Rework custom scans to work more like the new extensible node stuff.

Per discussion, the new extensible node framework is thought to be
better designed than the custom path/scan/scanstate stuff we added
in PostgreSQL 9.5.  Rework the latter to be more like the former.

This is not backward-compatible, but we generally don't promise that
for C APIs, and there probably aren't many people using this yet
anyway.

KaiGai Kohei, reviewed by Petr Jelinek and me.  Some further
cosmetic changes by me.

3 files changed, 76 insertions, 37 deletions

diff --git a/src/backend/nodes/extensible.c b/src/backend/nodes/extensible.c
index 2473b658b17..d61be58c45d 100644
--- a/src/backend/nodes/extensible.c
+++ b/src/backend/nodes/extensible.c
@@ -24,61 +24,87 @@
 #include "utils/hsearch.h"
 
 static HTAB *extensible_node_methods = NULL;
+static HTAB *custom_scan_methods = NULL;
 
 typedef struct
 {
 	char		extnodename[EXTNODENAME_MAX_LEN];
-	const ExtensibleNodeMethods *methods;
+	const void *extnodemethods;
 } ExtensibleNodeEntry;
 
 /*
- * Register a new type of extensible node.
+ * An internal function to register a new callback structure 
  */
-void
-RegisterExtensibleNodeMethods(const ExtensibleNodeMethods *methods)
+static void
+RegisterExtensibleNodeEntry(HTAB **p_htable, const char *htable_label,
+							const char *extnodename,
+							const void *extnodemethods)
 {
 	ExtensibleNodeEntry *entry;
 	bool		found;
 
-	if (extensible_node_methods == NULL)
+	if (*p_htable == NULL)
 	{
 		HASHCTL		ctl;
 
 		memset(&ctl, 0, sizeof(HASHCTL));
 		ctl.keysize = EXTNODENAME_MAX_LEN;
 		ctl.entrysize = sizeof(ExtensibleNodeEntry);
-		extensible_node_methods = hash_create("Extensible Node Methods",
-											  100, &ctl, HASH_ELEM);
+
+		*p_htable = hash_create(htable_label, 100, &ctl, HASH_ELEM);
 	}
 
-	if (strlen(methods->extnodename) >= EXTNODENAME_MAX_LEN)
+	if (strlen(extnodename) >= EXTNODENAME_MAX_LEN)
 		elog(ERROR, "extensible node name is too long");
 
-	entry = (ExtensibleNodeEntry *) hash_search(extensible_node_methods,
-												methods->extnodename,
+	entry = (ExtensibleNodeEntry *) hash_search(*p_htable,
+												extnodename,
 												HASH_ENTER, &found);
 	if (found)
 		ereport(ERROR,
 				(errcode(ERRCODE_DUPLICATE_OBJECT),
 				 errmsg("extensible node type \"%s\" already exists",
-						methods->extnodename)));
+						extnodename)));
 
-	entry->methods = methods;
+	entry->extnodemethods = extnodemethods;
 }
 
 /*
- * Get the methods for a given type of extensible node.
+ * Register a new type of extensible node.
  */
-const ExtensibleNodeMethods *
-GetExtensibleNodeMethods(const char *extnodename, bool missing_ok)
+void
+RegisterExtensibleNodeMethods(const ExtensibleNodeMethods *methods)
+{
+	RegisterExtensibleNodeEntry(&extensible_node_methods,
+								"Extensible Node Methods",
+								methods->extnodename,
+								methods);
+}
+
+/*
+ * Register a new type of custom scan node
+ */
+void
+RegisterCustomScanMethods(const CustomScanMethods *methods)
+{
+	RegisterExtensibleNodeEntry(&custom_scan_methods,
+								"Custom Scan Methods",
+								methods->CustomName,
+								methods);
+}
+
+/*
+ * An internal routine to get an ExtensibleNodeEntry by the given identifier
+ */
+static const void *
+GetExtensibleNodeEntry(HTAB *htable, const char *extnodename, bool missing_ok)
 {
 	ExtensibleNodeEntry *entry = NULL;
 
-	if (extensible_node_methods != NULL)
-		entry = (ExtensibleNodeEntry *) hash_search(extensible_node_methods,
+	if (htable != NULL)
+		entry = (ExtensibleNodeEntry *) hash_search(htable,
 													extnodename,
 													HASH_FIND, NULL);
-
 	if (!entry)
 	{
 		if (missing_ok)
@@ -89,5 +115,29 @@ GetExtensibleNodeMethods(const char *extnodename, bool missing_ok)
 						extnodename)));
 	}
 
-	return entry->methods;
+	return entry->extnodemethods;
+}
+
+/*
+ * Get the methods for a given type of extensible node.
+ */
+const ExtensibleNodeMethods *
+GetExtensibleNodeMethods(const char *extnodename, bool missing_ok)
+{
+	return (const ExtensibleNodeMethods *)
+		GetExtensibleNodeEntry(extensible_node_methods,
+							   extnodename,
+							   missing_ok);
+}
+
+/*
+ * Get the methods for a given name of CustomScanMethods
+ */
+const CustomScanMethods *
+GetCustomScanMethods(const char *CustomName, bool missing_ok)
+{
+	return (const CustomScanMethods *)
+		GetExtensibleNodeEntry(custom_scan_methods,
+							   CustomName,
+							   missing_ok);
 }
diff --git a/src/backend/nodes/outfuncs.c b/src/backend/nodes/outfuncs.c
index 32d03f7f257..83abaa68a38 100644
--- a/src/backend/nodes/outfuncs.c
+++ b/src/backend/nodes/outfuncs.c
@@ -632,11 +632,9 @@ _outCustomScan(StringInfo str, const CustomScan *node)
 	WRITE_NODE_FIELD(custom_private);
 	WRITE_NODE_FIELD(custom_scan_tlist);
 	WRITE_BITMAPSET_FIELD(custom_relids);
-	/* Dump library and symbol name instead of raw pointer */
+	/* CustomName is a key to lookup CustomScanMethods */
 	appendStringInfoString(str, " :methods ");
-	_outToken(str, node->methods->LibraryName);
-	appendStringInfoChar(str, ' ');
-	_outToken(str, node->methods->SymbolName);
+	_outToken(str, node->methods->CustomName);
 }
 
 static void
diff --git a/src/backend/nodes/readfuncs.c b/src/backend/nodes/readfuncs.c
index 6db0492e152..cb0752a6ad8 100644
--- a/src/backend/nodes/readfuncs.c
+++ b/src/backend/nodes/readfuncs.c
@@ -1827,8 +1827,7 @@ static CustomScan *
 _readCustomScan(void)
 {
 	READ_LOCALS(CustomScan);
-	char	   *library_name;
-	char	   *symbol_name;
+	char	   *custom_name;
 	const CustomScanMethods *methods;
 
 	ReadCommonScan(&local_node->scan);
@@ -1840,19 +1839,11 @@ _readCustomScan(void)
 	READ_NODE_FIELD(custom_scan_tlist);
 	READ_BITMAPSET_FIELD(custom_relids);
 
-	/*
-	 * Reconstruction of methods using library and symbol name
-	 */
+	/* Lookup CustomScanMethods by CustomName */
 	token = pg_strtok(&length);		/* skip methods: */
-	token = pg_strtok(&length);		/* LibraryName */
-	library_name = nullable_string(token, length);
-	token = pg_strtok(&length);		/* SymbolName */
-	symbol_name = nullable_string(token, length);
-
-	methods = (const CustomScanMethods *)
-		load_external_function(library_name, symbol_name, true, NULL);
-	Assert(strcmp(methods->LibraryName, library_name) == 0 &&
-		   strcmp(methods->SymbolName, symbol_name) == 0);
+	token = pg_strtok(&length);		/* CustomName */
+	custom_name = nullable_string(token, length);
+	methods = GetCustomScanMethods(custom_name, false);
 	local_node->methods = methods;
 
 	READ_DONE();