Re: fast-import fails in read-only tree

[Stefan Monnier <monnier@iro.umontreal.ca>]

>> I recently discovered that "git fast-import" signals an error if used in
>> a tree to which we do not have write-access, because it tries to create
>> a "objects/pack/tmp_pack_XXX" file even before starting to process
>> the commands.
> The primary goal of fast-import is to write that packfile.  It kind of
> sounds like you are using the wrong tool for the job.

Yes, I realize that.  But in some cases it's the best tool available.
`fast-import' is very close to being a "generic access API" which can be
used instead of something like libgit.  I think it'd be good to push it
yet a bit closer.

My earlier "cat-blob applied to a tree" issue is another such case.

> Can you elaborate on what you are sending to fast-import (preferably
> with a concrete example)?

I'm sending a stream of "progress <foo>; cat-blob <foo>", basically.

The concrete example is in [BuGit](https://gitlab.com/monnier/bugit),
see for example https://gitlab.com/monnier/bugit/commit/3678dcb8830a9c79c6f3404d75d63e6dd07bfe4c

> There may be a way to accomplish the same thing with read-only tools
> like cat-file.

Yes, I switched to using "cat-file --batch" instead, but it's less
convenient (I can't intersperse ad-hoc info in the output, the way I can
with "progress" in fast-import) and there are cases where the list of
files I need to extract cannot be determined without first looking at
some of those extracted files (I currently have been able to avoid
this in BuGit, luckily).

If I could use "cat-blob" on directories, there would be even more cases
where I'd want to use fast-import for read-only operations to reduce the
number of Git processes I fork.


        Stefan