Add macro to cast away volatile without allowing changes to underlying type

This adds unvolatize(), which works just like unconstify() but for volatile.

Discussion: https://www.postgresql.org/message-id/flat/7a5cbea7-b8df-e910-0f10-04014bcad701%402ndquadrant.com

src/backend/postmaster/pgstat.c

@@ -3311,7 +3311,7 @@ pgstat_read_current_status(void)
 			localentry->backendStatus.st_procpid = beentry->st_procpid;
 			if (localentry->backendStatus.st_procpid > 0)
 			{
-				memcpy(&localentry->backendStatus, (char *) beentry, sizeof(PgBackendStatus));
+				memcpy(&localentry->backendStatus, unvolatize(PgBackendStatus *, beentry), sizeof(PgBackendStatus));
 
 				/*
 				 * strcpy is safe even if the string is modified concurrently,

src/backend/storage/ipc/pmsignal.c

@@ -134,7 +134,7 @@ PMSignalShmemInit(void)
 
 	if (!found)
 	{
-		MemSet(PMSignalState, 0, PMSignalShmemSize());
+		MemSet(unvolatize(PMSignalData *, PMSignalState), 0, PMSignalShmemSize());
 		PMSignalState->num_child_flags = MaxLivePostmasterChildren();
 	}
 }

src/include/c.h

@@ -1122,7 +1122,7 @@ typedef union PGAlignedXLogBlock
 #endif
 
 /*
- * Macro that allows to cast constness away from an expression, but doesn't
+ * Macro that allows to cast constness and volatile away from an expression, but doesn't
  * allow changing the underlying type.  Enforcement of the latter
  * currently only works for gcc like compilers.
  *
@@ -1141,9 +1141,15 @@ typedef union PGAlignedXLogBlock
 	(StaticAssertExpr(__builtin_types_compatible_p(__typeof(expr), const underlying_type), \
 					  "wrong cast"), \
 	 (underlying_type) (expr))
+#define unvolatize(underlying_type, expr) \
+	(StaticAssertExpr(__builtin_types_compatible_p(__typeof(expr), volatile underlying_type), \
+					  "wrong cast"), \
+	 (underlying_type) (expr))
 #else
 #define unconstify(underlying_type, expr) \
 	((underlying_type) (expr))
+#define unvolatize(underlying_type, expr) \
+	((underlying_type) (expr))
 #endif
 
 /* ----------------------------------------------------------------