[Rpm-maint] [rpm-software-management/rpm] Allow database probing if _db_backend is not set (#1451)

[Panu Matilainen]

@pmatilai commented on this pull request.



> @@ -97,7 +98,7 @@ dbDetectBackend(rpmdb rdb)
 		rpmlog(RPMLOG_WARNING,
 			_("Converting database from %s to %s backend\n"),
 			ondisk->name, cfg->name);
-	    } else {
+	    } else if (*db_backend) {

Right, it does look a kinda suspect that way.

I can see how rpmdb2solv doesn't want that warning to be emitted, because it *wants* rpm to do a probe. But shouldn't this actually be:

```
/* On-disk database differs from configuration */
if (ondisk && ondisk != cfg) {
    if (cfg) {
        if (rdb->db_flags & RPMDB_FLAG_REBUILD) {
            rpmlog(RPMLOG_WARNING,
                    _("Converting database from %s to %s backend\n"),
                    ondisk->name, cfg->name);
        } else {
            rpmlog(RPMLOG_WARNING,
                _("Found %s %s database while attempting %s backend: "
                "using %s backend.\n"),
                ondisk->name, ondisk->path, db_backend, ondisk->name);
        }
    }
    rdb->db_ops = ondisk;
}   

```

Ie the warnings should only ever be emitted when there *is* a configuration, and I think conditionalizing on 'cfg' makes it more clear than the various different conditions (eg the `cfg->name` access *looks* like potential a NULL-pointer dereference now, that its guarded by RPMDB_FLAG_REBUILD is not immediately obvious, and looking at *db_backend is different from all the other surrounding conditions). Or am I missing something? (this is a surprisingly subtle little thing)?

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