TxDb: Walk method - to reuse cursors (erigontech#1548)

ethdb/tx_db.go

@@ -222,8 +222,20 @@ func (m *TxDb) IdealBatchSize() int {
 
 func (m *TxDb) Walk(bucket string, startkey []byte, fixedbits int, walker func([]byte, []byte) (bool, error)) error {
 	m.panicOnEmptyDB()
-	c := m.tx.Cursor(bucket) // create new cursor, then call other methods of TxDb inside MultiWalk callback will not affect this cursor
-	defer c.Close()
+	// get cursor out of pool, then calls txDb.Put/Get/Delete on same bucket inside Walk callback - will not affect state of Walk
+	c, ok := m.cursors[bucket]
+	if ok {
+		delete(m.cursors, bucket)
+	} else {
+		c = m.tx.Cursor(bucket)
+	}
+	defer func() { // put cursor back to pool if can
+		if _, ok = m.cursors[bucket]; ok {
+			c.Close()
+		} else {
+			m.cursors[bucket] = c
+		}
+	}()
 	return Walk(c, startkey, fixedbits, walker)
 }