Introduce extensible node types.

An extensible node is always tagged T_Extensible, but the extnodename
field identifies it more specifically; it may also include arbitrary
private data.  Extensible nodes can be copied, tested for equality,
serialized, and deserialized, but the core system doesn't know
anything about them otherwise.  Some extensions may find it useful to
include these nodes in fdw_private or custom_private lists in lieu of
arm-wrestling their data into a format that the core code can
understand.

Along the way, so as not to burden the authors of such extensible
node types too much, expose the functions for writing serialized
tokens, and for serializing and deserializing bitmapsets.

KaiGai Kohei, per a design suggested by me.  Reviewed by Andres Freund
and by me, and further edited by me.

1 files changed, 40 insertions, 0 deletions

diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index e4d41ee95b2..e6e6f2981c2 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -29,6 +29,7 @@
 #include <math.h>
 
 #include "fmgr.h"
+#include "nodes/extensible.h"
 #include "nodes/parsenodes.h"
 #include "nodes/plannodes.h"
 #include "nodes/readfuncs.h"
@@ -218,6 +219,14 @@ _readBitmapset(void)
 	return result;
 }
 
+/*
+ * for use by extensions which define extensible nodes
+ */
+Bitmapset *
+readBitmapset(void)
+{
+	return _readBitmapset();
+}
 
 /*
  * _readQuery
@@ -2220,6 +2229,35 @@ _readAlternativeSubPlan(void)
 }
 
 /*
+ * _readExtensibleNode
+ */
+static ExtensibleNode *
+_readExtensibleNode(void)
+{
+	const ExtensibleNodeMethods *methods;
+	ExtensibleNode *local_node;
+	const char	   *extnodename;
+	READ_TEMP_LOCALS();
+
+	token = pg_strtok(&length);		/* skip: extnodename */
+	token = pg_strtok(&length);		/* get extnodename */
+
+	extnodename = nullable_string(token, length);
+	if (!extnodename)
+		elog(ERROR, "extnodename has to be supplied");
+	methods = GetExtensibleNodeMethods(extnodename, false);
+
+	local_node = (ExtensibleNode *) newNode(methods->node_size,
+											T_ExtensibleNode);
+	local_node->extnodename = extnodename;
+
+	/* deserialize the private fields */
+	methods->nodeRead(local_node);
+
+	READ_DONE();
+}
+
+/*
  * parseNodeString
  *
  * Given a character string representing a node tree, parseNodeString creates
@@ -2447,6 +2485,8 @@ parseNodeString(void)
 		return_value = _readSubPlan();
 	else if (MATCH("ALTERNATIVESUBPLAN", 18))
 		return_value = _readAlternativeSubPlan();
+	else if (MATCH("EXTENSIBLENODE", 14))
+		return_value = _readExtensibleNode();
 	else
 	{
 		elog(ERROR, "badly formatted node string \"%.32s\"...", token);