Re: race condition... in cp?

[Gary Thomas <gary@mlbassoc.com>]

On 2012-03-16 09:19, Mark Hatle wrote:
> On 3/16/12 9:59 AM, Chris Larson wrote:
>> On Fri, Mar 16, 2012 at 4:35 AM, Paul Eggleton
>> <paul.eggleton@linux.intel.com> wrote:
>>> On Friday 16 March 2012 06:58:40 James Limbouris wrote:
>>>> Hi,
>>>>
>>>> I got a strange error when bitbaking two images after removing some files in
>>>> the deploy/images folder. It looks a whole lot like the cp's from the
>>>> individual tasks were racing... I didn't know this sort of thing could
>>>> happen.
>>>>
>>>> bitbake rica-dev-image rica-release-example-image
>>>> <...>
>>>> NOTE: Resolving any missing task queue dependencies
>>>> NOTE: multiple providers are available for runtime libssl
>>>> (openssl-nativesdk, openssl) NOTE: consider defining a PREFERRED_PROVIDER
>>>> entry to match libssl NOTE: Preparing runqueue
>>>> NOTE: Executing SetScene Tasks
>>>> NOTE: Executing RunQueue Tasks
>>>> NOTE: Running task 3673 of 3692 (ID: 23,
>>>> /home/james/oe/meta-rica5/recipes/images/rica-release-example-image.bb,
>>>> do_rootfs) NOTE: Running task 3685 of 3692 (ID: 8,
>>>> /home/james/oe/meta-rica5/recipes/images/rica-dev-image.bb, do_rootfs)
>>>> NOTE: package rica-release-example-image-1.0-r0: task do_rootfs: Started
>>>> NOTE: package rica-dev-image-1.0-r0: task do_rootfs: Started
>>>> ERROR: Function failed: do_rootfs (see
>>>> /home/james/oe/build/tmp-eglibc/work/rica5-rica-linux-gnueabi/rica-dev-imag
>>>> e-1.0-r0/temp/log.do_rootfs.4011 for further information) ERROR: Logfile of
>>>> failure stored in:
>>>> /home/james/oe/build/tmp-eglibc/work/rica5-rica-linux-gnueabi/rica-dev-imag
>>>> e-1.0-r0/temp/log.do_rootfs.4011
>>>> Log data follows:
>>>> | ERROR: Function failed: do_rootfs (see
>>>> | /home/james/oe/build/tmp-eglibc/work/rica5-rica-linux-gnueabi/rica-dev-im
>>>> | age-1.0-r0/temp/log.do_rootfs.4011 for further information) cp: cannot
>>>> | create regular file
>>>> | `/home/james/oe/build/tmp-eglibc/deploy/images/rica5/README_-_DO_NOT_DELE
>>>> | TE_FILES_IN_THIS_DIRECTORY.txt': File exists
>>>> NOTE: package rica-dev-image-1.0-r0: task do_rootfs: Failed
>>>> ERROR: Task 8 (/home/james/oe/meta-rica5/recipes/images/rica-dev-image.bb,
>>>> do_rootfs) failed with exit code '1' Waiting for 1 active tasks to finish:
>>>> 0: rica-release-example-image-1.0-r0 do_rootfs (pid 4008)
>>>> NOTE: package rica-release-example-image-1.0-r0: task do_rootfs: Succeeded
>>>> NOTE: Tasks Summary: Attempted 3685 tasks of which 3683 didn't need to be
>>>> rerun and 1 failed. pseudo: You must set the PSEUDO_PREFIX environment
>>>> variable to run pseudo. pseudo: You must set the PSEUDO_PREFIX environment
>>>> variable to run pseudo.
>>>>
>>>> Summary: 1 task failed:
>>>> /home/james/oe/meta-rica5/recipes/images/rica-dev-image.bb, do_rootfs
>>>> Summary: There was 1 ERROR message shown, returning a non-zero exit code.
>>>>
>>>> Perhaps we should be using cp -f, or discarding the result?
>>>
>>> I tried to use -n originally, but apparently that's not a standard option we
>>> can expect to be available everywhere so it had to be removed. I think in this
>>> case the easiest thing to do is just ignore the failure since if it's genuine
>>> it's not catastrophic and also it's highly unlikely you won't get a subsequent
>>> failure elsewhere. I'll prepare a fix.
>>
>> I had a fix for this, but apparently I never got it merged. As you
>> say, the easiest way is to ignore failure. You can't use -f, because
>> of how cp does its checking - the failure still occurs. And of course
>> you can't check for existence first, as that's a race. The fix I had
>> just used shell redirections (>) instead of cp, as they don't care if
>> the file already exists.
>
> Shell redirect has it's own race issues. If two processes happen to redirect at the same time, then you can get the contents mixed together.
>
> The way I've always addressed this is replace a "cp" or "cp -f" with a:
>
> tmpfile=`mktemp dest.XXXXX`
> cp source $tmpfile (or use a shell redirect here)
> mv $tmpfile dest
>
> The 'mv' operation is atomic, all other operations are not guaranteed to be...

'mv' will be atomic only as long as the two files are on the same file system.
What happens when "dest" is on a different file system than "/tmp/dest.XXXXX"?
To be fully safe and general purpose, I think you'd need to use something like this:
   tmpfile=`mktemp dest.XXXXX --tmpdir=$(dirname dest)`

-- 
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Gary Thomas                 |  Consulting for the
MLB Associates              |    Embedded world
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