_load_auth_store() treated every exception from reading auth.json as
corruption and returned an empty store. EMFILE under fd exhaustion,
EACCES, EIO and a stalled network mount all reached that branch. This
module does read-modify-write in roughly fifteen places, so the empty
store was one _save_auth_store() away from erasing every stored
credential.
Separate OSError from parse failure: a file that exists but cannot be
read now raises, naming the real cause and leaving the file on disk
untouched. Only a genuine parse failure takes the preserve-and-start-
empty branch, which is unchanged.
The backup was also unreliable in exactly the conditions that triggered
it: shutil.copy2 opens a file, so under EMFILE it failed too, its bare
except swallowed that, and the log still said "Corrupt file preserved
at ..." when nothing had been written. Track whether the copy landed
and say so accurately.
MaxFreedomPollard