Add UNCONSTIFY() macro.

Allows casting const-ness away from an expression, but doesn't allow changing the type. Enforcement of the latter currently only works for gcc-like compilers.

Note that it is not safe to cast const-ness away if the result will ever be modified (it would be undefined behavior). Doing so can cause compiler mis-optimizations or runtime crashes (by modifying read-only memory). It is only safe to use when the result will not be modified, but API design or language restrictions prevent you from declaring that (e.g. because a function returns both const and non-const variables).

Note that this only works in function scope, not for global variables (it would be nice, but not trivial, to improve that).

UNCONSTIFY() requires static assert which is a feature in its own right.

src/build.auto.h.in

@@ -4,5 +4,11 @@ Build Flags Generated by Configure
 // Are test code and asserts disabled?
 #undef NDEBUG
 
+// Does the compiler provide _Static_assert()?
+#undef HAVE_STATIC_ASSERT
+
+// Does the compiler provide __builtin_types_compatible_p()?
+#undef HAVE_BUILTIN_TYPES_COMPATIBLE_P
+
 // Is liblz4 present?
 #undef HAVE_LIBLZ4

src/build/configure.ac

@@ -28,6 +28,18 @@ case $host_os in
         ;;
 esac
 
+# Check if the C compiler supports _Static_assert()
+#
+# Test the syntax ({_Static_assert(...)}) because gcc-style compound expressions are needed to wrap it into macros.
+# ----------------------------------------------------------------------------------------------------------------------------------
+AC_LINK_IFELSE([AC_LANG_PROGRAM([], [({_Static_assert(1, "foo");})])], [AC_DEFINE(HAVE_STATIC_ASSERT)])
+
+# Check if the C compiler supports __builtin_types_compatible_p()
+# ----------------------------------------------------------------------------------------------------------------------------------
+AC_LINK_IFELSE(
+    [AC_LANG_PROGRAM([], [[int x; static int y[__builtin_types_compatible_p(__typeof__(x), int)];]])],
+    [AC_DEFINE(HAVE_BUILTIN_TYPES_COMPATIBLE_P)])
+
 # Set warnings and optimizations based on build type (i.e. production or test)
 # ----------------------------------------------------------------------------------------------------------------------------------
 AC_SUBST(CFLAGS, "${CFLAGS} -Wall -Wextra -Wno-missing-field-initializers")

src/common/macro.h

@@ -1,5 +1,8 @@
 /***********************************************************************************************************************************
 General Macros
+
+Portions Copyright (c) 1996-2020, PostgreSQL Global Development Group
+Portions Copyright (c) 1994, Regents of the University of California
 ***********************************************************************************************************************************/
 #ifndef COMMON_MACRO_H
 #define COMMON_MACRO_H
@@ -34,4 +37,52 @@ Useful for ensuring coverage in cases where compared values may be always ascend
     }                                                                                                                              \
     while (0)
 
+/***********************************************************************************************************************************
+If the "condition" (a compile-time-constant expression) evaluates to false then throw a compile error using the "message" (a string
+literal).
+
+gcc 4.6 and up supports _Static_assert(), but there are bizarre syntactic placement restrictions.  Macros STATIC_ASSERT_STMT() and
+STATIC_ASSERT_EXP() make it safe to use as a statement or in an expression, respectively.
+
+Otherwise we fall back on a kluge that assumes the compiler will complain about a negative width for a struct bit-field.  This will
+not include a helpful error message, but it beats not getting an error at all. Note that when std=c99 it looks like gcc is using the
+same kluge.
+
+Adapted from PostgreSQL src/include/c.h.
+***********************************************************************************************************************************/
+#ifdef HAVE_STATIC_ASSERT
+    #define STATIC_ASSERT_STMT(condition, message)                                                                                 \
+        do {_Static_assert(condition, message);} while(0)
+
+    #define STATIC_ASSERT_EXPR(condition, message)                                                                                 \
+        ((void)({STATIC_ASSERT_STMT(condition, message); true;}))
+#else
+    #define STATIC_ASSERT_STMT(condition, message)                                                                                 \
+        ((void)sizeof(struct {int static_assert_failure : (condition) ? 1 : -1;}))
+
+    #define STATIC_ASSERT_EXPR(condition, message)                                                                                 \
+        STATIC_ASSERT_STMT(condition, message)
+#endif
+
+/***********************************************************************************************************************************
+Allows casting const-ness away from an expression, but doesn't allow changing the type. Enforcement of the latter currently only
+works for gcc-like compilers.
+
+Note that it is not safe to cast const-ness away if the result will ever be modified (it would be undefined behavior). Doing so can
+cause compiler mis-optimizations or runtime crashes (by modifying read-only memory). It is only safe to use when the result will not
+be modified, but API design or language restrictions prevent you from declaring that (e.g. because a function returns both const and
+non-const variables).
+
+Note that this only works in function scope, not for global variables (it would be nice, but not trivial, to improve that).
+
+Adapted from PostgreSQL src/include/c.h.
+***********************************************************************************************************************************/
+#ifdef HAVE_BUILTIN_TYPES_COMPATIBLE_P
+    #define UNCONSTIFY(type, expression)                                                                                           \
+        (STATIC_ASSERT_EXPR(__builtin_types_compatible_p(__typeof(expression), const type), "invalid cast"), (type)(expression))
+#else
+    #define UNCONSTIFY(type, expression)                                                                                           \
+        ((type)(expression))
+#endif
+
 #endif

src/configure

@@ -3125,13 +3125,55 @@ case $host_os in
         ;;
 esac
 
+# Check if the C compiler supports _Static_assert()
+#
+# Test the syntax ({_Static_assert(...)}) because gcc-style compound expressions are needed to wrap it into macros.
+# ----------------------------------------------------------------------------------------------------------------------------------
+
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+({_Static_assert(1, "foo");})
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  $as_echo "#define HAVE_STATIC_ASSERT 1" >>confdefs.h
+
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+
+# Check if the C compiler supports __builtin_types_compatible_p()
+# ----------------------------------------------------------------------------------------------------------------------------------
+cat confdefs.h - <<_ACEOF >conftest.$ac_ext
+/* end confdefs.h.  */
+
+int
+main ()
+{
+int x; static int y[__builtin_types_compatible_p(__typeof__(x), int)];
+  ;
+  return 0;
+}
+_ACEOF
+if ac_fn_c_try_link "$LINENO"; then :
+  $as_echo "#define HAVE_BUILTIN_TYPES_COMPATIBLE_P 1" >>confdefs.h
+
+fi
+rm -f core conftest.err conftest.$ac_objext \
+    conftest$ac_exeext conftest.$ac_ext
+
 # Set warnings and optimizations based on build type (i.e. production or test)
 # ----------------------------------------------------------------------------------------------------------------------------------
 CFLAGS="${CFLAGS} -Wall -Wextra -Wno-missing-field-initializers"
 
 
 # -Wno-clobbered is not supported on all compilers
-
 { $as_echo "$as_me:${as_lineno-$LINENO}: checking whether C compiler accepts -Wno-clobbered" >&5
 $as_echo_n "checking whether C compiler accepts -Wno-clobbered... " >&6; }
 if ${ax_cv_check_cflags__Werror__Wno_clobbered+:} false; then :
@@ -5306,4 +5348,4 @@ if test -n "$ac_unrecognized_opts" && test "$enable_option_checking" != no; then
 $as_echo "$as_me: WARNING: unrecognized options: $ac_unrecognized_opts" >&2;}
 fi
 
-# Generated from src/build/configure.ac sha1 20715731e8e51aa203ea7b77c001d51175e2a797
+# Generated from src/build/configure.ac sha1 17c781e6a91b7e2b3fac63e875f04188d84876e8

test/lib/pgBackRestTest/Common/JobTest.pm

@@ -438,6 +438,8 @@ sub run
 
                 # Create build.auto.h
                 my $strBuildAutoH =
+                    ($self->{oTest}->{&TEST_VM} ne VM_CO6 ? "#define HAVE_STATIC_ASSERT\n" : '') .
+                    "#define HAVE_BUILTIN_TYPES_COMPATIBLE_P\n" .
                     (vmWithLz4($self->{oTest}->{&TEST_VM}) ? '#define HAVE_LIBLZ4' : '') . "\n";
 
                 buildPutDiffers($self->{oStorageTest}, "$self->{strGCovPath}/" . BUILD_AUTO_H, $strBuildAutoH);