Re: newbie questions about git design and features (some wrt hg)

From: Junio C Hamano <junkio@cox.net>
Date: Jan 31, 2007, 20:01 UTC
Message-ID: <7vabzzvud0.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <eppshi$1l4$1@sea.gmane.org>

Jakub Narebski <jnareb@gmail.com> writes:

Show 10 quoted lines

> Theodore Tso wrote:
>> 
>> To be fair hg modifies files using O_APPEND only.  That isn't quite as
>> safe as "only creating new files", but it is relatively safe.
>
> From (libc.info):
>  -- Macro: int O_APPEND
>  ...
> I don't quote understand how that would help hg (Mercurial) to have
> operations like commit, pull/fetch or push atomic, i.e. all or nothing.

If I remember correctly, thanks to their log-like file format, they can rely on O_APPEND to do the right thing when growing, and aborting the current transaction is just a truncate away (or a set of truncates on the files appended in the transaction, if hg touches more than one log-like file but I do not know if hg uses only one file or more than one). That's one of the things I found clean and beautiful (from theoretical point of view, at least) in their design. I do not think O_APPEND is not used to control concurrent operations.