branch locking mk2.

[Martin Pool]

On  8 Feb 2006, Jan Hudec <bulb at ucw.cz> wrote:

> Rename *IS* atomic on windows. Atomic means, in this case, it either
> happens, or not, but nothing in the middle. If the dst already exists,
> and is a directory (which is the case), directory rename never succeeds,
> no matter what system.

Perhaps surprisingly, directory rename overwrite will succeed on Unix if
the directory to be overwritten is empty:

  mkdir /tmp/a /tmp/b
  mv /tmp/a /tmp/b
  
succeeds - even if you write a small program that makes the rename
syscall directly.  This does trap properly:

  mkdir /tmp/a /tmp/b
  touch /tmp/b/bfile
  mv /tmp/a /tmp/b
  
In any case it is useful to put a file inside to carry information (as
ddaa asked) and to make sure which instance is really in place.

-- 
Martin
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