git patches may require copies to be handled out-of-order. For instance, take
the following sequence:
* modify a
* copy a into b
Here, we have to generate b from a before its modification. To do so,
applydiff() was scanning for copy metadata and performing the copies before
processing the other changes in-order. While smart and efficient, this approach
complicates things by handling file copies and file creations at different
places and times. While a new file must not exist before being patched a copied
file already exists before applying the first hunk.
Instead of copying the files at their final destination before patching, we
store them in a temporary file location and retrieve them when patching. The
filestore always stores file content in real files but nothing prevents adding
a cache layer. The filestore class was kept separate from fsbackend for at
least two reasons:
- This class is likely to be reused as a temporary result store for a future
repository patching call (entries just have to be extended to contain copy
sources).
- Delegating this role to backends might be more efficient in a repository
backend case: the source files are already available in the repository itself
and do not need to be copied again. It also means that third-parties backend
would have to implement two other methods. If we ever decide to merge the
filestore feature into backend, a minimalistic approach would be to compose
with filestore directly. Keep in mind this copy overhead only applies for
copy/rename sources, and may even be reduced to copy sources which have to
handled ahead of time.
