hgext/git/TODO.md

branchmap-v3: introduce a "stop_rev" argument to `headsrevs`

The `headsrevs` method of the revlog already have a `revs` argument to compute
the headrevs of a limited set of heads. However, it disable the use of the
native compiled code to compute the heads, which slows down the branchmap v3
code a lot.

The branchmap v3 usage is actually quite constrained as we will always only
ignores a part at the top of the graph. So we could be significantly faster.

We start by making small change to the python side to improve the situation and
introduce the new API. More collaboration with the native code are coming later.

This massively speedup operation and close most of the remaining gaps between
branchmap-v3 and branchmap-v2. especially on repository with many revs like
mozilla-try. A small overhead remains mostly because the `headrevs` logic
currently has some inefficiently. We will look into them from there.

### benchmark.name                                 = hg.command.unbundle
  # bin-env-vars.hg.py-re2-module                  = default
  # benchmark.variants.issue6528                   = disabled
  # benchmark.variants.resource-usage              = default
  # benchmark.variants.reuse-external-delta-parent = yes
  # benchmark.variants.revs                        = any-1-extra-rev
  # benchmark.variants.source                      = unbundle
  # benchmark.variants.validate                    = default
  # benchmark.variants.verbosity                   = quiet

 ## data-env-vars.name                             = netbeans-2018-08-01-zstd-sparse-revlog
  # bin-env-vars.hg.flavor                         = default
branch-v2:        0.233711  ~~~~~
branch-v3 before: 0.368769  (+57.79%, +0.14)
branch-v3 after:  0.239857   (+2.63%, +0.01)
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        0.235230  ~~~~~
branch-v3 before: 0.372460  (+58.34%, +0.14)
branch-v3 after:  0.240972   (+2.44%, +0.01)
 ## data-env-vars.name                             = netbeans-2018-08-01-ds2-pnm
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        0.255586  ~~~~~
branch-v3 before: 0.318907  (+24.78%, +0.06)
branch-v3 after:  0.268560   (+5.08%, +0.01)

 ## data-env-vars.name                             = mozilla-central-2024-03-22-zstd-sparse-revlog
  # bin-env-vars.hg.flavor                         = default
branch-v2:        0.339010  ~~~~~
branch-v3 before: 0.349752   (+3.17%, +0.01)
branch-v3 after:  0.349389   (+3.06%, +0.01)
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        0.346525  ~~~~~
branch-v3 before: 0.354300   (+2.24%, +0.01)
branch-v3 after:  0.355661   (+2.64%, +0.01)
 ## data-env-vars.name                             = mozilla-central-2024-03-22-ds2-pnm
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        0.380202  ~~~~~
branch-v3 before: 0.396293   (+4.23%, +0.02)
branch-v3 after:  0.408851   (+7.54%, +0.03)

 ## data-env-vars.name                             = mozilla-unified-2024-03-22-zstd-sparse-revlog
  # bin-env-vars.hg.flavor                         = default
branch-v2:        0.412165  ~~~~~
branch-v3 before: 0.424769   (+3.06%, +0.01)
branch-v3 after:  0.427782   (+3.79%, +0.02)
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        0.412397  ~~~~~
branch-v3 before: 0.421796   (+2.28%, +0.01)
branch-v3 after:  0.422354   (+2.41%, +0.01)
 ## data-env-vars.name                             = mozilla-unified-2024-03-22-ds2-pnm
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        0.429501  ~~~~~
branch-v3 before: 0.443849   (+3.34%, +0.01)
branch-v3 after:  0.443197   (+3.19%, +0.01)

 ## data-env-vars.name                             = mozilla-try-2024-03-26-zstd-sparse-revlog
  # bin-env-vars.hg.flavor                         = default
branch-v2:        3.403171  ~~~~~
branch-v3 before: 6.234055  (+83.18%, +2.83)
branch-v3 after:  3.819477  (+12.23%, +0.42)
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        3.454876  ~~~~~
branch-v3 before: 6.307813  (+82.58%, +2.85)
branch-v3 after:  3.590284   (+3.92%, +0.14)
 ## data-env-vars.name                             = mozilla-try-2024-03-26-ds2-pnm
  # bin-env-vars.hg.flavor                         = rust
branch-v2:        3.465435  ~~~~~
branch-v3 before: 5.176076  (+49.36%, +1.71)
branch-v3 after:  3.633278   (+4.84%, +0.17)

Octopus Merge Support
=====================

This will be moderately complicated, as we'll need to synthesize phony
changeset entries to explode the octopus into "revisions" that only
have two parents each. For today, we can probably just do something like

    aaaaaaaaaaaaaaaaaaXX{20 bytes of exploded node's hex sha}

where XX is a counter (so we could have as many as 255 parents in a
git commit - more than I think we'd ever see.) That means that we can
install some check in this extension to disallow checking out or
otherwise interacting with the `aaaaaaaaaaaaaaaaaa` revisions.


Interface Creation
====================

We at least need an interface definition for `changelog` in core that
this extension can satisfy, and again for `basicstore`.


Reason About Locking
====================

We should spend some time thinking hard about locking, especially on
.git/index etc. We're probably adequately locking the _git_
repository, but may not have enough locking correctness in places
where hg does locking that git isn't aware of (notably the working
copy, which I believe Git does not lock.)