The results of is_dir() are cached

So before calling is_dir() for the second time, clearstatcache of the dir
bug #1583 The results of is_dir() are cached (THofman)

This PR was merged into the 1.16-dev branch.

Discussion
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The results of is_dir() are cached

If a production server uses statcache, the results of the first and second call of is_dir() is always the same.
So before calling is_dir() for the second time, use clearstatcache() to clear the results of the first call of is_dir().

Commits
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df0d813 The results of is_dir() are cached