gh-89452: Prefer gdbm-compat over ndbm

This makes macOS gdbm provided by Homebrew not segfault through correct
selection of the linked library (-lgdbm_compat) *AND* the correct ndbm-style
header (gdbm-ndbm.h instead of the invalid ndbm.h).

Author: ambv

Misc/NEWS.d/next/Build/2022-05-03-03-36-47.gh-issue-89452.NIY0fF.rst

@@ -0,0 +1,2 @@
+gdbm-compat is now preferred over ndbm if both are available on the system.
+This allows avoiding the problematic ndbm.h on macOS.

Modules/_dbmmodule.c

@@ -12,10 +12,7 @@
 /* Some Linux systems install gdbm/ndbm.h, but not ndbm.h.  This supports
  * whichever configure was able to locate.
  */
-#if defined(USE_NDBM)
-  #include <ndbm.h>
-  static const char which_dbm[] = "GNU gdbm";  /* EMX port of GDBM */
-#elif defined(USE_GDBM_COMPAT)
+#if defined(USE_GDBM_COMPAT)
   #ifdef HAVE_GDBM_NDBM_H
     #include <gdbm/ndbm.h>
   #elif HAVE_GDBM_DASH_NDBM_H
@@ -24,6 +21,9 @@
     #error "No gdbm/ndbm.h or gdbm-ndbm.h available"
   #endif
   static const char which_dbm[] = "GNU gdbm";
+#elif defined(USE_NDBM)
+  #include <ndbm.h>
+  static const char which_dbm[] = "GNU gdbm";
 #elif defined(USE_BERKDB)
   #ifndef DB_DBM_HSEARCH
     #define DB_DBM_HSEARCH 1

configure

@@ -3850,7 +3850,7 @@ AC_CHECK_HEADERS([db.h], [
 AC_MSG_CHECKING(for --with-dbmliborder)
 AC_ARG_WITH(dbmliborder,
             AS_HELP_STRING([--with-dbmliborder=db1:db2:...], [override order to check db backends for dbm; a valid value is a colon separated string with the backend names `ndbm', `gdbm' and `bdb'.]),
-[], [with_dbmliborder=ndbm:gdbm:bdb])
+[], [with_dbmliborder=gdbm:ndbm:bdb])
 
 have_gdbm_dbmliborder=no
 as_save_IFS=$IFS
@@ -3865,7 +3865,7 @@ for db in $with_dbmliborder; do
 done
 IFS=$as_save_IFS
 AS_VAR_IF([with_dbmliborder], [error], [
-  AC_MSG_ERROR([proper usage is --with-dbmliborder=db1:db2:... (ndbm:gdbm:bdb)])
+  AC_MSG_ERROR([proper usage is --with-dbmliborder=db1:db2:... (gdbm:ndbm:bdb)])
 ])
 AC_MSG_RESULT($with_dbmliborder)
 

configure.ac

@@ -1207,7 +1207,7 @@ def detect_dbm_gdbm(self):
             if dbm_args:
                 dbm_order = [arg.split('=')[-1] for arg in dbm_args][-1].split(":")
             else:
-                dbm_order = "ndbm:gdbm:bdb".split(":")
+                dbm_order = "gdbm:ndbm:bdb".split(":")
             dbmext = None
             for cand in dbm_order:
                 if cand == "ndbm":