[PATCH v3 0/3] batch blob diff generation

Through git-diff(1) it is possible to generate a diff directly between two blobs. This is particularly useful when the pre-image and post-image blobs are known and we only care about the diff between them. Unfortunately, if a user has a batch of known blob pairs to compute diffs for, there is currently not a way to do so via a single Git process.

To enable support for batch diffs of multiple blob pairs, this series introduces a new diff plumbing command git-diff-pairs(1) based on a previous patch series submitted by Peff[1]. This command uses NUL-delimited raw diffs as its source of input to control exactly which filepairs are diffed. The advantage of using the raw diff format is that it already has diff status type and object context information embedded in each line making it more efficient to generate diffs with as we can avoid having to peel revisions to get some the same info.

For example:

    git diff-tree -r -z -M $old $new |
    git diff-pairs -p -z

Here the output of git-diff-tree(1) is fed to git-diff-pairs(1) to generate the same output that would be expected from `git diff-tree -p -M`. While by itself not particularly useful, this means it is possible to split git-diff-tree(1) output across multiple git-diff-pairs(1) processes. Such a feature is useful on the server-side where diffs bewteen a large set of changes may not be feasible all at once due to timeout concerns.

This command can be viewed as a backend tool that exposes Git's diff machinery. In its current form, the frontend that generates the raw diff lines used as input is expected to most of the heavy lifting (ie. pathspec limiting, tree object expansion).

This series is structured as follows:

    - Patch 1 adds some new helper functions to get access to the queued
      `diff_filepair` after `diff_queue()` is invoked.

    - Patch 2 introduces the new git-diff-pairs(1) plumbing command.

    - Patch 3 allows git-diff-pairs(1) to immediately compute diffs
      queued on stdin when a NUL-byte is written after a raw input line
      instead of waiting for stdin to close.

Changes since V2:

    - Pathspecs are not supported and thus rejected when provided as
      arguments. It should be possible in a future series to add support
      though.

    - Tree objects present in `diff-pairs` input are rejected. Support
      for tree objects could be added in the future, but for now they
      are rejected to enable to future support in a backwards compatible
      manner.

    - The -z option is required by git-diff-pairs(1). The NUL-delimited
      raw diff format is the only accepted form of input. Consequently,
      NUL-delimited output is the only option in the `--raw` mode.

    - git-diff-pairs(1) defaults to patch output instead of raw output.
      This better fits the intended usecase of the command.

    - A NUL-byte is now always used as the delimiter between batches of
      file pair diffs when queued diffs are explicitly computed by
      writing a NUL-byte on stdin.

    - Several other small cleanups and fixes along with documentation
      changes.

Changes since V1:

    - Changed from git-diff-blob(1) to git-diff-pairs(1) based on a
      previously submitted series.

    - Instead of each line containing a pair of blob revisions, the raw
      diff format is used as input which already has diff status and
      object context embedded.

-Justin

[1]: <20161201204042.6yslbyrg7l6ghhww@sigill.intra.peff.net>

Justin Tobler (3):
  diff: return diff_filepair from diff queue helpers
  builtin: introduce diff-pairs command
  builtin/diff-pairs: allow explicit diff queue flush

 .gitignore                        |   1 +
 Documentation/git-diff-pairs.adoc |  60 +++++++++
 Documentation/meson.build         |   1 +
 Makefile                          |   1 +
 builtin.h                         |   1 +
 builtin/diff-pairs.c              | 206 ++++++++++++++++++++++++++++++
 command-list.txt                  |   1 +
 diff.c                            |  70 +++++++---
 diff.h                            |  25 ++++
 git.c                             |   1 +
 meson.build                       |   1 +
 t/meson.build                     |   1 +
 t/t4070-diff-pairs.sh             |  83 ++++++++++++
 13 files changed, 432 insertions(+), 20 deletions(-)
 create mode 100644 Documentation/git-diff-pairs.adoc
 create mode 100644 builtin/diff-pairs.c
 create mode 100755 t/t4070-diff-pairs.sh