[PATCH 0/2] worktree: copy-on-write creation and shared-branch worktrees

["Jason Newton via GitGitGadget" <gitgitgadget@gmail.com>]

When many worktrees share one repository -- e .g. a fleet of agents each
needing an isolated checkout -- "git worktree add" is costly at scale.
Objects are shared via the common dir, but the working tree is not: each add
rewrites every tracked file, so N worktrees cost N full checkouts of disk
and I/O. And a branch can only be checked out in one worktree.

Patch 1 adds "git worktree add --reflink": on a copy-on-write filesystem it
populates the new worktree by reflinking the current worktree's files and
index, then "git reset --hard" rewrites only the paths that differ from . A
reflink_file() helper in copy.c uses FICLONE (Linux) and clonefile()
(macOS); elsewhere (other filesystems, Windows) it is probed up front and
falls back to a normal checkout. Defaulting is via the worktree.reflink
config (true/false/auto); --no-reflink overrides.

Patch 2 lets a branch be checked out in several worktrees, for parallel work
on one checkout. A branch mid-rebase or mid-bisect elsewhere is still
refused.

Benchmark (Linux-kernel fork, 93k files, ~33 GB tree incl. build output,
btrfs): a normal add allocates ~0.9 GB of real disk per worktree (~5.3 GB
for four, linear); --reflink allocates ~0 at any count and also carries the
untracked build tree. ("Real disk" = btrfs exclusive bytes.)

worktree-reflink-bench
[https://github.com/user-attachments/assets/e3e721c8-2206-4b78-ad08-21677ef30753]

Note: patch 2 changes a default (same-branch checkout now allowed); two
t2400 assertions were updated accordingly.

Jason Newton (2):
  worktree: add --reflink for copy-on-write worktree creation
  worktree: allow sharing a checked-out branch across worktrees

 Documentation/config/worktree.adoc |  10 ++
 Documentation/git-worktree.adoc    |  47 +++++-
 builtin/worktree.c                 | 257 ++++++++++++++++++++++++++++-
 copy.c                             |  65 ++++++++
 copy.h                             |  13 ++
 t/t2400-worktree-add.sh            | 119 ++++++++++++-
 6 files changed, 493 insertions(+), 18 deletions(-)


base-commit: c69baaf57ba26cf117c2b6793802877f19738b0d
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-2317%2Fnevion%2Fworktree-reflink-cow-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-2317/nevion/worktree-reflink-cow-v1
Pull-Request: https://github.com/git/git/pull/2317
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