Re: First cut at git port to Cygwin

From: Daniel Barkalow <barkalow@iabervon.org>
Date: Oct 10, 2005, 20:27 UTC
Message-ID: <Pine.LNX.4.63.0510101620290.23242@iabervon.org>
In-Reply-To: <Pine.LNX.4.63.0510102100010.7688@wbgn013.biozentrum.uni-wuerzburg.de>

On Mon, 10 Oct 2005, Johannes Schindelin wrote:

Show 15 quoted lines

> Hi,
> 
> On Mon, 10 Oct 2005, H. Peter Anvin wrote:
> 
> > Junio C Hamano wrote:
> > > 
> > > PROT_WRITE is true, but we do MAP_PRIVATE, and if I recall
> > > correctly we do not write file via mmap -- at least we do not
> > > intend to.
> > > 
> > 
> > Then PROT_READ probably makes more sense?
> 
> Not necessarily. Sometimes you need to annotate the data from the index, 
> and this does not need to be written back to the index file.

In fact, it is intentional that we open the file O_RDONLY, and mmap it PROT_READ | PROT_WRITE, MAP_PRIVATE. We prepare the next index in the memory where we mapped the old index, but we don't want to change what's on the disk using the mapping; we write that later to a different file using write().

Show 9 quoted lines

> > > Yes.  It might have been overkill that you supported writing
> > > changes back, though.
> > 
> > Not just overkill; if we do MAP_PRIVATE it's actively WRONG.
> 
> See above.
> 
> BTW, is there a mechanism to make sure that the index file is locked 
> between reading and writing?

There's definitely locking; the new file is written to "(filename).lock", which is openned O_CREAT | O_EXCL, and is moved to the destination when it's complete. I believe everything that intends to write a new index gets the lock before reading the old index, although I haven't actually checked.

	-Daniel
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