This is a lot more complex than it sounds like it will be.
We shut down the cache flush when a holder is closed, but if you're
deleting an index, we don't check for that, and can have a cache flush
still creating cache files in an index which could conceivably result
in os.RemoteAll() failing. This shouldn't happen often, but it's happened
at least once.
To address this, first, we make sure that every tier of this operation
bails as quickly as it can after the thing it's working on closes. Second,
we retry RemoveAll.
Unfortunately, some things get reopened, so we have to handle that,
have mutexes covering the access to the channel, and so on. Also, some
things were getting double-closed, which was previously harmless but
could now fail. So, first, catch all the existing double-closes and
remove them, second, make the double-close fail with an error. Note
that virtually none of the tests check for errors on close.
This passes tests and should be unable to hit the original problem.
Unfortunately, it's unreasonably hard to check that, because it
requires an incredible coincidence of timing on the delete aligning
with a cache flush.
i used this script, a little clunky but it got the job done
```bash
for file in `find . -type f -print | grep '\.go'`; do
sed '1,/^\/\/ limitations under the License.$/d' $file > $file.tmp;
result=`cat $file.tmp`
if [[ result != "" ]]; then
gofmt $file.tmp &> /dev/null;
if [[ $? == 0 ]]; then
mv $file.tmp $file && gofmt -w $file;
else
rm $file.tmp;
fi
else
rm $file.tmp;
fi
done
```
