windows.rename: eliminate temp name race (
issue2571)
On Windows, os.rename reliably raises OSError with errno.EEXIST if the
destination already exists (even on shares served by Samba).
Windows does *not* silently overwrite the destination of a rename.
So there is no need to first call os.path.exists on the chosen temp path.
Trusting os.path.exists is actually harmful, since using it enables the
following racy sequence of actions:
1) os.path.exists(temp) returns False
2) some evil other process creates a file with name temp
3) os.rename(dst, temp) now fails because temp has been taken
Not using os.path.exists and directly trying os.rename(dst, temp)
eliminates this race.
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