A Spinel statement-cache miss no longer scans every cached statement

[namespaceMarcello]

What

On Spinel, DbConn#prepare_cached (runtime/spinel/db.rb) found a statement by scanning @entries from the tail, and new SQL is cached without a cap until trim! runs at lease end. SQL that inlines its values misses on nearly every read, so a lease that prepares K distinct statements did about K²/2 string comparisons.

That is what made the sidebar in #496 slow. With RH_SQL_TRACE=1, one /users/me/sidebar request for a user with 10,000 direct rooms prepares 30,011 statements, all different: three per room (the memberships.room_id = N AND NOT users.id = N existence check, the same count, and the user row). It also replays 30,021 reads from the request's query cache.

This change adds @entry_by_sql, a Hash from SQL to its cached Stmt, so a miss costs one Hash lookup. A hit still scans from the tail and moves the entry there, so recency and trim! work as before. trim!, discard_closed and finalize_all remove the key whenever they drop the entry.

How it relates to the open bind series: #403 moves @entries.push into prepare_owned, so whichever PR lands second has to carry the @entry_by_sql[sql] = entry line along. I measured ROUNDHOUSE_PARAM_BINDS=1 on main (not the #403–#405 branches). The same request still prepares 30,010 distinct statements with their values written into the SQL, because these reads go through the Relation. Only the user lookup switches to a bound users WHERE id = ?. The first request stays at 14.4 s (table below).

Repro and test

• Repro: the SQL in Campfire on Spinel: the sidebar takes 0.46 s with 2,000 direct rooms and 8 s with 10,000, and repeat requests are no faster #496 (user 1 with 10,000 direct rooms) against the Campfire archive, then one cold GET /users/me/sidebar: restart the binary, sign in, one request.
• Test: tests/spinel_stmt_cache_lru.rb gains a misses case, run by a_miss_does_not_scan_the_cache. It prepares 12,000 distinct statements in one lease and times the last 500 misses against the first 500; it fails when the last block takes more than 5× the first plus 2 ms. On main: first 500 took 3.2 ms, last 500 took 48.6 ms, and it raised a miss scans the cache. With the change: 2.0 ms and 1.4 ms. The case also checks that a statement dropped by trim! is prepared and cached again.

Verification

Run locally, with Spinel master f3da0151f:

• SPINEL=… cargo test --test spinel_stmt_cache_lru -- --ignored: 4 passed with the change. On main the new case fails and the other three pass.
• SPINEL=… cargo test --test param_binds --test spinel_db_lease --test db_sqlite_concurrency -- --ignored: 5 passed (bind_runtime_spinel, raw_where_substitution_spinel, varying_binds_spinel, a_request_that_raises_releases_its_connection, spinel_shim_policy).
• The Ruby floor jobs, replicated in a Linux container: build, store-check, unit (4,135 passed, 152 ignored), compare-ruby, the Campfire suite (405 tests, 69 files), campfire-compare and db-differential on the Ruby target, and clippy on the changed files. All green.
• Campfire at the CI pin (90b3300), built by scripts/build-campfire-archive from main (607bb24f) and from this branch, with the same Spinel. Each binary came from its own docker.tgz and ran on 4 CPUs (AMD Ryzen 9 7940HX, Docker Desktop on Windows 11), one at a time, each against its own copy of the same database.

/users/me/sidebar with 10,000 direct rooms, on the #496 repro database:

	main	this PR	main, ROUNDHOUSE_PARAM_BINDS=1
cold first request, 3 runs	14,544 / 14,932 / 14,409 ms	1,543 / 1,486 / 1,581 ms	14,430 / 14,407 / 14,171 ms
repeat requests, median of 11	609 ms	563 ms	563 ms

All three serve the same 7,367,670-byte page.

On #496's larger database (2M messages, 10,008 users; median of 11, first request in parentheses):

	main	this PR
sidebar, 10,000 direct rooms	1,190 ms (38,544)	1,247 ms (2,203)
sidebar, 2,000 direct rooms	88 ms (1,334)	79 ms (263)
POST /rooms/directs for a user with 10,000 direct rooms	3,248 ms	730 ms
post to a room with 10,003 members	77 ms	46 ms

POST /rooms/directs is faster on every request, not just the first, because finding the existing direct room runs one statement per room. Traced on the repro database for the last of the 10,000 rooms: 10,031 statements, 10,026 of them different.

About #496 itself: on main, repeat requests are already around 0.6 s, while in the published archive (0ac82f5) they took 8 s. I haven't looked for what changed between the two. What #496 still showed on main was the first request after a restart, or after fragments go stale, and this change fixes that.

Not run: hosted CI; the Spinel and Campfire jobs (build-spinel, toolchain-spinel, framework-tests-spinel, compare-spinel, campfire-compare), except the Spinel tests listed above.

Fixes #496

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Written by Claude Code (AI assistant) on behalf of @namespaceMarcello, who directs this work.

Summary by CodeRabbit

• Performance
  • Prepared-statement cache lookups are faster, particularly when the cache contains many distinct statements.
  • Preparing new statements remains responsive as the number of cached statements grows, including during extended runs with many distinct queries.
• Reliability
  • Cache entries are maintained consistently as the cache is trimmed and connections are released.
  • Statements removed during trimming can be prepared and cached again when needed.

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📝 Walkthrough

Walkthrough

DbConn now indexes cached statements by SQL text. The new miss-performance probe checks caching during a connection lease and after entries are trimmed.

Changes

Statement cache lookup

Layer / File(s)	Summary
SQL lookup index and lifecycle runtime/spinel/db.rb	DbConn uses an SQL map to avoid scanning cached entries for a matching statement. It updates the map when statements are added, discarded, trimmed, or cleared.
Miss performance and cache retention checks tests/spinel_stmt_cache_lru.rb, tests/spinel_stmt_cache_lru.rs	The probe prepares 12,000 distinct statements, times batches of cache misses, and checks cache retention and re-caching after trimming. An ignored integration test runs the probe.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: bunnykong

Merge Risk: ⚪ Minimal · up to f53d9

The change adds indexed statement-cache lookup, and the inspected ownership and cleanup paths do not show a remaining merge-blocking risk.

Security Architecture Review

Security architecture risk: ⚪ Minimal · up to f53d9

The change improves statement-cache lookup without expanding database access or changing connection ownership. The inspected insertion, eviction, and cleanup paths maintain the new index consistently; no introduced security concern was identified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The changed state is confined to the already-selected connection's statement cache. The inspected change does not add cross-connection lookup, broaden tenant routing, or grant additional database authority.

Trust Boundaries and Controls

• inferred — SQL reaching Db.prepare follows the same connection-selection and preparation path as before. The index changes lookup cost, not SQL authorization or parameterization. The existing non-leased fallback connection behavior predates this PR and is not expanded by it.

Resilience and Maintainability Implications

• inferred — Under the existing single-holder lease invariant, inspected failure and recovery paths preserve statement containment: failed cached statements become closed, closed entries are unindexed, and lease cleanup releases abandoned statements before trimming and returning the connection. No introduced stale or cross-owned reuse path was identified.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 63.64% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: statement-cache misses now avoid scanning every cached statement.
Linked Issues check	✅ Passed	The change addresses the coding objective in [#496]. DbConn adds an SQL-to-Stmt index, uses it for cache lookup, and keeps LRU recency updates. trim!, closed-statement cleanup, and finalization …
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to runtime/spinel/db.rb and the Spinel statement-cache tests. The index maintenance supports the [#496] cache objective. The new Ruby probe and Rust integration test…

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Inline comments:
Review comments at @runtime/spinel/db.rb:
- Around line 531-535: Update Db.prepare’s Stmt.new call to pass a copy of the
SQL string, so later caller mutations cannot change Stmt#sql or prevent trim!
from removing the matching @entry_by_sql key.

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