Simplify rpmquery.unpack_string

There is no need to unpack a single byte because it is not
affected by (byte) endianness (and that's what struct.unpack is
about). Moreover, rpmquery.unpack_string now supports an optional
encoding parameter, which could be used by the python3 port to
decode a string. Note: in general we cannot assume that all strings
in a rpm are utf-8 encoded (it is possible to build a rpm that
contains illegal utf-8 sequences).

osc/util/rpmquery.py

@@ -370,16 +370,15 @@ class RpmQuery(packagequery.PackageQuery, packagequery.PackageQueryResult):
     def filename(name, epoch, version, release, arch):
         return '%s-%s-%s.%s.rpm' % (name, version, release, arch)
 
-def unpack_string(data):
+def unpack_string(data, encoding=None):
     """unpack a '\\0' terminated string from data"""
-    val = ''
-    for c in data:
-        c, = struct.unpack('!c', c)
-        if c == '\0':
-            break
-        else:
-            val += c
-    return val
+    idx = data.find(b'\0')
+    if idx == -1:
+        raise ValueError('illegal string: not \\0 terminated')
+    data = data[:idx]
+    if encoding is not None:
+        data = data.decode(encoding)
+    return data
 
 if __name__ == '__main__':
     import sys