Merge DDB file, return the absolute path of the new database in workdir.

def merge(self, workdir, ddb_files, out_ddb, description, delete_source_ddbs=True):
        """Merge DDB file, return the absolute path of the new database in workdir."""
        # We work with absolute paths.
        ddb_files = [os.path.abspath(s) for s in list_strings(ddb_files)]
        if not os.path.isabs(out_ddb):
            out_ddb = os.path.join(os.path.abspath(workdir), os.path.basename(out_ddb))

        if self.verbose:
            print("Will merge %d files into output DDB %s" % (len(ddb_files), out_ddb))
            for i, f in enumerate(ddb_files):
                print(" [%d] %s" % (i, f))

        # Handle the case of a single file since mrgddb uses 1 to denote GS files!
        if len(ddb_files) == 1:
            with open(ddb_files[0], "r") as inh, open(out_ddb, "w") as out:
                for line in inh:
                    out.write(line)
            return out_ddb

        self.stdin_fname, self.stdout_fname, self.stderr_fname = \
            map(os.path.join, 3 * [os.path.abspath(workdir)], ["mrgddb.stdin", "mrgddb.stdout", "mrgddb.stderr"])

        inp = StringIO()
        inp.write(out_ddb + "\n")              # Name of the output file.
        inp.write(str(description) + "\n")     # Description.
        inp.write(str(len(ddb_files)) + "\n")  # Number of input DDBs.

        # Names of the DDB files.
        for fname in ddb_files:
            inp.write(fname + "\n")

        self.stdin_data = [s for s in inp.getvalue()]

        with open(self.stdin_fname, "wt") as fh:
            fh.writelines(self.stdin_data)
            # Force OS to write data to disk.
            fh.flush()
            os.fsync(fh.fileno())

        retcode = self.execute(workdir, exec_args=['--nostrict'])
        if retcode == 0 and delete_source_ddbs:
            # Remove ddb files.
            for f in ddb_files:
                try:
                    os.remove(f)
                except IOError:
                    pass

        return out_ddb