Re: Options for getting mandatory locking in cygwin1.dll (was: Promote sqlite 3.7.13-1 from test status?)

[Warren Young <warren at etr-usa dot com>]

On 8/16/2012 6:26 AM, Corinna Vinschen wrote:
> On Aug 16 06:01, Warren Young wrote:
> > Advisory locking only works when all players cooperate.  We can't
> > assume that on Windows, unless we set up an insular Cygwin ghetto.
>
> So, are you saying that Cygwin should use mandatory file locking?

Of course not.  That would be a disaster.

> You are aware that there's no chance at all to implement the UNIX
> locking API calls correctly this way since Windows locking has nothing
> in common with UNIX locking except for the word "locking".

Not even on a per-process basis (cygwin_set_mandatory_locking(1)), in 
conjunction with fcntl(F_SETLK)?  That's how SQLite's unixFileLock() 
function is implemented.

That is, this proposal would put the DLL in a mode where it called 
LockFileEx() to establish the reader/writer lock byte ranges instead of 
using its private advisory locking scheme.

Alternately, Cygwin could follow Linux and implement 'mount -o mand'. 
You could get mandatory locking on just your svn checkout tree this way, 
for example.  It would apply to all Cygwin programs, not just 
svn/SQLite, but it shouldn't be any more problematic than normal day to 
day Windows use.  Cygwin programs not run within that mount wouldn't be 
affected.

I'm aware that fcntl(F_SETLK) isn't thread-safe[1] but we're arguing "if 
it's good enough for Unix, it's good enough for Cygwin" here, aren't we? 
 The other objection in reference [1] is that it doesn't work with NFS, 
which doesn't matter to us here, I think.

If neither of those proposals will work, I think we'll have no choice 
but to continue to try and chase a SQLite specific solution.  You can't 
fix it on the Windows native side of things.  I doubt you can fix it in 
Subversion, and even if you could, that's no better than fixing it in 
SQLite.  Worse, actually, because then you've got a fix that affects 
only one program, not all SQLite dependents.


[1] http://0pointer.de/blog/projects/locking.html

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