Windows: hardlink support is broken on shared drives

[Matt Mackall]

On Tue, 2010-08-17 at 19:49 +0200, Patrick Mézard wrote:
> Le 17/08/10 16:32, Matt Mackall a écrit :
> > See this thread:
> > 
> > http://mercurial.markmail.org/thread/4hvungefkgmq3cum
> > 
> > And this bug:
> > 
> > http://mercurial.selenic.com/bts/issue761
> > 
> > If we remotely commit to a repo that has hardlinks because it was
> > locally cloned, we get a mess in the clone. So we either need to:
> > 
> > a) figure out how to reliably get an accurate link count over the wire
> > 
> > or 
> > 
> > b) figure out how to detect we're on a network share and assume
> > -everything- is a linked file when committing (slow!)
> > 
> > 
> > Even something as drastic as disabling hardlink support is insufficient
> > because there will still be repos with hardlinks on many users' systems.
> > 
> > I've upped the priority on this bug to critical - we've gotten several
> > reports of it that we've failed to properly identify the cause of.
> > 
> > The interesting code bits are here:
> > 
> > http://www.selenic.com/hg/file/0b84864d1325/mercurial/win32.py#l23
> > 
> > http://www.selenic.com/hg/file/0b84864d1325/mercurial/win32.py#l53
> 
> And the interesting quote from:
> 
>     http://msdn.microsoft.com/en-us/library/aa364952%28VS.85%29.aspx
> 
> "Depending on the underlying network features of the operating system
> and the type of server connected to, the GetFileInformationByHandle
> function may fail, return partial information, or full information for
> the given file."

So that gives us three cases:

a) works - return real nNumberOfLinks
b) partial - pretend there are 2 to force link breaking
c) fails - pretend there are 2 to force link breaking

The trick is to distinguish case (a) from (b). If nNumberOfLinks > 1,
then we can assume it worked. But we'll probably have to look at other
fields in the structure to sanity-check it in other cases. But we're
going to need people with Windows (ie not me) to do some testing with
different setups to figure out if we can actually reliably detect case
(b).

Step one here is probably to write a test script that people can run to:

a) create a hardlink
b) report the result details of GetFileInformationByHandle

-- 
Mathematics is the supreme nostalgia of our time.