fix: concurrent callers each spawned their own git rev-parse

[pftg]

Follow-up to #250, found while auditing parallel test execution.

#250 caches git rev-parse --show-toplevel per root, removing ~200 process spawns from a 200-screenshot suite. Under threads it removed none.

MRI releases the GVL for the duration of Open3.capture3, so concurrent callers all miss key? before any of them writes. Measured on master, 8 threads asking about one root:

git spawns for ONE root, 8 concurrent callers: 8

That is the cost the cache exists to remove, still fully paid — in parallelize(with: :threads), which is the default parallel mode on JRuby. JRuby also has no GVL, so the unsynchronized Hash#[]= is unsafe there as well as wasteful.

The fix synchronizes the lookup rather than the write, so the spawn happens once. Holding the lock across the subprocess is deliberate: callers almost always share a single root, so the remaining threads wait once and then read the cache.

After: git spawns for ONE root, 8 concurrent callers: 1

Guard: test/unit/vcs_test.rb drives 8 concurrent checkout_vcs calls through a stubbed capture3 that sleeps (standing in for the GVL-releasing subprocess) and asserts exactly one lookup. Mutation-checked — replacing the mutex with a lock that merely yields reds the test; restored, green.

588 unit runs / 0 failures, standardrb clean.

Summary by Sourcery

Synchronize Git-root caching so concurrent VCS operations share a single repository lookup.

Bug Fixes:

• Prevent concurrent callers from spawning duplicate git rev-parse processes when resolving the same repository root.
• Make cached Git-root lookups safe for parallel execution, including runtimes without a global VM lock.

Tests:

• Add coverage verifying that eight concurrent VCS lookups perform exactly one Git lookup.

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📥 Commits

Reviewing files that changed from the base of the PR and between d5b7a9e and e347e74.

📒 Files selected for processing (2)

• lib/snap_diff/vcs.rb
• test/unit/vcs_test.rb

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Reviewer's Guide

Synchronizes git root caching in SnapDiff::Vcs so concurrent thread callers share a single git rev-parse invocation, and adds a unit test that exercises and mutation-checks the concurrent behavior via a stubbed capture3.

Sequence diagram for synchronized git root lookup

sequenceDiagram
    participant T1 as Thread 1
    participant T2 as Thread 2
    participant Vcs as SnapDiff::Vcs
    participant Git as Open3.capture3

    par Concurrent callers
        T1->>Vcs: git_root_for(root_path)
        T2->>Vcs: git_root_for(root_path)
    end
    Vcs->>Vcs: @git_roots_lock.synchronize
    Vcs->>Git: Open3.capture3(git, -C, root_path, rev-parse, --show-toplevel)
    Git-->>Vcs: git_root, status
    Vcs->>Vcs: @git_roots[root_path] = status.success? && git_root.chomp
    Vcs-->>T1: cached git root
    Vcs-->>T2: cached git root
    Vcs->>Vcs: @git_roots_lock.synchronize
    Vcs-->>Vcs: @git_roots.key?(root_path)

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Flow diagram for concurrent git root cache behavior

flowchart TD
    A[Concurrent git_root_for calls] --> B{Cache lookup under mutex}
    B -->|miss| C[Open3.capture3 git rev-parse]
    C --> D[Store result in @git_roots]
    D --> E[Return cached git root]
    B -->|hit| E

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File-Level Changes

Change	Details	Files
Make git root lookup and caching thread-safe so concurrent callers only spawn one git rev-parse subprocess and share the cached result.	Introduce a class-level Mutex to guard access to the git root cache Wrap git_root_for cache lookup and potential git rev-parse call in a synchronized block Return cached git root immediately when present within the synchronized section, otherwise perform git rev-parse and write the result back into the cache	lib/snap_diff/vcs.rb
Add a concurrency-focused unit test that verifies multiple threads share a single git root lookup via the cache.	Create a new test that spawns multiple threads calling checkout_vcs against the same root directory Stub Open3.capture3 with a counting function that sleeps to simulate GVL-releasing subprocess behavior Use a Mutex-protected counter to track how many times the git rev-parse stub is invoked and assert it is exactly one	test/unit/vcs_test.rb

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