Add -Wmissing-variable-declarations to the standard compilation flags

This warning flag detects global variables not declared in header
files.  This is similar to what -Wmissing-prototypes does for
functions.  (More correctly, it is similar to what
-Wmissing-declarations does for functions, but -Wmissing-prototypes is
a superset of that in C.)

This flag is new in GCC 14.  Clang has supported it for a while.

Several recent commits have cleaned up warnings triggered by this, so
it should now be clean.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://www.postgresql.org/message-id/flat/e0a62134-83da-4ba4-8cdb-ceb0111c95ce@eisentraut.org

configure

@@ -741,6 +741,7 @@ CXXFLAGS_SL_MODULE
 CFLAGS_SL_MODULE
 CFLAGS_VECTORIZE
 CFLAGS_UNROLL_LOOPS
+PERMIT_MISSING_VARIABLE_DECLARATIONS
 PERMIT_DECLARATION_AFTER_STATEMENT
 LLVM_BINPATH
 LLVM_CXXFLAGS
@@ -6022,6 +6023,54 @@ if test x"$pgac_cv_prog_CXX_cxxflags__Wformat_security" = x"yes"; then
 fi
 
 
+  # gcc 14+, clang for a while
+  # (Supported in C++ by clang but not gcc.  For consistency, omit in C++.)
+  save_CFLAGS=$CFLAGS
+
+{ $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -Wmissing-variable-declarations, for CFLAGS" >&5
+$as_echo_n "checking whether ${CC} supports -Wmissing-variable-declarations, for CFLAGS... " >&6; }
+if ${pgac_cv_prog_CC_cflags__Wmissing_variable_declarations+:} false; then :
+  $as_echo_n "(cached) " >&6
+else
+  pgac_save_CFLAGS=$CFLAGS
+pgac_save_CC=$CC
+CC=${CC}
+CFLAGS="${CFLAGS} -Wmissing-variable-declarations"
+ac_save_c_werror_flag=$ac_c_werror_flag
+ac_c_werror_flag=yes
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_compile "$LINENO"; then :
+  pgac_cv_prog_CC_cflags__Wmissing_variable_declarations=yes
+else
+  pgac_cv_prog_CC_cflags__Wmissing_variable_declarations=no
+fi
+rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext
+ac_c_werror_flag=$ac_save_c_werror_flag
+CFLAGS="$pgac_save_CFLAGS"
+CC="$pgac_save_CC"
+fi
+{ $as_echo "$as_me:${as_lineno-$LINENO}: result: $pgac_cv_prog_CC_cflags__Wmissing_variable_declarations" >&5
+$as_echo "$pgac_cv_prog_CC_cflags__Wmissing_variable_declarations" >&6; }
+if test x"$pgac_cv_prog_CC_cflags__Wmissing_variable_declarations" = x"yes"; then
+  CFLAGS="${CFLAGS} -Wmissing-variable-declarations"
+fi
+
+
+  PERMIT_MISSING_VARIABLE_DECLARATIONS=
+  if test x"$save_CFLAGS" != x"$CFLAGS"; then
+    PERMIT_MISSING_VARIABLE_DECLARATIONS=-Wno-missing-variable-declarations
+  fi
+
   # Disable strict-aliasing rules; needed for gcc 3.3+
 
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether ${CC} supports -fno-strict-aliasing, for CFLAGS" >&5

configure.ac

@@ -518,6 +518,15 @@ if test "$GCC" = yes -a "$ICC" = no; then
   # This was included in -Wall/-Wformat in older GCC versions
   PGAC_PROG_CC_CFLAGS_OPT([-Wformat-security])
   PGAC_PROG_CXX_CFLAGS_OPT([-Wformat-security])
+  # gcc 14+, clang for a while
+  # (Supported in C++ by clang but not gcc.  For consistency, omit in C++.)
+  save_CFLAGS=$CFLAGS
+  PGAC_PROG_CC_CFLAGS_OPT([-Wmissing-variable-declarations])
+  PERMIT_MISSING_VARIABLE_DECLARATIONS=
+  if test x"$save_CFLAGS" != x"$CFLAGS"; then
+    PERMIT_MISSING_VARIABLE_DECLARATIONS=-Wno-missing-variable-declarations
+  fi
+  AC_SUBST(PERMIT_MISSING_VARIABLE_DECLARATIONS)
   # Disable strict-aliasing rules; needed for gcc 3.3+
   PGAC_PROG_CC_CFLAGS_OPT([-fno-strict-aliasing])
   PGAC_PROG_CXX_CFLAGS_OPT([-fno-strict-aliasing])

meson.build

@@ -1986,6 +1986,16 @@ if cc.has_argument('-Wdeclaration-after-statement')
   cflags_no_decl_after_statement += '-Wno-declaration-after-statement'
 endif
 
+# Some code is not clean for -Wmissing-variable-declarations, so we
+# make the "no" option available.  Also, while clang supports this
+# option for C++, gcc does not, so for consistency, leave it off for
+# C++.
+cflags_no_missing_var_decls = []
+if cc.has_argument('-Wmissing-variable-declarations')
+  cflags_warn += '-Wmissing-variable-declarations'
+  cflags_no_missing_var_decls += '-Wno-missing-variable-declarations'
+endif
+
 
 # The following tests want to suppress various unhelpful warnings by adding
 # -Wno-foo switches.  But gcc won't complain about unrecognized -Wno-foo

src/Makefile.global.in

@@ -266,6 +266,7 @@ CFLAGS_POPCNT = @CFLAGS_POPCNT@
 CFLAGS_CRC = @CFLAGS_CRC@
 CFLAGS_XSAVE = @CFLAGS_XSAVE@
 PERMIT_DECLARATION_AFTER_STATEMENT = @PERMIT_DECLARATION_AFTER_STATEMENT@
+PERMIT_MISSING_VARIABLE_DECLARATIONS = @PERMIT_MISSING_VARIABLE_DECLARATIONS@
 CXXFLAGS = @CXXFLAGS@
 
 LLVM_CPPFLAGS = @LLVM_CPPFLAGS@

src/interfaces/ecpg/test/Makefile.regress

@@ -3,7 +3,7 @@
 
 override CPPFLAGS := -I../../include -I$(top_srcdir)/src/interfaces/ecpg/include \
 	-I$(libpq_srcdir) $(CPPFLAGS)
-override CFLAGS += $(PTHREAD_CFLAGS)
+override CFLAGS += $(PTHREAD_CFLAGS) $(PERMIT_MISSING_VARIABLE_DECLARATIONS)
 
 LDFLAGS_INTERNAL += -L../../ecpglib -lecpg -L../../pgtypeslib -lpgtypes $(libpq)
 

src/interfaces/ecpg/test/meson.build

@@ -27,6 +27,7 @@ testprep_targets += pg_regress_ecpg
 
 # create .c files and executables from .pgc files
 ecpg_test_exec_kw = {
+  'c_args': cflags_no_missing_var_decls,
   'dependencies': [frontend_code, libpq],
   'include_directories': [ecpg_inc],
   'link_with': [ecpglib_so, ecpg_compat_so, ecpg_pgtypes_so],

src/makefiles/meson.build

@@ -98,6 +98,8 @@ pgxs_kv = {
   'CXXFLAGS_SL_MODULE': ' '.join(cxxflags_mod),
   'PERMIT_DECLARATION_AFTER_STATEMENT':
     ' '.join(cflags_no_decl_after_statement),
+  'PERMIT_MISSING_VARIABLE_DECLARATIONS':
+    ' '.join(cflags_no_missing_var_decls),
 
   'CFLAGS_CRC': ' '.join(cflags_crc),
   'CFLAGS_POPCNT': ' '.join(cflags_popcnt),

src/tools/pg_bsd_indent/Makefile

@@ -25,6 +25,8 @@ OBJS = \
 	parse.o \
 	pr_comment.o
 
+$(OBJS): CFLAGS += $(PERMIT_MISSING_VARIABLE_DECLARATIONS)
+
 all: pg_bsd_indent
 
 pg_bsd_indent: $(OBJS) | submake-libpgport

src/tools/pg_bsd_indent/meson.build

@@ -18,6 +18,7 @@ endif
 
 pg_bsd_indent = executable('pg_bsd_indent',
   pg_bsd_indent_sources,
+  c_args: cflags_no_missing_var_decls,
   dependencies: [frontend_code],
   include_directories: include_directories('.'),
   kwargs: default_bin_args + {