feat(mcp): bounded row consumer for component_mcp run_query (Phase 5) by askdba · Pull Request #257 · askdba/myvector

askdba · 2026-11-02T09:53:55Z

First Phase 5 item from #256: bounded row consumer.

Problem

The command service's default (DOM) consumer collects every row while a statement runs. sql_select_limit only bounds statements without their own LIMIT, so run_query with SELECT … LIMIT 1000000 or with wide rows buffered the whole result in mysqld before the reply was cut (CodeRabbit finding on #253).

Change

component_mcp provides its own mysql_text_consumer_row_factory_v1 and get_* services (….mcp) and selects them on each run_as() handle.
They pass rows on to the default ….mysql_server implementations (required with REQUIRES_SERVICE_IMPLEMENTATION_AS). So store_result / fetch_row work as before, until the statement has produced max_rows + 1 (1001) rows or 4 MiB of values.
After that, rows are dropped as they arrive. A row that crosses the byte budget is aborted in the DOM consumer, so it is never half stored. The result is then reported as truncated.
The budget lives in a thread_local, because the statement runs on the calling thread. Without a budget, the services pass everything on.
The statement still runs to its end. mcp.max_execution_time_ms bounds its time when set.

Tests

scripts/test-mcp-endpoint.sh on mysql:26.10.0: 37 passed, 0 failed (was 30).

Check	Without this PR	With it
peak RSS (VmHWM) growth, 100 rows × 1 MB	+61 MiB (FAIL)	+0 MiB
peak RSS growth, `LIMIT 1000000`	+144 MiB (FAIL)	+0 MiB

Also new:

the replies are bounded and marked truncated;
a NULL-only result isn't cut;
an error raised after some rows, or past the bound, is reported as an error rather than a partial result (this exercises abort_row).

Docs: docs/MCP_COMPONENTS.md (replaces the "known spike limit" note) and CHANGELOG.

Part of #256.

🤖 Generated with Claude Code

Summary by CodeRabbit

New Features
- Query results are limited to 1,001 rows or 4 MiB of estimated row and value data, including per-row and per-cell overhead. Additional rows are omitted, and the response indicates when results are truncated. Rows that exceed the remaining byte limit are not partially included.
- Queries continue to completion unless an execution-time limit is configured. Errors are reported rather than returning partial results.
Documentation
- Updated the MCP component guide with details about result limits, truncation, and execution behavior.

run_query sessions select the component's own text consumer row and value services (mysql_text_consumer_*_v1.mcp). They pass rows on to the server's default (DOM) consumer until the statement has produced max_rows + 1 rows or 4 MiB of values, then drop later rows as they arrive; a row that crosses the byte budget is aborted in the DOM consumer. A large explicit LIMIT or wide rows no longer buffer the whole result in mysqld. test-mcp-endpoint.sh: peak RSS checks for LIMIT 1000000 (+0 MiB, was +144) and 100 x 1 MB rows (+0 MiB, was +61), NULL-only rows, and errors raised after some rows / past the bound. 37 checks. Part of #256 (RFC 0001 Phase 5). Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

chatgpt-codex-connector · 2026-11-02T09:54:10Z

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📝 Code Review	✅ Completed 2026-11-02T09:56:38.104060Z	`144d884`	PR opened

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coderabbitai · 2026-11-02T09:54:13Z

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scripts/test-mcp-endpoint.sh
src/mcp/endpoint/component_mcp.cc

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Walkthrough

component_mcp now bounds retained run_query results by row count and value bytes. It records truncation when rows are dropped. Endpoint checks cover wide rows, large result sets, NULL-heavy results, memory use, and query errors.

Changes: Bounded query results

Layer / File(s)	Summary
Bounded consumers and DOM delegation `src/mcp/endpoint/component_mcp.cc`	The component adds row and value consumers that delegate accepted values to the server’s default DOM consumers. The consumers account for row and cell overhead, retain at most 1,001 rows or 4 MiB of values, and do not retain a row that crosses the byte limit.
Query integration `src/mcp/endpoint/component_mcp.cc`	`run_as` configures the bounded consumers before connecting. It executes SQL through a fresh budget scope and records whether rows were dropped. The component registers the consumer services and requires the default DOM services.
Endpoint validation and documentation `scripts/test-mcp-endpoint.sh`, `docs/MCP_COMPONENTS.md`, `CHANGELOG.md`	Endpoint checks cover truncation, peak RSS, NULL-heavy results, `DOUBLE` formatting, and errors before or beyond the row bound. Documentation and the changelog describe the limits and test coverage.

Estimated code review effort: 3 (Moderate) | ~25 minutes

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant run_as
  participant bounded_query
  participant bounded_consumers
  participant default_DOM_consumers
  run_as->>bounded_query: Execute SQL with a fresh row budget
  bounded_query->>bounded_consumers: Consume query rows and values
  bounded_consumers->>default_DOM_consumers: Forward values accepted by the budget
  bounded_consumers-->>bounded_query: Report whether rows were dropped
  bounded_query-->>run_as: Return query status

Possibly related issues

component_mcp Phase 5: production hardening of the endpoint (RFC 0001) #256: Requires bounded row retention and a byte budget for large LIMIT values and wide rows, which this change implements.

Possibly related PRs

askdba/myvector#253: Adds the run_query path that this change extends with bounded consumers and endpoint checks.
askdba/myvector#252: Changes guard classification for component-created local sessions, including sessions opened through component_mcp’s run_as.

Merge Risk: Low · up to b557f

Result limits are in place. Confirm DOUBLE accounting and the NULL-memory regression check on the target server as follow-up.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documenta…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title names the feature this PR adds: a bounded row consumer for component_mcp run_query. The changeset matches it. component_mcp.cc installs a bounded MySQL text consumer that keeps at most 1,001…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

Full details: Title check

Explanation

The title names the feature this PR adds: a bounded row consumer for component_mcp run_query. The changeset matches it. component_mcp.cc installs a bounded MySQL text consumer that keeps at most 1,001 rows and 4 MiB of values, and the changelog, docs, and endpoint test script all describe that behavior. The title's "(Phase 5)" tag is project-specific context and does not hurt the match.

Full details: Docstring Coverage

Explanation

Docstring coverage is 60.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 5 functions across 2 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches

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_{A rabbit counts the rows with care,
And weighs each value in the air.
When budgets fill, no row slips through,
The query runs; the count stays true.
Short doubles hop, long ones stay whole,
Then errors reach the waiting console.}

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chatgpt-codex-connector · 2026-11-02T09:56:35Z

+}
+
+DEFINE_BOOL_METHOD(bounded_get_null, (SRV_CTX_H ctx)) {
+  return skip_value(0) ? false : dom_get_null->get(ctx);


Count NULL cells in the retention budget

When run_query returns a wide result made mostly of NULLs, skip_value(0) never advances the byte counter, so the consumer can retain all 1001 rows regardless of the 4 MiB limit. The default consumer still needs per-column row structure, so a sufficiently wide projection can use substantial memory during query execution despite this bound; account for row/cell structure or impose a separate width limit.

Useful? React with 👍 / 👎.

Fixed in 810ed90: the budget now counts a pointer per cell (NULL cells included) and the per-row list node and end pointer the DOM consumer keeps, so a wide all-NULL result is bounded by the same 4 MiB. New check: 2000 NULL columns over 5000 rows comes back bounded and truncated.

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