kvdb/sqlbase: optimize Delete for the rw cursor

Similar to the write bucket, we can optimize for the case where the key
already exists, allowing us to skip the extra `SELECT` statement.

Author: Roasbeef

kvdb/sqlbase/readwrite_cursor.go

@@ -4,6 +4,8 @@ package sqlbase
 
 import (
 	"database/sql"
+	"errors"
+	"fmt"
 
 	"github.com/btcsuite/btcwallet/walletdb"
 )
@@ -230,3 +232,62 @@ func (c *readWriteCursor) Delete() error {
 
 	return err
 }
+
+// Delete removes the current key/value pair the cursor is at without
+// invalidating the cursor. Returns ErrIncompatibleValue if attempted when the
+// cursor points to a nested bucket.
+func (c *readWriteCursor) Delete() error {
+	if c.currKey == nil {
+		// Cursor might not be positioned on a valid key.
+		return errors.New("cursor not positioned on a key")
+	}
+
+	// Attempt to delete the key the cursor is pointing at, but only if it's
+	// a value.
+	result, err := c.bucket.tx.Exec(
+		"DELETE FROM "+c.bucket.table+" WHERE "+
+			parentSelector(c.bucket.id)+
+			" AND key=$1 AND value IS NOT NULL",
+		c.currKey,
+	)
+	if err != nil {
+		panic(err)
+	}
+
+	rows, err := result.RowsAffected()
+	if err != nil {
+		return fmt.Errorf("error getting rows affected "+
+			"for cursor key %x: %w", c.currKey, err)
+	}
+
+	// If deletion succeeded, we are done.
+	if rows == 1 {
+		return nil
+	}
+
+	// If rows == 0, the key either didn't exist anymore (concurrent
+	// delete?) or it was a bucket. Check if it's a bucket.
+	var existsAsBucket int
+	rowCheck, cancelCheck := c.bucket.tx.QueryRow(
+		"SELECT 1 FROM "+c.bucket.table+" WHERE "+
+			parentSelector(c.bucket.id)+
+			" AND key=$1 AND value IS NULL", c.currKey,
+	)
+	defer cancelCheck()
+
+	errCheck := rowCheck.Scan(&existsAsBucket)
+	if errCheck == nil {
+		// Found it, and value IS NULL -> It's a bucket.
+		return walletdb.ErrIncompatibleValue
+	}
+
+	if errCheck == sql.ErrNoRows {
+		return nil
+	}
+
+	// Some other error during the check.
+	//
+	// TODO(roasbeef): panic here like above/
+	return fmt.Errorf("error checking if cursor key %x is "+
+		"bucket: %w", c.currKey, errCheck)
+}