gh-115119: Remove implicit fallback to the bundled libmpdec (#134078)

skirpichev · vstinner · hugovk · web-flow · commit 17cf0a343b13 · 2025-07-01T17:50:51.000+02:00
Co-authored-by: Victor Stinner &lt;vstinner@python.org&gt;
Co-authored-by: Hugo van Kemenade &lt;1324225+hugovk@users.noreply.github.com&gt;
diff --git a/.github/workflows/posix-deps-apt.sh b/.github/workflows/posix-deps-apt.sh
@@ -25,3 +25,10 @@ apt-get -yq install \
     uuid-dev \
     xvfb \
     zlib1g-dev
+
+# Workaround missing libmpdec-dev on ubuntu 24.04:
+# https://launchpad.net/~ondrej/+archive/ubuntu/php
+# https://deb.sury.org/
+sudo add-apt-repository ppa:ondrej/php
+apt-get update
+apt-get -yq install libmpdec-dev
diff --git a/Doc/whatsnew/3.15.rst b/Doc/whatsnew/3.15.rst
@@ -310,6 +310,11 @@ that may require changes to your code.
 Build changes
 =============
 
+* Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
+  Now this should be explicitly enabled with :option:`--with-system-libmpdec`
+  set to ``no`` or with :option:`!--without-system-libmpdec`.
+  (Contributed by Sergey B Kirpichev in :gh:`115119`.)
+
 
 C API changes
 =============
diff --git a/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst b/Misc/NEWS.d/next/Build/2025-05-16-07-46-06.gh-issue-115119.ALBgS_.rst
@@ -0,0 +1,4 @@
+Removed implicit fallback to the bundled copy of the ``libmpdec`` library.
+Now this should be explicitly enabled via :option:`--with-system-libmpdec`
+set to ``no`` or :option:`!--without-system-libmpdec`.  Patch by Sergey
+B Kirpichev.
diff --git a/configure b/configure
diff --git a/configure.ac b/configure.ac
@@ -4169,31 +4169,30 @@ fi
 
 # Check for use of the system libmpdec library
 AC_MSG_CHECKING([for --with-system-libmpdec])
+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])
 AC_ARG_WITH(
   [system_libmpdec],
   [AS_HELP_STRING(
     [--with-system-libmpdec],
     [build _decimal module using an installed mpdecimal library, see Doc/library/decimal.rst (default is yes)]
   )],
-  [],
+  [AS_IF([test "x$with_system_libmpdec" = xno],
+         [USE_BUNDLED_LIBMPDEC()])],
   [with_system_libmpdec="yes"])
 AC_MSG_RESULT([$with_system_libmpdec])
 
-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()])
+     LIBMPDEC_INTERNAL=])])
 
 AS_VAR_IF([with_system_libmpdec], [yes],
   [WITH_SAVE_ENV([
@@ -4209,16 +4208,7 @@ AS_VAR_IF([with_system_libmpdec], [yes],
       ], [const char *x = mpd_version();])],
       [have_mpdec=yes],
       [have_mpdec=no])
-  ])],
-  [AC_MSG_WARN([m4_normalize([
-     the bundled copy of libmpdecimal is scheduled for removal in Python 3.15;
-     consider using a system installed mpdecimal library.])])])
-
-AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
-      [AC_MSG_WARN([m4_normalize([
-         no system libmpdecimal found; falling back to bundled libmpdecimal
-         (deprecated and scheduled for removal in Python 3.15)])])
-       USE_BUNDLED_LIBMPDEC()])
+  ])])
 
 # Disable forced inlining in debug builds, see GH-94847
 AS_VAR_IF(
@@ -8136,6 +8126,16 @@ PY_STDLIB_MOD([_curses_panel],
 PY_STDLIB_MOD([_decimal],
   [], [test "$have_mpdec" = "yes"],
   [$LIBMPDEC_CFLAGS], [$LIBMPDEC_LIBS])
+
+AS_VAR_IF([with_system_libmpdec], [no],
+          [AC_MSG_WARN([m4_normalize([
+     the bundled copy of libmpdecimal is scheduled for removal in Python 3.16;
+     consider using a system installed mpdecimal library.])])])
+AS_IF([test "$with_system_libmpdec" = "yes" && test "$have_mpdec" = "no"],
+      [AC_MSG_WARN([m4_normalize([
+         no system libmpdecimal found; falling back to pure-Python version
+         for the decimal module])])])
+
 PY_STDLIB_MOD([_dbm],
   [test -n "$with_dbmliborder"], [test "$have_dbm" != "no"],
   [$DBM_CFLAGS], [$DBM_LIBS])
