comparison tests/test-git-import.t @ 14452:ee574cfd0c32

patch: use temporary files to handle intermediate copies

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.

14451:c78d41db6f88

  > @@ -1,1 +1,2 @@
  > a
  > +b
  > EOF
  applying patch from stdin
  abort: cannot create b: destination already exists
  [255]
  $ cat b
  b
  $ cd ..

14452:ee574cfd0c32

  > @@ -1,1 +1,2 @@
  > a
  > +b
  > EOF
  applying patch from stdin
  cannot create b: destination already exists
  1 out of 1 hunks FAILED -- saving rejects to file b.rej
  abort: patch failed to apply
  [255]
  $ cat b
  b
  $ cd ..