replace chdir with permission check on ftp_CWD #661
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Perform directory permission check instead of
chdir, addressing #475 and #483 .An user of my application found that stressing pyftpdlib ftp_CWD calls can lead to race conditions ( https://gitlab.com/ergoithz/umftpd/-/issues/7 ) which shouldn't be theoretically possible using ioloop (might be a n issue in cpython), but regardless of the reason,
chdirbeing global is itself problematic, this PR addresses that.As a side note, the full POSIX equivalent to
os.chdirwould beos.close(os.open(path, os.R_OK | os.O_DIRECTORY)), but it's 2x slower and not portable, so I'm checking access and inode type instead (which isn't substantially slower thanos.chdirin my testing).