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.

src/backend/nodes/Makefile

@@ -13,7 +13,7 @@ top_builddir = ../../..
 include $(top_builddir)/src/Makefile.global
 
 OBJS = nodeFuncs.o nodes.o list.o bitmapset.o tidbitmap.o \
-       copyfuncs.o equalfuncs.o makefuncs.o \
+       copyfuncs.o equalfuncs.o extensible.o makefuncs.o \
        outfuncs.o readfuncs.o print.o read.o params.o value.o
 
 include $(top_srcdir)/src/backend/common.mk

src/backend/nodes/copyfuncs.c

@@ -23,6 +23,7 @@
 #include "postgres.h"
 
 #include "miscadmin.h"
+#include "nodes/extensible.h"
 #include "nodes/plannodes.h"
 #include "nodes/relation.h"
 #include "utils/datum.h"
@@ -4165,6 +4166,27 @@ _copyList(const List *from)
 	return new;
 }
 
+/* ****************************************************************
+ *					extensible.h copy functions
+ * ****************************************************************
+ */
+static ExtensibleNode *
+_copyExtensibleNode(const ExtensibleNode *from)
+{
+	ExtensibleNode	   *newnode;
+	const ExtensibleNodeMethods *methods;
+
+	methods = GetExtensibleNodeMethods(from->extnodename, false);
+	newnode = (ExtensibleNode *) newNode(methods->node_size,
+										 T_ExtensibleNode);
+	COPY_STRING_FIELD(extnodename);
+
+	/* copy the private fields */
+	methods->nodeCopy(newnode, from);
+
+	return newnode;
+}
+
 /* ****************************************************************
  *					value.h copy functions
  * ****************************************************************
@@ -4544,6 +4566,13 @@ copyObject(const void *from)
 			retval = list_copy(from);
 			break;
 
+			/*
+			 * EXTENSIBLE NODES
+			 */
+		case T_ExtensibleNode:
+			retval = _copyExtensibleNode(from);
+			break;
+
 			/*
 			 * PARSE NODES
 			 */

src/backend/nodes/equalfuncs.c

@@ -29,6 +29,7 @@
 
 #include "postgres.h"
 
+#include "nodes/extensible.h"
 #include "nodes/relation.h"
 #include "utils/datum.h"
 
@@ -871,6 +872,25 @@ _equalPlaceHolderInfo(const PlaceHolderInfo *a, const PlaceHolderInfo *b)
 	return true;
 }
 
+/*
+ * Stuff from extensible.h
+ */
+static bool
+_equalExtensibleNode(const ExtensibleNode *a, const ExtensibleNode *b)
+{
+	const ExtensibleNodeMethods  *methods;
+
+	COMPARE_STRING_FIELD(extnodename);
+
+	/* At this point, we know extnodename is the same for both nodes. */
+	methods = GetExtensibleNodeMethods(a->extnodename, false);
+
+	/* compare the private fields */
+	if (!methods->nodeEqual(a, b))
+		return false;
+
+	return true;
+}
 
 /*
  * Stuff from parsenodes.h
@@ -2873,6 +2893,13 @@ equal(const void *a, const void *b)
 			retval = _equalValue(a, b);
 			break;
 
+			/*
+			 * EXTENSIBLE NODES
+			 */
+		case T_ExtensibleNode:
+			retval = _equalExtensibleNode(a, b);
+			break;
+
 			/*
 			 * PARSE NODES
 			 */

src/backend/nodes/extensible.c

@@ -0,0 +1,92 @@
+/*-------------------------------------------------------------------------
+ *
+ * extensible.c
+ *	  Support for extensible node types
+ *
+ * Loadable modules can define what are in effect new types of nodes using
+ * the routines in this file.  All such nodes are flagged T_ExtensibleNode,
+ * with the extnodename field distinguishing the specific type.  Use
+ * RegisterExtensibleNodeMethods to register a new type of extensible node,
+ * and GetExtensibleNodeMethods to get information about a previously
+ * registered type of extensible node.
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/nodes/extensible.c
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "nodes/extensible.h"
+#include "utils/hsearch.h"
+
+static HTAB *extensible_node_methods = NULL;
+
+typedef struct
+{
+	char		extnodename[EXTNODENAME_MAX_LEN];
+	const ExtensibleNodeMethods *methods;
+} ExtensibleNodeEntry;
+
+/*
+ * Register a new type of extensible node.
+ */
+void
+RegisterExtensibleNodeMethods(const ExtensibleNodeMethods *methods)
+{
+	ExtensibleNodeEntry *entry;
+	bool		found;
+
+	if (extensible_node_methods == NULL)
+	{
+		HASHCTL		ctl;
+
+		memset(&ctl, 0, sizeof(HASHCTL));
+		ctl.keysize = NAMEDATALEN;
+		ctl.entrysize = sizeof(ExtensibleNodeEntry);
+		extensible_node_methods = hash_create("Extensible Node Methods",
+											  100, &ctl, HASH_ELEM);
+	}
+
+	Assert(strlen(methods->extnodename) <= EXTNODENAME_MAX_LEN);
+
+	entry = (ExtensibleNodeEntry *) hash_search(extensible_node_methods,
+												methods->extnodename,
+												HASH_ENTER, &found);
+	if (found)
+		ereport(ERROR,
+				(errcode(ERRCODE_DUPLICATE_OBJECT),
+				 errmsg("extensible node type \"%s\" already exists",
+						methods->extnodename)));
+
+	entry->methods = methods;
+}
+
+/*
+ * Get the methods for a given type of extensible node.
+ */
+const ExtensibleNodeMethods *
+GetExtensibleNodeMethods(const char *extnodename, bool missing_ok)
+{
+	ExtensibleNodeEntry *entry = NULL;
+
+	if (extensible_node_methods != NULL)
+		entry = (ExtensibleNodeEntry *) hash_search(extensible_node_methods,
+													extnodename,
+													HASH_FIND, NULL);
+
+	if (!entry)
+	{
+		if (missing_ok)
+			return NULL;
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_OBJECT),
+				 errmsg("ExtensibleNodeMethods \"%s\" was not registered",
+						extnodename)));
+	}
+
+	return entry->methods;
+}

src/backend/nodes/outfuncs.c

@@ -26,6 +26,7 @@
 #include <ctype.h>
 
 #include "lib/stringinfo.h"
+#include "nodes/extensible.h"
 #include "nodes/plannodes.h"
 #include "nodes/relation.h"
 #include "utils/datum.h"
@@ -140,6 +141,13 @@ _outToken(StringInfo str, const char *s)
 	}
 }
 
+/* for use by extensions which define extensible nodes */
+void
+outToken(StringInfo str, const char *s)
+{
+	_outToken(str, s);
+}
+
 static void
 _outList(StringInfo str, const List *node)
 {
@@ -196,6 +204,13 @@ _outBitmapset(StringInfo str, const Bitmapset *bms)
 	appendStringInfoChar(str, ')');
 }
 
+/* for use by extensions which define extensible nodes */
+void
+outBitmapset(StringInfo str, const Bitmapset *bms)
+{
+	_outBitmapset(str, bms);
+}
+
 /*
  * Print the value of a Datum given its type.
  */
@@ -2112,6 +2127,27 @@ _outPlannerParamItem(StringInfo str, const PlannerParamItem *node)
 	WRITE_INT_FIELD(paramId);
 }
 
+/*****************************************************************************
+ *
+ *	Stuff from extensible.h
+ *
+ *****************************************************************************/
+
+static void
+_outExtensibleNode(StringInfo str, const ExtensibleNode *node)
+{
+	const ExtensibleNodeMethods  *methods;
+
+	methods = GetExtensibleNodeMethods(node->extnodename, false);
+
+	WRITE_NODE_TYPE("EXTENSIBLENODE");
+
+	WRITE_STRING_FIELD(extnodename);
+
+	/* serialize the private fields */
+	methods->nodeOut(str, node);
+}
+
 /*****************************************************************************
  *
  *	Stuff from parsenodes.h.
@@ -3367,6 +3403,10 @@ _outNode(StringInfo str, const void *obj)
 				_outPlannerParamItem(str, obj);
 				break;
 
+			case T_ExtensibleNode:
+				_outExtensibleNode(str, obj);
+				break;
+
 			case T_CreateStmt:
 				_outCreateStmt(str, obj);
 				break;

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
@@ -2219,6 +2228,35 @@ _readAlternativeSubPlan(void)
 	READ_DONE();
 }
 
+/*
+ * _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
  *
@@ -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);

src/include/nodes/extensible.h

@@ -0,0 +1,72 @@
+/*-------------------------------------------------------------------------
+ *
+ * extensible.h
+ *    Definitions for extensible node type
+ *
+ *
+ * Portions Copyright (c) 1996-2016, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/nodes/extensible.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef EXTENSIBLE_H
+#define EXTENSIBLE_H
+
+#include "nodes/nodes.h"
+
+#define EXTNODENAME_MAX_LEN					64
+
+/*
+ * An extensible node is a new type of node defined by an extension.  The
+ * type is always T_ExtensibleNode, while the extnodename identifies the
+ * specific type of node.  extnodename can be looked up to find the
+ * ExtensibleNodeMethods for this node type.
+ */
+typedef struct ExtensibleNode
+{
+	NodeTag		type;
+	const char *extnodename;	/* identifier of ExtensibleNodeMethods */
+} ExtensibleNode;
+
+/*
+ * node_size is the size of an extensible node of this type in bytes.
+ *
+ * nodeCopy is a function which performs a deep copy from oldnode to newnode.
+ * It does not need to copy type or extnodename, which are copied by the
+ * core system.
+ *
+ * nodeEqual is a function which performs a deep equality comparison between
+ * a and b and returns true or false accordingly.  It does not need to compare
+ * type or extnodename, which are compared by the core system.
+ *
+ * nodeOut is a serialization function for the node type.  It should use the
+ * output conventions typical for outfuncs.c.  It does not need to output
+ * type or extnodename; the core system handles those.
+ *
+ * nodeRead is a deserialization function for the node type.  It does not need
+ * to read type or extnodename; the core system handles those.  It should fetch
+ * the next token using pg_strtok() from the current input stream, and then
+ * reconstruct the private fields according to the manner in readfuncs.c.
+ *
+ * All callbacks are mandatory.
+ */
+typedef struct ExtensibleNodeMethods
+{
+	const char *extnodename;
+	Size		node_size;
+	void	  (*nodeCopy)(struct ExtensibleNode *newnode,
+						  const struct ExtensibleNode *oldnode);
+	bool	  (*nodeEqual)(const struct ExtensibleNode *a,
+						   const struct ExtensibleNode *b);
+	void	  (*nodeOut)(struct StringInfoData *str,
+						 const struct ExtensibleNode *node);
+	void	  (*nodeRead)(struct ExtensibleNode *node);
+} ExtensibleNodeMethods;
+
+extern void RegisterExtensibleNodeMethods(const ExtensibleNodeMethods *method);
+extern const ExtensibleNodeMethods *GetExtensibleNodeMethods(const char *name,
+						 bool missing_ok);
+
+#endif	/* EXTENSIBLE_H */

src/include/nodes/nodes.h

@@ -275,6 +275,11 @@ typedef enum NodeTag
 	T_IntList,
 	T_OidList,
 
+	/*
+	 * TAGS FOR EXTENSIBLE NODES (extensible.h)
+	 */
+	T_ExtensibleNode,
+
 	/*
 	 * TAGS FOR STATEMENT NODES (mostly in parsenodes.h)
 	 */
@@ -527,10 +532,17 @@ extern PGDLLIMPORT Node *newNodeMacroHolder;
  */
 extern char *nodeToString(const void *obj);
 
+struct Bitmapset;		/* not to include bitmapset.h here */
+struct StringInfoData;	/* not to include stringinfo.h here */
+extern void outToken(struct StringInfoData *str, const char *s);
+extern void outBitmapset(struct StringInfoData *str,
+						 const struct Bitmapset *bms);
+
 /*
  * nodes/{readfuncs.c,read.c}
  */
 extern void *stringToNode(char *str);
+extern struct Bitmapset *readBitmapset(void);
 
 /*
  * nodes/copyfuncs.c