[PATCH 0/3] git for-each-ref: is-base atom and base branches

This change introduces a new 'git for-each-ref' atom, 'is-base', in a very similar way to the 'ahead-behind' atom. As detailed carefully in the first change, this is motivated by the need to detect the concept of a "base branch" in a repository with multiple long-lived branches.

This change is motivated by a third-party tool created to make this detection with the same optimization mechanism, but using a much slower technique due to the limitations of the Git CLI not presenting this information. The existing algorithm involves using git rev-list --first-parent -<N> in batches for the collection of considered references, comparing those lists, and increasing <N> as needed until finding a collision. This new use of 'git for-each-ref' will allow determining this mechanism within a single process and walking a minimal number of commits.

There are benefits to users both on client-side and server-side. In an internal monorepo, this base branch detection algorithm is used to determine a long-lived branch based on the HEAD commit, mapping to a group within the organizational structure of the repository, which determines a set of projects that the user will likely need to build; this leads to automatically selecting an initial sparse-checkout definition based on the build dependencies required. An upcoming feature in Azure Repos will use this algorithm to automatically create a pull request against the correct target branch, reducing user pain from needing to select a different branch after a large commit diff is rendered against the default branch. This atom unlocks that ability for Git hosting services that use Git in their backend.

Thanks, -Stolee

Derrick Stolee (3):
  commit-reach: add get_branch_base_for_tip
  for-each-ref: add 'is-base' token
  p1500: add is-base performance tests

 commit-reach.c              | 118 ++++++++++++++++++++++++++++++++++++
 commit-reach.h              |  17 ++++++
 ref-filter.c                |  78 +++++++++++++++++++++++-
 ref-filter.h                |  15 +++++
 t/helper/test-reach.c       |   2 +
 t/perf/p1500-graph-walks.sh |  31 ++++++++++
 t/t6600-test-reach.sh       |  94 ++++++++++++++++++++++++++++
 7 files changed, 354 insertions(+), 1 deletion(-)

base-commit: bea9ecd24b0c3bf06cab4a851694fe09e7e51408
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1768%2Fderrickstolee%2Ftarget-ref-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1768/derrickstolee/target-ref-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/1768