perf(parser): skip O(n) frozenset in get_all_sessions on cache hit (#859)

[microsasa]

Closes #859

Problem

Every call to get_all_sessions unconditionally built a frozenset of all (Path, file_id) pairs — O(n) allocations and O(n) hash operations — before checking _sorted_sessions_cache. When the discovery cache hits (is_cache_hit=True) and no sessions were re-parsed or had name updates (deferred_sessions=[]), this work was entirely wasted since the sorted cache is guaranteed valid.

Fix

Added a cheap early-return check using the already-available is_cache_hit flag before the frozenset construction:

if is_cache_hit and not deferred_sessions and _sorted_sessions_cache is not None:
    return list(_sorted_sessions_cache.summaries)

The existing frozenset path is preserved for cache-miss cases (root changed or sessions re-parsed), so _sorted_sessions_cache is still updated correctly on misses.

Tests

Added TestSortedSessionsCacheFastPath with two tests:

• test_frozenset_not_built_on_warm_cache_hit — uses a counting wrapper around _SortedSessionsCache to verify it is NOT constructed on the second call when the session state is unchanged (spy/counter approach, no wall-clock assertions).
• test_frozenset_built_when_session_added — verifies the frozenset path IS still taken when the session set changes (cache miss).

All 1238 tests pass with 99% coverage.

Generated by Issue Implementer · ● 11.2M · ◷

[Copilot]

Pull request overview

This PR targets a steady-state performance issue in copilot_usage.parser.get_all_sessions() by avoiding redundant O(n) work when internal discovery/session caches indicate nothing has changed.

Changes:

• Adds an early-return fast path in get_all_sessions() to skip building the (Path, file_id) frozenset fingerprint on warm discovery-cache hits.
• Adds new tests intended to verify the fast path behavior and that the fingerprint path still runs when the session set changes.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
src/copilot_usage/parser.py	Adds a new early-return fast path before fingerprint construction.
tests/copilot_usage/test_parser.py	Adds tests for the warm-cache fast path and cache-miss behavior.

[github-actions]

Commit pushed: 4edd9f4

Generated by CI Fixer

[microsasa]

CI Fix Applied

Root cause: The fast-path early return in get_all_sessions checked is_cache_hit and _sorted_sessions_cache is not None but didn't verify the cache belonged to the current base_path. When e2e tests used different paths in succession (e.g. test_empty_directory_graceful with a temp dir → FIXTURES-based tests), the fast path returned stale empty results from the wrong root, causing 20 e2e test failures ("No sessions found").

Fix: Added a root field to _SortedSessionsCache and guarded the fast path with _sorted_sessions_cache.root == resolved_root. Also hoisted resolved_root before the stale-prune block to eliminate a duplicate resolve() call.

All checks pass: ruff ✅ | pyright ✅ | bandit ✅ | unit tests (99% cov) ✅ | e2e tests (86 passed) ✅

Generated by CI Fixer · ● 16.5M · ◷

[microsasa]

CI fix already attempted once — stopping to prevent loops. Manual intervention needed.

Generated by CI Fixer · ● 420.7K · ◷

[github-actions]

Commit pushed: 505ddce

Generated by Review Responder

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

[github-actions]

Commit pushed: d98a738

Generated by Review Responder

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

[github-actions]

Commit pushed: fd8293b

Generated by Review Responder

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

[github-actions]

Commit pushed: 543b17f

Generated by Review Responder

[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

[github-actions]

Quality Gate: APPROVED (auto-merge eligible)

Impact: MEDIUM — Internal performance refactoring of private cache logic (_SortedSessionsCache, _discover_with_identity) with unchanged public API.

What was evaluated:

• Optimization logic: The 5-condition early-return guard (is_cache_hit, not deferred_sessions, len(summaries) == len(discovered), cache exists, same root) is sound — when all hold, the sorted order cannot have changed, so the O(n) frozenset construction is correctly skipped.
• Return type change: _discover_with_identity now returns the resolved root as a 3-tuple element. All 30+ existing call sites updated correctly.
• _build_fingerprint extraction: Clean refactor — the inline frozenset comprehension is now a named, testable function.
• _SortedSessionsCache.root field: Added correctly to the dataclass(slots=True) and threaded through construction.
• Tests: Two new tests (test_fingerprint_skipped_on_warm_cache_hit, test_fingerprint_built_when_session_added) use spy/counter approach consistent with repo conventions — no wall-clock timing.
• CI: All 9 checks pass (lint, typecheck, security, tests, CodeQL).