Hi! > >>User wants to test for a file with name "foo.txt" > >> > >>* create "foo.txt~" (or whatever) > >>* write contents into "foo.txt~" > >>* rename "foo.txt~" to "foo.txt" > >> > >>Until rename is done, the file does not exists and is not complete. > >>You will potentially have a garbage file to clean up if the program > >>(or system) crashes, but that is not racy in a classic sense, right? > >Well. If people rsync from you, they will start fetching incomplete > >foo.txt~. Plus the garbage issue. > > That is not racy, just garbage (not trying to be pedantic, just > trying to understand). I can see that the "~" file is annoying, but > we have dealt with it for a *long* time :) Ok, so lets keep it at "~" is annoying :-). [But... I was wrong. openat(..., AT_UNLINKED) is not enough to solve this: we do not have flink() and it is not easily possible to link deleted file "back to life" from /proc/self/fd: pavel@amd:/tmp$ > delme pavel@amd:/tmp$ bash 3< delme & [2] 32667 [2]+ Stopped bash 3< delme pavel@amd:/tmp$ fg bash 3< delme pavel@amd:/tmp$ ls -al delme -rw-r--r-- 1 pavel pavel 0 Apr 1 01:36 delme pavel@amd:/tmp$ ls -al /proc/self/fd/3 lr-x------ 1 pavel pavel 64 Apr 1 01:37 /proc/self/fd/3 -> /tmp/delme pavel@amd:/tmp$ rm delme pavel@amd:/tmp$ ls -al /proc/self/fd/3 lr-x------ 1 pavel pavel 64 Apr 1 01:37 /proc/self/fd/3 -> /tmp/delme (deleted) pavel@amd:/tmp$ ln /proc/self/fd/3 delme2 ln: creating hard link `delme2' => `/proc/self/fd/3': Invalid cross-device link ] > >>This is more of a garbage clean up issue? > >Also. Plus sometimes you want temporary "file" that is > >deleted. Terminals use it for history, etc... > > There you would have a race, you can create a file and unlink it of > course and still write to it, but you would have a potential empty > file issue? Yes. openat(..., AT_UNLINKED) solves that -- you'll no longer get those files. (Not sure they'd be always empty. How do you ensure rm hits the disk? fsync() on parent directory? Sounds expensive.) Pavel -- (english) http://www.livejournal.com/~pavelmachek (cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
