- set errno to ENOENT by default so that we don't leave stale errno values
  around in error cases

Index: b/src/db-XXX.c
===================================================================
--- a/src/db-XXX.c
+++ b/src/db-XXX.c
@@ -122,6 +122,7 @@
     }
 
   /* Succeed iff it matches a value that parses correctly.  */
+  *errnop = ENOENT;
   value.flags = 0;
   err = db->get (db, NULL, key, &value, 0);
   if (err)