gh-115119: removed implicit fallback to the bundled libmpdec

Author: skirpichev

.github/workflows/build.yml

@@ -90,7 +90,7 @@ jobs:
     name: 'Check if generated files are up to date'
     # Don't use ubuntu-latest but a specific version to make the job
     # reproducible: to get the same tools versions (autoconf, aclocal, ...)
-    runs-on: ubuntu-24.04
+    runs-on: ubuntu-22.04
     timeout-minutes: 60
     needs: build-context
     if: needs.build-context.outputs.run-tests == 'true'
@@ -110,7 +110,9 @@ jobs:
           # Include env.pythonLocation in key to avoid changes in environment when setup-python updates Python
           key: ${{ github.job }}-${{ env.IMAGE_OS_VERSION }}-${{ needs.build-context.outputs.config-hash }}-${{ env.pythonLocation }}
       - name: Install dependencies
-        run: sudo ./.github/workflows/posix-deps-apt.sh
+        run: |
+          sudo ./.github/workflows/posix-deps-apt.sh
+          sudo apt-get -yq install libmpdec-dev
       - name: Add ccache to PATH
         run: echo "PATH=/usr/lib/ccache:$PATH" >> "$GITHUB_ENV"
       - name: Configure ccache action

Doc/whatsnew/3.15.rst

@@ -163,6 +163,11 @@ that may require changes to your code.
 Build changes
 =============
 
+* Removed implicit fallback to the buildled 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
 =============

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 buildled 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`.  Patch by Sergey
+B Kirpichev.

configure

@@ -4092,31 +4092,30 @@ 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([
@@ -4132,16 +4131,17 @@ AS_VAR_IF([with_system_libmpdec], [yes],
       ], [const char *x = mpd_version();])],
       [have_mpdec=yes],
       [have_mpdec=no])
-  ])],
-  [AC_MSG_WARN([m4_normalize([
+  ])])
+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 bundled libmpdecimal
-         (deprecated and scheduled for removal in Python 3.16)])])
-       USE_BUNDLED_LIBMPDEC()])
+         no system libmpdecimal found; falling back to pure-Python version
+         for the decimal module])])
+       AS_VAR_SET([py_cv_module_]_decimal, [n/a])])
 
 # Disable forced inlining in debug builds, see GH-94847
 AS_VAR_IF(