fix(memory): re-read and merge on save_to_disk to prevent concurrent data loss

[michielhdoteth]

What does this PR do?

Fixes silent MEMORY.md data loss when two Hermes instances (or a Hermes instance + any external writer — daemon, cron, script) share one profile and write concurrently. save_to_disk() now re-reads the file under the already-held lock from #1726 and merges on-disk entries with the session's pending entries before writing. Last-writer-wins on content is eliminated; no entry from any writer is silently dropped.

Closes #85858.

Related Issue

Fixes #85858

Type of Change

• 🐛 Bug fix (non-breaking change that fixes an issue)
• ✅ Tests (adding or improving test coverage)

Changes Made

• tools/memory_tool.py:387 — save_to_disk() now re-reads the file under the already-held _file_lock and merges on-disk entries with the session's pending entries before writing. New _merge_disk_with_pending() method implements the merge (union-where-session-wins, deduplicated). +58 lines.
• tests/tools/test_memory_concurrent_85858.py — new file. 6 tests reproducing the concurrent-snapshot-staleness bug and verifying the fix:
  • test_concurrent_adds_both_survive — direct reproduction of MEMORY.md lost under concurrent Hermes instances sharing one profile (frozen-snapshot + full-file rewrite = last-writer-wins) #85858
  • test_concurrent_add_replace_both_survive — replacement entry survives stale save
  • test_concurrent_adds_no_duplicate — dedup on merge
  • test_external_append_preserved — daemon append without lock preserved
  • test_external_single_entry_preserved — external full-file replace preserved
  • test_multiple_external_entries_preserved — multiple external appends preserved
• tests/tools/test_memory_tool.py — updated test_mutations_read_the_file_exactly_once docstring/assertions to distinguish the drift-detection single-read invariant (still fixed) from the new intentional merge-read in save_to_disk. -1/+19 lines.

Total: 3 files, +290/-6 lines, 47 tests pass, 0 regressions.

How to Test

1. Clone the PR branch: gh pr checkout 96195
2. Run the full test suite: pytest tests/ -q — all tests pass
3. Run the new concurrent tests specifically: pytest tests/tools/test_memory_concurrent_85858.py -v — 6/6 pass
4. Reproduction of the original bug (before fix): the test test_concurrent_adds_both_survive encodes the exact reproduction from MEMORY.md lost under concurrent Hermes instances sharing one profile (frozen-snapshot + full-file rewrite = last-writer-wins) #85858 — session A adds X, session B adds Y, session A saves from stale snapshot. Before the fix this test fails (Y is lost); after the fix it passes (both X and Y survive).

Checklist

Code

• I've read the Contributing Guide
• My commit messages follow Conventional Commits (fix(memory):, etc.)
• I searched for existing PRs to make sure this isn't a duplicate
• My PR contains only changes related to this fix (no unrelated commits)
• I've run pytest tests/ -q and all tests pass
• I've added tests for my changes (required for bug fixes)
• I've tested on my platform: Windows 11

Documentation & Housekeeping

• N/A — no tool behavior change that needs doc update; the memory tool's contract ("Mid-session writes update files on disk immediately (durable)") is now actually true under concurrency, which the docstring at tools/memory_tool.py:11-14 already claims.
• N/A — no new config keys, no CLI changes
• N/A — CONTRIBUTING.md and AGENTS.md unchanged
• N/A — cross-platform: fix is Windows + Unix compatible (uses the existing _file_lock from fix(memory): concurrent writes silently drop entries — add file locking #1726 and stdlib dict.fromkeys)
• N/A — tool description/schema unchanged

Screenshots / Logs

N/A — behavior change, not UI. Test output demonstrates the fix:

tests/tools/test_memory_concurrent_85858.py::TestConcurrentSnapshotStaleness::test_concurrent_adds_both_survive PASSED
tests/tools/test_memory_concurrent_85858.py::TestConcurrentSnapshotStaleness::test_concurrent_add_replace_both_survive PASSED
tests/tools/test_memory_concurrent_85858.py::TestConcurrentSnapshotStaleness::test_concurrent_adds_no_duplicate PASSED
tests/tools/test_memory_concurrent_85858.py::TestExternalWriterPreservation::test_external_append_preserved PASSED
tests/tools/test_memory_concurrent_85858.py::TestExternalWriterPreservation::test_external_single_entry_preserved PASSED
tests/tools/test_memory_concurrent_85858.py::TestExternalWriterPreservation::test_multiple_external_entries_preserved PASSED