fix: streaming callback returns non-opt StreamingCallbackHttpResponse#96
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The HTTP gateway decodes the streaming-callback reply via ic-agent into a non-optional StreamingCallbackHttpResponse. We returned `opt ...` — which the written HTTP Gateway Protocol spec specifies, but which the gateway does not accept: candid cannot coerce wire-type `opt T` into the expected `T`, so the reply failed to decode and every multi-chunk asset served HTTP 503. Single-chunk assets skip the callback, so no existing test caught it. Drop the Option/opt wrapper from the callback return type in the method, the candid interface, and the StreamingStrategy callback func type. Add an e2e test that round-trips a ~5 MB (3-chunk) asset through the local gateway, plus canister-core unit tests covering the multi-chunk streaming paths (3-chunk continuation token, non-identity encoding, single-chunk no-strategy, and the callback's unknown-key / unknown-encoding errors). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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## Summary The streaming callback in the HTTP gateway protocol spec must return a non-opt `StreamingCallbackHttpResponse`. The spec incorrectly wrapped the return type in `opt`, which no real gateway accepts — candid cannot coerce wire-type `opt T` into the expected `T`, so serving any multi-chunk (streamed) asset fails to decode. Fixed all three occurrences: - `public/references/http-gateway.did` (downloadable interface) - `docs/references/http-gateway-protocol-spec.md` — full Canister HTTP Interface block - `docs/references/http-gateway-protocol-spec.md` — Response Body Streaming Interface block ```diff - callback: func (StreamingToken) -> (opt StreamingCallbackHttpResponse) query; + callback: func (StreamingToken) -> (StreamingCallbackHttpResponse) query; ``` Note: only the callback's **return type** changes. The `token : opt Token` field inside `StreamingCallbackHttpResponse` stays optional — a null *token* (not a null response) terminates the stream. ## Verification The real gateway decodes the callback reply into a non-opt `StreamingCallbackHttpResponse`: `ic-gateway` → `ic-http-gateway-protocol` (tag `v0.5.1`) → `ic-agent` / `ic-utils` - [`ic-http-gateway-protocol` @ v0.5.1 — `response_handler.rs#L116-L120`](https://github.com/dfinity/ic-http-gateway-protocol/blob/v0.5.1/packages/ic-http-gateway-protocol/src/response/response_handler.rs#L116-L120): calls `http_request_stream_callback` and matches `Ok((StreamingCallbackHttpResponse { body, token },))` — the bare struct, no `Option`. - [`ic-agent` — `ic-utils/src/interfaces/http_request.rs`](https://github.com/dfinity/agent-rs/blob/main/ic-utils/src/interfaces/http_request.rs): `http_request_stream_callback -> SyncCall<Value = (StreamingCallbackHttpResponse,)>`, and the callback CandidType `func((ArgToken) -> (StreamingCallbackHttpResponse) query)`. Detected while adding e2e streaming tests for certified-assets (dfinity/certified-assets#96). `npm run build` passes.
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What
Serving any multi-chunk asset (>
MAX_CHUNK_SIZE, 1.9 MB) through the HTTP gateway returned HTTP 503. The fix:http_request_streaming_callbacknow returns a bareStreamingCallbackHttpResponseinstead ofopt StreamingCallbackHttpResponse.Root cause
The HTTP gateway follows the streaming callback via
ic-agent, which decodes the reply into a non-optional(StreamingCallbackHttpResponse,). We returnedopt ..., and candid cannot coerce wire-typeopt Tinto the expectedT(subtype-incomparable), so the gateway failed withAgentError::CandidError("Candid returned an error") and served 503. The firsthttp_requestresponse decodes fine, so single-chunk assets worked and no existing test exercised the callback decode path.This means
optwas a real bug even though it matches the written HTTP Gateway Protocol spec. The spec text is stale: ic-agent and the SDK reference asset canister's actual wasm both use non-opt (the SDK's hand-written.didsaysoptbut its Rust returns non-opt). A separate docs PR will correct the spec.Changes
Option/optfrom the callback return type in three places: the#[query]method,certified-assets.did, and theStreamingStrategycallback func type (define_function!).crates/e2e/tests/streaming.rs): generates a ~5 MB (3-chunk) incompressible asset on the fly, deploys it, and asserts it round-trips byte-for-byte through the local gateway. A >2 MB body can only return intact via streaming callbacks, and the gateway validates the certificate before responding — so this also proves certified reassembly.Testing
cargo test -p canister-core— 104 passcargo test -p canister candid_interface_compatibility— passcargo test -p e2e— full suite pass (streaming round-trips through the gateway; previously 503)cargo fmt --check,cargo clippy --workspace --all-targets— clean🤖 Generated with Claude Code