view tests/test-rebase-empty-successor.t @ 47120:7109a38830c9

dirstate-tree: Fold "tracked descendants" counter update in main walk

For the purpose of implementing `has_tracked_dir` (which means "has tracked
descendants) without an expensive sub-tree traversal, we maintaing a counter
of tracked descendants on each "directory" node of the tree-shaped dirstate.

Before this changeset, mutating or inserting a node at a given path would
involve:

* Walking the tree from root through ancestors to find the node or the spot
  where to insert it
* Looking at the previous node if any to decide what counter update is needed
* Performing any node mutation
* Walking the tree *again* to update counters in ancestor nodes

When profiling `hg status` on a large repo, this second walk takes times
while loading a the dirstate from disk.

It turns out we have enough information to decide before he first tree walk
what counter update is needed. This changeset merges the two walks, gaining
~10% of the total time for `hg update` (in the same hyperfine benchmark as
the previous changeset).

---

Profiling was done by compiling with this `.cargo/config`:

    [profile.release]
    debug = true

then running with:

    py-spy record -r 500 -n -o /tmp/hg.json --format speedscope -- \
    ./hg status -R $REPO --config experimental.dirstate-tree.in-memory=1

then visualizing the recorded JSON file in https://www.speedscope.app/

Differential Revision: https://phab.mercurial-scm.org/D10554

  $ cat << EOF >> $HGRCPATH
  > [extensions]
  > rebase=
  > [alias]
  > tglog = log -G -T "{rev} '{desc}'\n"
  > EOF

  $ hg init

  $ echo a > a; hg add a; hg ci -m a
  $ echo b > b; hg add b; hg ci -m b1
  $ hg up 0 -q
  $ echo b > b; hg add b; hg ci -m b2 -q

  $ hg tglog
  @  2 'b2'
  |
  | o  1 'b1'
  |/
  o  0 'a'
  

With rewrite.empty-successor=skip, b2 is skipped because it would become empty.

  $ hg rebase -s 2 -d 1 --config rewrite.empty-successor=skip --dry-run
  starting dry-run rebase; repository will not be changed
  rebasing 2:6e2aad5e0f3c tip "b2"
  note: not rebasing 2:6e2aad5e0f3c tip "b2", its destination already has all its changes
  dry-run rebase completed successfully; run without -n/--dry-run to perform this rebase

With rewrite.empty-successor=keep, b2 will be recreated although it became empty.

  $ hg rebase -s 2 -d 1 --config rewrite.empty-successor=keep
  rebasing 2:6e2aad5e0f3c tip "b2"
  note: created empty successor for 2:6e2aad5e0f3c tip "b2", its destination already has all its changes
  saved backup bundle to $TESTTMP/.hg/strip-backup/6e2aad5e0f3c-7d7c8801-rebase.hg

  $ hg tglog
  @  2 'b2'
  |
  o  1 'b1'
  |
  o  0 'a'