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wasi-http p3: reused instance leaks handles until it traps when a `content-length` response body is dropped after `content-length` bytes

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Valutazione

Difficoltà
5/5
Tempo stimato
Più di una settimana
Idoneità per principianti
8/100
Tipo di issue
Bug
Chiarezza
Abbastanza chiara
Stato di attività
Attiva
Stack tecnologico
rust, wasm
Ambito
backend, networking

Direzione di ricerca

Start by running ./run.sh in the linked wasi-repro, then read LimitedGuestBodyConsumer and GuestBody::size_hint in src/p3/body.rs, where the host drops a content-length body early. The guest side is http_into_wasi_response and BodyWriter::send_http_body, which spawn a task that may never finish. Done means 300,000 requests succeed on one instance with until-length and 2 requests in flight.

Scritto dal modello di indicizzazione a partire dal testo della issue.

Descrizione

bug

A wasi:http/[email protected] instance that is reused for many requests leaks handles until it traps, if all three of these are true:

  1. the guest's response sets content-length;
  2. the host reads exactly content-length bytes of the response body and then drops the body without polling it to the end. hyper does this when it serves a response with content-length, so wasmtime serve-style embeddings will hit it;
  3. at least 2 requests are in flight on the instance at once.

If any one of these is false, 300,000 requests run fine on one instance.

The trap is raised while the guest creates the next response, so it looks like resources from earlier responses are never released:

wasmtime Fails after Error
50.0.0-rc.1 ~100,000 requests resource table has no free keys, in the guest's Response::new
49.0.3 ~25,400 requests cannot allocate another handle: store handle limit exceeded, in the guest's BodyWriter::new
Test Case

Repro: https://github.com/avkonst/wasi-repro (./run.sh runs the failing case and the three passing ones). In summary:

  • guest: a wasip3::http_compat component with two routes that differ only in whether the response sets content-length.
  • host: one instance in one Store, with up to N calls in flight, each started with handler::Prepared inside Store::run_concurrent. Each response body is either read to the end (full) or read until content-length bytes have arrived and then dropped (until-length, like hyper).
Steps to Reproduce
git clone https://github.com/avkonst/wasi-repro && cd wasi-repro
./run.sh   # rust-toolchain.toml pins rustc 1.99.0 + wasm32-wasip2

# or by hand:
W=guest/target/wasm32-wasip2/release/guest.wasm
H=host/target/release/host

# fails
$H $W /with-content-length    300000 until-length 2 direct
# pass
$H $W /with-content-length    300000 until-length 1 direct
$H $W /with-content-length    300000 full         2 direct
$H $W /without-content-length 300000 until-length 2 direct
Expected Results

All 300,000 requests succeed on one instance, as they do when any one of the three conditions is removed.

Actual Results
instance failed after 99998 requests: error while executing at wasm backtrace:
    0:  0x3ee1d - <unknown>!<wasm function 9>
    1:   0x8ead - guest.wasm!wasip3::service::wasi::http::types::Response::new
    2:   0x53db - guest.wasm!wasip3::http_compat::conversions::http_into_wasi_response
    3:   0x5924 - guest.wasm!wasip3::service::exports::wasi::http::handler::_export_handle_cabi::{{closure}}
    4:  0x112ca - guest.wasm!<futures_util::stream::futures_unordered::FuturesUnordered<Fut> as futures_core::stream::Stream>::poll_next
    5:  0x14a0c - guest.wasm!wit_bindgen::rt::async_support::spawn::Tasks::poll_next
    6:  0x11876 - guest.wasm!wit_bindgen::rt::async_support::TaskState::callback
    7:  0x116a2 - guest.wasm!wit_bindgen::rt::async_support::callback
    8:   0x55bf - guest.wasm!wit_bindgen::rt::async_support::start_task
    9:   0x7e65 - guest.wasm![async-lift]wasi:http/[email protected]#handle

Caused by:
    resource table has no free keys
On 49.0.3 
instance failed after 25394 requests: error while executing at wasm backtrace:
    0:  0x10262 - guest.wasm!wasip3::http_compat::body_writer::BodyWriter::new
    1:   0x538a - guest.wasm!wasip3::http_compat::conversions::http_into_wasi_response
    ...
Caused by:
    cannot allocate another handle: store handle limit exceeded

To build the host against 49.0.3, change =50.0.0-rc.1 to =49.0.3 in host/Cargo.toml, and TaskGroupId to GuestTaskId in host/src/main.rs.

Results (/with-content-length unless noted):

Driver Body In flight 50.0.0-rc.1 49.0.3
direct until-length 1 300,000 ok 300,000 ok
direct until-length 2 traps after 99,998 traps after 25,394
direct until-length 64 traps after 99,968 traps after 25,344
direct full 64 300,000 ok 300,000 ok
direct, /without-content-length until-length 64 300,000 ok 300,000 ok
ProxyHandler (reuse, never expire) until-length 64 6,000,000 ok over 25 instances worker trapped or panicked after 2,263,112 over 27 instances
ProxyHandler (reuse, never expire) full 64 3,000,000 ok over 27 instances 3,000,000 ok over 26 instances

The ProxyHandler only starts a concurrent request on an instance whose tasks are blocked on I/O, so each instance sees less overlap. On 49.0.3 that only delays the failure.

Versions and Environment
  • Wasmtime version or commit: wasmtime, wasmtime-wasi, wasmtime-wasi-http 50.0.0-rc.1 (component-model-async); also 49.0.3
  • Guest: wasip3 0.9.0+wasi-0.3.0 (http-compat), wit-bindgen 0.62.0
  • rustc 1.99.0, target wasm32-wasip2
  • Operating system: Linux
  • Architecture: x86_64
Extra Info
  • With content-length, wasmtime-wasi-http consumes the guest body with LimitedGuestBodyConsumer (src/p3/body.rs), not UnlimitedGuestBodyConsumer. GuestBody::size_hint is then exact, which is what lets hyper stop after the declared length and drop the body.
  • The guest writes the body and then the trailers from a task that http_into_wasi_response spawns (spawn_local, then BodyWriter::send_http_body). That task, or the stream and future ends it holds, may be what never completes once the host drops the body. If so, it may be partly or wholly a wasip3/wit-bindgen issue and not only a host-side one.
  • Workaround attempt on 49.0.3: polling the dropped body to the end on a spawned host task made the trap less frequent but did not stop it.
Guest

The whole guest, for reference (the host is in the repo):

//! A `wasi:http/[email protected]` component with two routes that differ only in
//! whether the response carries a `content-length` header:
//!
//! - `GET /with-content-length`
//! - `GET /without-content-length`

use bytes::Bytes;
use http::header;
use http_body_util::Full;
use wasip3::http::types::{self, ErrorCode};
use wasip3::http_compat::{http_from_wasi_request, http_into_wasi_response};

wasip3::http::service::export!(Guest);

struct Guest;

impl wasip3::exports::http::handler::Guest for Guest {
    async fn handle(request: types::Request) -> Result<types::Response, ErrorCode> {
        let request = http_from_wasi_request(request)?;
        let body = "hello\n";
        let mut response = http::Response::new(Full::new(Bytes::from_static(body.as_bytes())));
        if request.uri().path() == "/with-content-length" {
            response
                .headers_mut()
                .insert(header::CONTENT_LENGTH, body.len().into());
        }
        http_into_wasi_response(response)
    }
}
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