[llvm-dev] [RFC] One or many git repositories?

[via llvm-dev]

Mehdi Amini <mehdi.amini at apple.com> writes:

>     It could be problematic for us depending on how the monorepository
>     is structured. We reference the LLVM git repository directly and
>     use it to migrate to new versions, pick patches, etc. If LLVM
>     proper were part of a larger repository that becomes more
>     difficult to do because the commit file paths won't match. We'd be
>     back to essentially manual diff+patch which is quite a step
>     backward from the smoth git-oriented process we use now.
>
> Can you clarify what you mean? Which part of the process would quite
> manual patch that wouldn’t otherwise?

If the monorepository is not using submodules but is instead a weaving
of the histories of each component, that means each tree item pointing
to a blob will have a different path.  For example,
lib/Target/X86/X86InstrInfo.cpp would become
llvm/lib/Target/X86/X86InstrInfo.cpp or something similar.  IME git
doesn't deal well with applying changes to blobs that exist in different
paths in the repository.  That makes sense since the hashes directly
depend on the information in the trees.

>     The document says that the individual git repositories will
>     remain.  Does that mean the monorepository is using git-submodule
>     to manage the aggregate repository?
>
> First, have you read this document: https://reviews.llvm.org/D24167 ?

Yes, though I was only able to figure out how to see an actual document
by clicking "download raw diff."  I'm not sure that's giving me the
latest version.  Is there another convenient way to view the document,
preferable with the Markdown rendered?

It's not completely clear to me how the monorepository would be created,
and thus, how it would be structured.  I understand each component gets
its own subdirectory.  I'm talking about how the underlying history is
represented.

> TLDR: The answer is no: you have to see it as it is today, i.e. a
> single SVN repo containing all the sub-projects, and “exports” in
> individual repositories.

So the SVN version isn't using externals?  I haven't ever looked at that
repository.  I didn't even know it existed until reading the document.

> The same thing after: a single git repo containing all the subprojects
> side-by-side and the *same* “exports” in individual repositories.

How are those exports managed?  Do you use a tool to filter the history
for a directory in the monorepository and then export that to its own
repository?

>     I completely understand the benefits of a monorepository. One of
>     the biggest for us was the ability to git-bisect across
>     components.  How does git-bisect work with submodules? I have very
>     little experience with submodules but would like to learn more.
>     
>
> Fairly easy, the document mentions it in the examples.

Ok, I probably skimmed that part since it wasn't directly related to
describing how the repository would be structured.  I'll go back and
read it in more detail.

Thanks!

                              -David