Support multiple iterators in the Red-Black Tree implementation.

While we don't need multiple iterators at the moment, the interface is
nicer and less dangerous this way.

Aleksander Alekseev, with some changes by me.

src/backend/access/gin/ginbulk.c

@@ -255,7 +255,7 @@ qsortCompareItemPointers(const void *a, const void *b)
 void
 ginBeginBAScan(BuildAccumulator *accum)
 {
-	rb_begin_iterate(accum->tree, LeftRightWalk);
+	rb_begin_iterate(accum->tree, LeftRightWalk, &accum->tree_walk);
 }
 
 /*
@@ -271,7 +271,7 @@ ginGetBAEntry(BuildAccumulator *accum,
 	GinEntryAccumulator *entry;
 	ItemPointerData *list;
 
-	entry = (GinEntryAccumulator *) rb_iterate(accum->tree);
+	entry = (GinEntryAccumulator *) rb_iterate(&accum->tree_walk);
 
 	if (entry == NULL)
 		return NULL;			/* no more entries */

src/include/access/gin_private.h

@@ -919,6 +919,7 @@ typedef struct
 	GinEntryAccumulator *entryallocator;
 	uint32		eas_used;
 	RBTree	   *tree;
+	RBTreeIterator tree_walk;
 } BuildAccumulator;
 
 extern void ginInitBA(BuildAccumulator *accum);

src/include/lib/rbtree.h

@@ -22,7 +22,6 @@
  */
 typedef struct RBNode
 {
-	char		iteratorState;	/* workspace for iterating through tree */
 	char color;					/* node's current color, red or black */
 	struct RBNode *left;		/* left child, or RBNIL if none */
 	struct RBNode *right;		/* right child, or RBNIL if none */
@@ -41,6 +40,22 @@ typedef enum RBOrderControl
 	InvertedWalk				/* postorder: left child, right child, node */
 } RBOrderControl;
 
+/*
+ * RBTreeIterator holds state while traversing a tree.  This is declared
+ * here so that callers can stack-allocate this, but must otherwise be
+ * treated as an opaque struct.
+ */
+typedef struct RBTreeIterator RBTreeIterator;
+
+struct RBTreeIterator
+{
+	RBTree	   *rb;
+	RBNode	   *(*iterate) (RBTreeIterator *iter);
+	RBNode	   *last_visited;
+	char		next_step;
+	bool		is_over;
+};
+
 /* Support functions to be provided by caller */
 typedef int (*rb_comparator) (const RBNode *a, const RBNode *b, void *arg);
 typedef void (*rb_combiner) (RBNode *existing, const RBNode *newdata, void *arg);
@@ -60,7 +75,8 @@ extern RBNode *rb_leftmost(RBTree *rb);
 extern RBNode *rb_insert(RBTree *rb, const RBNode *data, bool *isNew);
 extern void rb_delete(RBTree *rb, RBNode *node);
 
-extern void rb_begin_iterate(RBTree *rb, RBOrderControl ctrl);
-extern RBNode *rb_iterate(RBTree *rb);
+extern void rb_begin_iterate(RBTree *rb, RBOrderControl ctrl,
+				 RBTreeIterator *iter);
+extern RBNode *rb_iterate(RBTreeIterator *iter);
 
 #endif   /* RBTREE_H */