From ext/SDBM_File/sdbm/README:

 The entire sdbm  library package, as authored by me, Ozan S.  Yigit,
 is  hereby placed in the public domain. As such, the author is not
 responsible for the  consequences  of  use  of this  software, no
 matter how awful, even if they arise from defects in it. There is no
 expressed or implied warranty for the sdbm library.

 Since the sdbm library package is in the public domain, this   original
 release  or  any  additional  public-domain releases of the modified
 original cannot possibly (by definition) be withheld from you. Also
 by definition, You (singular) have all the rights to this code
 (including  the  right to sell without permission, the right to
 hoard[3]  and  the right  to  do  other  icky  things as you see fit)
 but those rights are also granted to everyone else.

 Please note that all  previous  distributions  of  this software
 contained  a  copyright  (which is now dropped) to protect its origins
 and its  current  public  domain  status against any possible claims
 and/or challenges.