bug: serialize mixed testing.invoke calls around cwd changes

[codeforester]

Goal

Prevent a testing.invoke(cwd=None) call from running inside another invocation's temporary process-wide working directory.

Background

#11 serialized pairs where both invocations specify cwd. The current lock is acquired only in the cwd is not None branch. Because os.chdir() is process-global, a concurrent no-cwd invocation bypasses the lock and observes the directory temporarily selected by the other call.

Reproduction and evidence

Framework reference: 263d893 on main.

Patch CliRunner.invoke to hold the first call open, start invoke(app, cwd=tempdir), then concurrently start invoke(app).

original=/Users/rameshhp/work/base-cli
observed=<tempdir>,<tempdir>

The no-cwd call should have observed the caller's original directory, not the other test's fixture directory. Existing coverage only starts two calls that both pass cwd.

Scope

• Make all helper invocations coordinate with any process-wide cwd mutation.
• Preserve parallelism only where it cannot observe or overlap a temporary os.chdir().
• Restore cwd on every success/failure path.
• Document the mixed-call concurrency contract.
• Add a deterministic mixed cwd/no-cwd regression test.

Acceptance criteria

• A no-cwd invocation cannot enter while another helper call has changed process cwd.
• Mixed calls observe their intended directories.
• The original process cwd is restored after exceptions and normal exits.
• Existing same-cwd and invocation-parity tests pass.

Project fields

• Status: Backlog
• Priority: P2
• Area: Python
• Initiative: v1.0 Readiness
• Size: S

Ownership

• Assignee: @codeforester