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To: python-list@python.org
From: Dennis Lee Bieber <wlfraed@ix.netcom.com>
Subject: Re: How to safely maintain a status file
Date: Sun, 08 Jul 2012 15:35:30 -0400
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Xref: csiph.com comp.lang.python:25051

On Sun, 8 Jul 2012 21:29:41 +1000, Richard Baron Penman
<richardbp@gmail.com> declaimed the following in
gmane.comp.python.general:


> and then on startup read from tmp_file if status_file does not exist.
> But this seems awkward.
>
	It also violates your requirement -- since the "crash" could take
place with a partial "temp file".

	I'd suggest that, rather than deleting the old status file, you
rename IT -- and only delete it IF you successfully rename the temp
file.

#pseudo code -- initialization
if exists("old-status.file"):
	#assume system crashed
	if exists("status.file"):
		delete("status.file")
		rename("old-status.file", "status.file")
	
if exists("status.file"):
	read("status.file")
	rename("status.file", "old-status.file")
else:
	#start-up with no status history

# -- snapshot
if exists("status.file"):
	if exists("old-status.file"):
		delete("old-status.file")
	rename("status.file", "old-status.file")
write("temp.file", "snapshot-data")
rename("temp.file", "status.file")
delete("old-status.file")
-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/