Mark constantly allocated dest receiver as const.

This allows the compiler / linker to mark affected pages as read-only.

Doing so requires casting constness away, as CreateDestReceiver()
returns both constant and non-constant dest receivers. That's fine
though, as any modification of the statically allocated receivers
would already have been a bug (and would now be caught on some
platforms).

Discussion: https://postgr.es/m/20181015200754.7y7zfuzsoux2c4ya@alap3.anarazel.de

diff --git a/src/backend/tcop/dest.c b/src/backend/tcop/dest.c
index c95a4d519de15539931320733d37a80f689bffc5..4091ca9e4b9be1b3df48101d6c00f400bdf00710 100644 (file)
--- a/src/backend/tcop/dest.c
+++ b/src/backend/tcop/dest.c
@@ -67,29 +67,33 @@ donothingCleanup(DestReceiver *self)
  *             static DestReceiver structs for dest types needing no local state
  * ----------------
  */
-static DestReceiver donothingDR = {
+static const DestReceiver donothingDR = {
        donothingReceive, donothingStartup, donothingCleanup, donothingCleanup,
        DestNone
 };
 
-static DestReceiver debugtupDR = {
+static const DestReceiver debugtupDR = {
        debugtup, debugStartup, donothingCleanup, donothingCleanup,
        DestDebug
 };
 
-static DestReceiver printsimpleDR = {
+static const DestReceiver printsimpleDR = {
        printsimple, printsimple_startup, donothingCleanup, donothingCleanup,
        DestRemoteSimple
 };
 
-static DestReceiver spi_printtupDR = {
+static const DestReceiver spi_printtupDR = {
        spi_printtup, spi_dest_startup, donothingCleanup, donothingCleanup,
        DestSPI
 };
 
-/* Globally available receiver for DestNone */
-DestReceiver *None_Receiver = &donothingDR;
-
+/*
+ * Globally available receiver for DestNone.
+ *
+ * It's ok to cast the constness away as any modification of the none receiver
+ * would be a bug (which gets easier to catch this way).
+ */
+DestReceiver *None_Receiver = (DestReceiver *) &donothingDR;
 
 /* ----------------
  *             BeginCommand - initialize the destination at start of command
@@ -108,6 +112,11 @@ BeginCommand(const char *commandTag, CommandDest dest)
 DestReceiver *
 CreateDestReceiver(CommandDest dest)
 {
+       /*
+        * It's ok to cast the constness away as any modification of the none receiver
+        * would be a bug (which gets easier to catch this way).
+        */
+
        switch (dest)
        {
                case DestRemote:
@@ -115,16 +124,16 @@ CreateDestReceiver(CommandDest dest)
                        return printtup_create_DR(dest);
 
                case DestRemoteSimple:
-                       return &printsimpleDR;
+                       return unconstify(DestReceiver *, &printsimpleDR);
 
                case DestNone:
-                       return &donothingDR;
+                       return unconstify(DestReceiver *, &donothingDR);
 
                case DestDebug:
-                       return &debugtupDR;
+                       return unconstify(DestReceiver *, &debugtupDR);
 
                case DestSPI:
-                       return &spi_printtupDR;
+                       return unconstify(DestReceiver *, &spi_printtupDR);
 
                case DestTuplestore:
                        return CreateTuplestoreDestReceiver();
@@ -146,7 +155,7 @@ CreateDestReceiver(CommandDest dest)
        }
 
        /* should never get here */
-       return &donothingDR;
+       pg_unreachable();
 }
 
 /* ----------------