Re: [PATCH] fast-import: Allow filemodify to set the root

[Johannes Sixt <j.sixt@viscovery.net>]

Am 10/8/2010 9:05, schrieb Jonathan Nieder:
> Johannes Sixt wrote:
>> Am 10/7/2010 22:28, schrieb Jonathan Nieder:
> 
>>> | For a command (like filter-branch --subdirectory-filter) that wants
>>> | to commit a lot of trees that already exist in the object db, writing
>>> | undeltified objects as loose files only to repack them later can
>>> | involve a significant amount[*] of overhead.
>>
>> 1. But when an object already exists in the db, it won't be written again,
>> will it?
> 
> In David's application, the trees already exist, but the commits are new.

But then what has this to do with "allow filemodify to set the root"?

> I suppose supporting M 040000 <tree> "" and C <path> "" could still
> be a good idea in that case anyway, for the convenience of front-end
> authors.

What is the special new thing here? That "" means 'empty string' == 'tree
at the root'? If so:

1. Then this is the missing piece in the justification. Then I could buy
that the observed speed-up is due to the reuse of an existing tree object
(which avoids parsing it and re-constructing it from its pieces because
fast-imports syntax didn't allow it otherwise). But it has nothing to do
with new loose objects (the re-constructed object would be identical to an
existing one).

2. Without this patch, would this syntax create a tree object with a name
consisting of two double-quotes in the root? Or would it be a syntax
error? How would one construct such an entry with this patch?

-- Hannes