maniadd() can make files writeable even to make no effective change.

[kentfredric]

I'm just going to start cataloguing the bugs as I find then in test re-factoring.

There's a test in Manifest.t that ensures maniadd() does not attempt to write to the file if an entry already exists, and that adding an entry that doesnt exist causes a fatal exception to occur.

However, maniadd inside it calls this horrible function called _fix_manifest, which munges the state of the manifest on disk prior to maniadd actually doing anything.

This in turn can silently cause the file to be marked writeable, and the expected failure of maniadd vanishes.

repro steps:

   path("MANIFEST")->spew_raw("foo #bar");
   chmod(0400, "MANIFEST");
   maniadd({ foo => "bar" });  # no fatals
   die "Is now writeable" if -w "MANIFEST"