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

[Junio C Hamano <junkio@cox.net>]

Jakub Narebski <jnareb@gmail.com> writes:

> 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.