_CFLAGS-""}
	LIBMPDEC_LIBS=${LIBMPDEC_LIBS-"-lmpdec -lm"}
	LIBMPDEC_INTERNAL=])],
	[USE_BUNDLED_LIBMPDEC()])

If PKG_CHECK_MODULES() fails - USE_BUNDLED_LIBMPDEC() is used with with_system_libmpdec=no.

USE_BUNDLED_LIBMPDEC() is the argument to AS_VAR_IF() so it won't fall through if PKG_CHECK_MODULES() fails. I've also verified this on a Centos Stream 9 machine on the 3.14 branch, no bundled object files and _decimal.so is linked against the system library (assuming of course I tested properly).