feat(mock-worker): --capture writes each Generate request as a JSON line

[hello-alexmcc]

Description

Problem

mock-worker serves the TokenSpeed scheduler service over gRPC, and the gateway detects that runtime on its own. So the mock can stand in for the engine behind a real smg when replaying recorded fixtures (bellwether). But its Generate handler reads only request_id and drops the rest of the request. Nothing records what the gateway actually put on the wire: the prompt ids and text, the sampling values after defaults, the stop fields, or the constraint.

Solution

--capture PATH appends every Generate request to PATH as one JSON line.

• The line is written before generate returns its stream, so it is in the file before the gateway can read a frame, and killing the worker loses no line.
• The mapping is written by hand, because the prost types do not derive Serialize and SamplingParams holds a prost_types::Struct.
• A capture path that cannot be opened fails at startup with exit code 2.
• Responses are unchanged, and without the flag nothing changes.

This is the first of three mock-worker changes for gateway replay tests. --script (token chunks scripted per request_id) and --default-sampling-params come next and join the same ReplayConfig, so the six Config struct literals change only once.

Changes

• crates/mock_worker/src/config.rs:
  • Config.replay: ReplayConfig (Default), holding capture: Option<PathBuf>;
  • a --capture PATH arm and one usage() line;
  • from_args() now delegates to a private parse(args), so the flag is unit-tested.
• crates/mock_worker/src/replay.rs (new): Capture, an append-mode Mutex<File> that writes one line per request with one write_all under the lock. The JSON mapping:
  • request_id, input_ids, original_text, stream;
  • every SamplingParams scalar, and logit_bias as an object. Unset optionals are null. Each f32 is the shortest decimal that reads back as the same f32 (0.7, not 0.699999988079071); NaN and infinities are strings, since null means unset;
  • constraint: {"kind": "regex" | "json_schema" | "ebnf_grammar" | "structural_tag", "value": <the string as sent>}, or null;
  • return_logprob, logprob_start_len, top_logprobs_num, token_ids_logprob;
  • presence flags has_custom_params, has_mm_inputs, has_encode_bootstrap_info, has_kv_bootstrap_info, has_data_parallel_rank.
• crates/mock_worker/src/grpc.rs: MockScheduler holds Option<Arc<Capture>>, opened in serve_with_listener (signature unchanged). generate records the request first, then runs exactly as before. A failed write fails that request with INTERNAL. Called directly, serve_with_listener logs and returns when the file cannot be opened.
• crates/mock_worker/src/main.rs: after parsing flags, opens the capture file once. On failure it prints mock-worker: cannot open capture file <path>: <err> to stderr and exits with code 2 before any worker starts. Otherwise a worker that could not open it would stop while the process kept running and served no gRPC.
• crates/mock_worker/src/lib.rs: pub mod replay.
• crates/mock_worker/README.md: a "Capturing requests" section.
• The six Config struct literals gain replay: Default::default():
  • model_gateway/tests/{grpc_context_length_test,tenant_rate_limiting_grpc_test,grpc_pd_fanout_test,zmq_backend_test}.rs;
  • model_gateway/src/routers/grpc/regular/streaming/eof_tests.rs;
  • the mock's own ZMQ test.

Test Plan

crates/mock_worker/tests/capture.rs serves the real TokenSpeed service with serve_with_listener on port 0 and drives it with the gateway's TokenSpeedSchedulerClient:

• writes_one_line_per_request_in_order: three requests give three lines, in order, with exact input_ids (empty, 0, u32::MAX), original_text (a newline, quotes, a backslash, a tab, CJK) and stream.
• unset_optionals_are_null_and_set_values_come_back_as_sent: whole-line equality for a request with nothing set and one with everything set (set zeros, logprob_start_len: 0, a u64::MAX seed, 0.7).
• constraint_is_captured_with_its_kind_and_value: all four oneof arms; values kept byte for byte.
• line_is_on_disk_before_the_first_frame: the file is read after generate() returns and before the first frame. The realistic engine's 200 ms decode step holds that frame back; a canned-mode version passed 20 of 20 runs against an implementation that writes after the last frame, because the server drains canned frames at once.
• canned_output_is_unchanged_with_capture_on: frames equal those of a worker without capture, streamed and not.
• unopenable_capture_file_stops_the_worker: serve_with_listener, given a path in a missing directory, returns instead of serving.
• unopenable_capture_file_fails_at_startup: the built binary, run with --grpc-count 1 --grpc-base-port <free port> --capture <path in a missing directory>, exits with code 2 within 10 s, names the path on stderr, and leaves nothing listening on the port.
• Unit: config::tests::capture_flag_sets_the_capture_path, replay::tests::non_finite_floats_are_not_captured_as_unset.

Each test was seen failing before its implementation: unknown flag; no capture file; missing keys; 0.699999988079071 != 0.7; constraint absent; null != "NaN"; the binary still running after 10 s. Mutations confirm that the ordering, unchanged-output and open-failure tests each catch their own regression.

cargo +nightly fmt --all -- --check                                  # exit 0
cargo +1.98.0 clippy -p mock-worker --all-targets -- -D warnings     # exit 0
cargo +1.98.0 clippy -p smg --all-targets -- -D warnings             # exit 0
cargo +1.98.0 test -p mock-worker                                    # 10 + 7 passed
cargo +1.98.0 check -p smg --tests                                   # exit 0
cargo +1.98.0 test -p smg --test grpc_context_length_test            # 17 passed
cargo +1.98.0 test -p smg --test tenant_rate_limiting_grpc_test      # 14 passed
cargo +1.98.0 test -p smg --test grpc_pd_fanout_test                 # 13 passed
cargo +1.98.0 test -p smg --test zmq_backend_test                    # 13 passed
cargo +1.98.0 test -p smg --lib eof_tests                            # 12 passed

End to end. A debug build of smg ran in front of this branch's mock-worker --capture, with the model's local tokenizer and HF_HUB_OFFLINE=1, everything on 127.0.0.1. bellwether's verify (smg-project/bellwether#48) sent every committed render fixture with its id as rid and compared the captured input_ids with the reference.

Model	Cases	match	differs	rejected	missing
Qwen/Qwen3-8B	24	20	2	2	0
deepseek-ai/DeepSeek-R1	23	16	2	5	0

Every case got its capture line, so the rid reached the engine as request_id and the line was written before the answer each time. The cases that differ or are rejected are known gateway issues: #2779, #2780, #2222 (still reproducing on main) and #2783. The one exception is a call whose arguments are an object, which the gateway refuses as vLLM does. A sample line:

{"request_id":"capture-demo-1","input_ids":[151644,872,198,45764,15588,304,825,3409,13,151645,198,151644,77091,198],"original_text":"<|im_start|>user\nSay hi in one word.<|im_end|>\n<|im_start|>assistant\n","stream":false,"temperature":0.7,"top_p":0.95,"top_k":-1,"min_p":0.0,"repetition_penalty":1.0,"frequency_penalty":0.0,"presence_penalty":0.0,"max_new_tokens":64,"min_new_tokens":0,"stop":["\n\n"],"stop_token_ids":[],"ignore_eos":false,"no_stop_trim":false,"skip_special_tokens":true,"spaces_between_special_tokens":true,"n":1,"sampling_seed":null,"logit_bias":{},"constraint":null,"return_logprob":false,"logprob_start_len":-1,"top_logprobs_num":0,"token_ids_logprob":[],"has_custom_params":false,"has_mm_inputs":false,"has_encode_bootstrap_info":false,"has_kv_bootstrap_info":false,"has_data_parallel_rank":false}

Checklist

• cargo +nightly fmt passes
• cargo clippy -p mock-worker --all-targets -- -D warnings and cargo clippy -p smg --all-targets -- -D warnings pass (not run with --all-features)
• (Optional) Documentation updated
• (Optional) Please join us on Slack #sig-smg to discuss, review, and merge PRs

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📝 Summary

Summary by CodeRabbit

• New Features
  • Added optional request capture for mock gRPC workers. Use --capture <path> to save each Generate request as a JSONL record with request and sampling details.
  • Records are written before the first response frame, and capture does not change responses. Capture is unavailable for HTTP and ZMQ workers.
  • New capture files use restrictive permissions; existing files retain their permissions.
• Bug Fixes
  • If the capture file cannot be opened at startup, the worker stops before serving requests and the binary exits with status 2.

Walkthrough

The mock worker adds an optional --capture path that records each gRPC Generate request as a JSONL line before response generation. Startup stops if the path cannot be opened. Configuration, serialization, tests, and documentation are updated.

Changes

gRPC Request Capture

Layer / File(s)	Summary
Capture configuration and JSONL serialization crates/mock_worker/src/config.rs, crates/mock_worker/src/replay.rs, crates/mock_worker/src/lib.rs	Configuration adds an optional capture path and the --capture flag. Capture appends serialized request records, including constraint variants and representations for non-finite floats.
Startup validation and request recording crates/mock_worker/src/main.rs, crates/mock_worker/src/grpc.rs	The executable checks the capture path before starting workers. The gRPC scheduler records each request before response generation and returns an internal status if recording fails.
Capture coverage and worker configuration crates/mock_worker/tests/capture.rs, crates/mock_worker/src/zmq.rs, model_gateway/src/routers/grpc/regular/streaming/eof_tests.rs, model_gateway/tests/*, crates/mock_worker/README.md	Integration tests cover captured fields, order, response behavior, and capture failures. Existing mock worker configurations initialize replay settings, and the README documents capture behavior.

Priority: ⬇️ Low

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

Change: Feature

Sequence Diagram(s)

sequenceDiagram
  participant Main as mock_worker main
  participant Grpc as gRPC service
  participant Capture
  participant File as JSONL file
  participant Client
  Main->>Capture: open configured path before worker startup
  Client->>Grpc: send Generate request
  Grpc->>Capture: record request
  Capture->>File: append JSON line
  Grpc-->>Client: return response stream

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Merge Risk: 🟡 Moderate · up to b6bea

With capture enabled, slow file writes can delay requests. Move recording off the async executor before merging, unless that latency risk is explicitly accepted.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly describes the main change: mock-worker captures each Generate request as a JSON line.
Description check	✅ Passed	The description explains the capture feature, its behavior, implementation, and tests. It is directly related to the changeset.
Docstring Coverage	✅ Passed	Docstring coverage is 88.24% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 34 functions across 12 files. (1 skipped: 1…
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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🤖 Prompt to fix review comments

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Inline comments:
Review comments at @crates/mock_worker/src/grpc.rs:
- Around line 105-106: Offload the synchronous `capture.record(&req)` call in
the `generate` method to a Tokio blocking task and await its result before
returning the stream. Preserve the existing `INTERNAL` response when recording
fails.

Review comments at @crates/mock_worker/src/replay.rs:
- Line 23: Update the OpenOptions call that opens the capture file to create it
with mode 0o600 on Unix, and also restrict permissions on an existing capture
file before writing original_text; creation mode alone does not change existing
file permissions.

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📒 Files selected for processing (13)

• crates/mock_worker/README.md
• crates/mock_worker/src/config.rs
• crates/mock_worker/src/grpc.rs
• crates/mock_worker/src/lib.rs
• crates/mock_worker/src/main.rs
• crates/mock_worker/src/replay.rs
• crates/mock_worker/src/zmq.rs
• crates/mock_worker/tests/capture.rs
• model_gateway/src/routers/grpc/regular/streaming/eof_tests.rs
• model_gateway/tests/grpc_context_length_test.rs
• model_gateway/tests/grpc_pd_fanout_test.rs
• model_gateway/tests/tenant_rate_limiting_grpc_test.rs
• model_gateway/tests/zmq_backend_test.rs

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[slin1237]

Code-owner review at b6bea46: approve. Sections 3, 4, 7, 8, 9 and 10 of the checklist were worked through (the mock worker is a gRPC and ZMQ transport path), and an independent reader built, tested and ran both heads end to end; what I state below I reran or read myself at b6bea46.

What I ran

• cargo +nightly fmt --all -- --check 0; cargo clippy -p mock-worker --all-targets -- -D warnings 0; cargo test -p mock-worker 0 (11 unit, 8 in tests/capture.rs). The full --all-features clippy is blocked on this machine by llm-multimodal (opencv); CI's pre-commit ran it green on fce9aa2.
• The reader ran the five gateway suites the one-line struct-literal additions touch (grpc_context_length_test 17, grpc_pd_fanout_test 13, tenant_rate_limiting_grpc_test 14, zmq_backend_test 13, eof_tests 12, all passing), and a debug smg --enable-igw in front of mock-worker --capture with the local Qwen3-8B tokenizer: 6 requests, 6 lines, 32 keys in one order, request_id == rid on every line; temperature, top_p, stop, seed, logprobs, n, stream, a json_schema constraint and a raw /v1/completions prompt all captured as sent; a request rejected at parsing produced no line.

Checklist

• Worker lifecycle and dispatch (3, 4): nothing in the handshake, registration or selection path changes; the capture is per listener (grpc.rs:49-67), opened before serving and recorded before the first frame goes out (grpc.rs:105), so a consumer that reads the file after the response has the line. request_id is written verbatim, and the gateway passes a client rid through unchanged outside prefill-decode (helpers.rs:175-183), which is what bellwether #48 joins on.
• Error handling (7): no unwrap or expect in production code; a capture that cannot be opened ends the process with exit 2 and mock-worker: cannot open capture file <path>: <err> (tested through the built binary); a failed write answers INTERNAL, which the gateway turns into a 500 with start_generation_failed, the right outcome for a tool whose whole point is the file.
• The mapping: all 12 GenerateRequest fields and all 21 SamplingParams fields, plus the four constraint arms, are covered (reader, against the proto); the hand-written JSON is justified, since the prost types do not derive Serialize and serde_json::Value::from(f32) would widen 0.7 to 0.699999988079071; every line is valid JSON for every input (proto strings are UTF-8, control characters escaped, NaN and infinity as strings); logit_bias keys sorted at aa6389d keep lines byte-stable.
• Testing (8): full flow through gRPC, the before-first-frame ordering, the open failure, the file mode, byte-identical lines; the existing struct literals carry the new field. ZMQ is out of scope and the README says so.
• Code quality (9): three conventional commits, each with the DCO sign-off and no AI trailer; no generated-with footer; #[expect] where a lint is silenced; tracing for runtime errors.

Nits

• 🟡 Nit (crates/mock_worker/src/replay.rs:42-51): no test sends concurrent requests, and none shares one file between two listeners (--grpc-count N opens one Capture per listener, so the mutex does not span them; what keeps lines whole across them is one write per line on an append-mode file). The reader's 96-request probe shows that two mutants, writing a line in two calls with or without the lock, pass every test here. A concurrent test, and the comment naming the single-write invariant, would hold it; a follow-up is fine.
• 🟡 Nit (replay.rs:111): data_parallel_rank is an optional int32 written as has_data_parallel_rank, while the other optional scalars are written as null or their value. A consumer that routes by rank cannot read it back.
• 🟡 Nit: --capture with --grpc-count 0 is accepted, creates the file and captures nothing; a startup warning would save a bellwether user a puzzled minute.
• 🟡 Nit (README.md): the prefill-decode suffix is mentioned; {rid}-p{i} for array-prompt completions (helpers.rs:231-236) is not.

Nothing blocks; the maintainer or Chang merges.