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From Stefan Ruppert ...@myarm.com>
Subject Re: exclusive file access under Windows (Was Re: svn commit: r1534139 - in /apr/apr/branches/1.5.x: ./ CHANGES file_io/win32/readwrite.c)
Date Tue, 22 Oct 2013 10:04:34 GMT
Am 21.10.2013 20:39, schrieb Jeff Trawick:
> On Mon, Oct 21, 2013 at 12:41 PM, Stefan Ruppert <sr@myarm.com
> <mailto:sr@myarm.com>> wrote:
>
>     Am 21.10.2013 16:22, schrieb Jeff Trawick:
>
>         On Mon, Oct 21, 2013 at 8:57 AM, <trawick@apache.org
>         <mailto:trawick@apache.org>
>         <mailto:trawick@apache.org <mailto:trawick@apache.org>>> wrote:
>
>              Author: trawick
>              Date: Mon Oct 21 12:57:05 2013
>              New Revision: 1534139
>
>              URL: http://svn.apache.org/r1534139
>              Log:
>              Merge r960671 from trunk:
>
>              Only deal with the mutex when XTHREAD is enabled. This
>         increases the
>              performance of buffered reads/writes tremendously.
>
>              * file_io/win32/readwrite.c:
>                 (apr_file_read, apr_file_write): only manipulate mutex
>         when XTHREAD
>
>              Submitted by: Ivan Zhakov <ivan visualsvn.com
>         <http://visualsvn.com> <http://visualsvn.com>>
>
>
>         Trunk continues to allocate a mutex if buffered, even if the XTHREAD
>         flag is on (a minor detail I suppose).  That presumably is a
>         simple fix
>         after double checking all the references to mutex or buffered in the
>         code used on Windows.  ISTR other concerns about the mutex or
>         XTHREAD,
>         but I think this is an orthogonal issue.
>
>
>     Regarding exclusive access to a file under windows I filed a bug in
>     2010: https://issues.apache.org/__bugzilla/show_bug.cgi?id=50058
>     <https://issues.apache.org/bugzilla/show_bug.cgi?id=50058>
>
>     Using the apr_file_lock()/apr_file___unlock() under Windows in
>     append mode will deadlock the current thread! In the time of 2010 I
>     just removed the apr_file_lock()/apr_file___unlock() code within the
>     readwrite.c module. But a better solution is to support nesting
>     within apr_file_lock()/apr_file___unlock() API calls!
>
>     Any comments?
>
>     Stefan
>
>
> I thought it was this simple for append:
>
> On Unix a lock isn't needed because the APR implementation there uses
> O_APPEND, which is atomic (subject to the size of the write I suppose)*;
> on Windows there's no such feature and APR has to use a lock to make it
> equivalent.  So the app shouldn't be getting a lock.
>
> Is that consistent with what you see?

The problem arise when you want to use the 
apr_file_lock()/apr_file_unlock() calls to protected multiple calls to 
apr_file_write():

1) apr_file_open(FOPEN_APPEND);
2) apr_file_lock();
3) apr_file_write();
4) apr_file_write();
5) apr_file_write();
6) apr_file_unlock();
7) apr_file_close();

Under Unix all works perfect. But under Windows the step 3) call to 
apr_file_write() will deadlock, because the LockFileEx() should not be 
called recursively...

However, in the APR API docs there is nothing said about an atomic write 
within apr_file_write(), thus from my point of view its up to the 
application to make it atomic with the apr_file_lock()/apr_file_unlock() 
calls. On Unix its a nice side effect that each call to apr_file_write() 
is atomic....

The easist way to make it conistent is to support a nesting counter 
within apr_file_lock()/apr_file_unlock() which will also reflect the APR 
API docs for that calls which are documented to be used recursively!

Regards,
Stefan


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