Use a transaction when adding new bookmarks

During testing it was discovered that the `first-result' procedure
throws an exception, which left the database in a somewhat
inconsistent state, the bookmark had been added, but (at least one of)
the tags weren't added.  This should make it all succeed or all fail.

linkwave.scm

@@ -26,7 +26,13 @@
                     (set! tag-id (last-insert-rowid db)))
                   (execute db "INSERT INTO bookmark_tag VALUES (?, ?)"
                            bookmark-id tag-id)))
-              tags)))
+              tags))
+  #t)
+
+(define (url-string? str)
+  (and (> (string-length str) 7)
+       (or (string= (substring str 0 7) "http://")
+           (string= (substring str 0 8) "https://"))))
 
 (define db (open-database (data-file "linkwave.db")))
 
@@ -34,8 +40,9 @@
   (if (null? cla)
       (for-each-row print-row db "select * from bookmark")
       (cond
-       ((and (>= (string-length (car cla)) 7) (string= (substring (car cla) 0 7) "http://"))
-        (add-bookmark (car cla) (cadr cla) (caddr cla) (cdddr cla)))
+       ((url-string? (car cla))
+        (with-transaction
+         db (lambda () (add-bookmark (car cla) (cadr cla) (caddr cla) (cdddr cla)))))
        (else
         (format #t "Unrecognized option: ~a~%" (car cla))))))