Memory mapped files on remote volumes (from Windows)

Hi All,

We have a few users reporting Metakit database corruption.  Having 
investigated, they are all opening a database that is residing on a 
remote volume - in particular one user is opening a database from 
Windows XP that resides on a Mac running 10.3.8 (the Mac uses SAMBA to 
share files to PC clients).

After doing some investigation, the problem seems to be with memory 
mapped files.  If I turn off memory mapping for Windows, we get no 
problems.

Looking at the Metakit sources, I think I see why.  MK maps the file, 
but then also uses fread() and fwrite() to access the file at the same 
time.  This confuses me somewhat - I would have thought that data would 
have been modified in-place rather than using fwrite().

Reading up on Windows file mapping, I find this comment in the docs for 
CreateFileMapping():

>  Creating a file mapping object creates the potential for mapping a 
> view of the file, but does not map the view. The MapViewOfFile and  
> MapViewOfFileEx functions map a view of a file into a process address 
> space.
>
>  With one important exception, file views derived from a single file 
> mapping object are coherent or identical at a specific time. If 
> multiple processes have handles of the same file mapping object, they 
> see a coherent view of the data when they map a view of the file.
>
>  The exception is related to remote files. Although CreateFileMapping 
> works with remote files, it does not keep them coherent. For example, 
> if two computers both map a file as writable, and both change the same 
> page, each computer only sees its own writes to the page. When the 
> data gets updated on the disk, it is not merged.
>
>  A mapped file and a file that is accessed by using the input and 
> output (I/O) functions (ReadFile and  WriteFile) are not necessarily 
> coherent.

That last bit is the killer - Windows does not guarantee that files 
accessed by ReadFile and  WriteFile (which I suspect fread() and 
fwrite() eventually call) are coherent with memory mapped on those 
files.

So, I can think of a couple of ways of solving this:

(a) Turn off file mapping for remote volumes.

(b) Modify MK so that writes inside the mapped area are done using 
memmove() (or whatever) rather than fwrite().

This is not guaranteed to work on Unix either:

http://www.opengroup.org/onlinepubs/009695399/functions/mmap.html

> The application must ensure correct synchronization when using mmap() 
> in conjunction with any other file access method, such as read() and 
> write(), standard input/output, and shmat().

Has anyone come across this before?  Any fixes?

Cheers,

Steve.

Stephen Baxter
Software Development Manager
Improvision
[email protected]
+44-2476-692229

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