[JacobStephens2]

Summary

macOS shards spend ~33 s building and 990–1220 s running tests (main run 39337614995). Locally, Run tests are bound by spawning processes, and ~82% of the spawns are git commands from Worktree ownership inspection. That inspection runs at least twice around every ordinary Worktree operation, and spawning costs more on macOS.

 Checkout::inspect
   verify_repository                       # launch: rev-parse --git-common-dir
   registrations                           # worktree list --porcelain -z
-  rev-parse --verify HEAD
-  rev-parse --show-toplevel
-  rev-parse --absolute-git-dir
-  rev-parse --git-common-dir
+  Git::checkout_location                  # one rev-parse for all four
+    if output is not four lines (a path holds a newline)
+      read each value with its own rev-parse, as before
   rev-parse --symbolic-full-name HEAD
   for-each-ref refs/heads/<branch>
   verify_repository

The comparisons are unchanged. Each value is trimmed as Git::run trims.

Evidence

• Before: spec_run a_ticket_needs_only_a_trusted_author_not_a_ready_label_of_its_own ran 5,415 programs (strace) in 19.6 s (12.3 s system time) on x86_64 Linux. One ordinary Worktree::head() ran 32 git processes.
  After: 4,221 programs (22% fewer) in ~17.2 s. One head() runs 23 git processes; an_ordinary_operation_inspects_ownership_in_few_git_processes fails at 32 and passes at 23.

• The Spec review found a regression in the first commit: a newline in the path. Before the fix, a_checkout_path_with_a_newline_passes_ownership_inspection failed (test result: FAILED. 0 passed; 1 failed). It passes now. a_checkout_location_reads_a_linked_worktree_whatever_its_path checks paths with a newline and with a trailing space against the separate observations.

• Full suite: cargo nextest run gives 2012 passed, 0 failed. cargo clippy --all-targets -- -D warnings and cargo fmt --check are clean.

• macOS, test-step seconds per shard:

  Run	Shards 1 / 2 / 3 / 4	Sum	Slowest
  main 39337614995	1107 / 1120 / 969 / 1191	4387	1191
  main 39328254506	1193 / 937 / 1174 / 972	4276	1193
  this PR 39351810586	709 / 916 / 1089 / 905	3619	1089

  That is about 16% less macOS test time in total and about 9% off the slowest shard, from one run. Shard times vary by ±15% between runs.

Merge Danger

Door: two-way

A pure revert restores the four separate calls. Nothing is persisted.

Blast Radius: Worktree-ownership

Every Run's worktree inspection goes through Git::checkout_location. A wrong parse would refuse the Run's worktree operations (fail closed); it cannot authorize a different checkout, because every value is still compared with the pinned identity.

Unaddressed findings

Standards

• Keeping the PR mergeable: merge the Base branch and conflict Repair #8 (rationale repeated in three places): partly addressed. The ownership.rs comment left with the parsing. The src/git.rs doc comment, the test's doc comment and docs/ci-performance.md each keep one sentence for their own reader.
• Pre-flight checks #4, The Failed run path #5, Continuation of an existing Issue branch #6: notes, not findings. The reviewer called them not a smell, correct behaviour, and not a violation. The Failed run path #5 is superseded by the newline fallback.

Spec

• Walking skeleton: a fresh run on the happy path #3 (the larger --git-common-dir share is untouched): declined; filed as Decide whether Worktree ownership inspection may recheck the launch repository less often #858. Those calls are the launch-side repository rechecks (Directory::verify_repository, src/worktree/ownership.rs) before and after the Git observations, under ADR 0014's pinned-ownership model. Removing or caching them changes what inspection checks. That is a security design decision for the Day shift.
• Pre-flight checks #4 (no macOS evidence): not a code defect. This PR's own CI run supplies it; see Evidence.

Security review outcome

No unaddressed introduced findings.

Review coverage

The reviewers' reports cover commit b729ef0 and read every file it changed. The fixes in 2dcb944 came after the review. The changed file src/git/execution_tests.rs was left unread by both axes. Standards also did not read the new CheckoutLocation hunk in src/git.rs or the test appended to src/worktree/acquisition_tests.rs.

Closes #846

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