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serve: the post-launch smoke test spins forever at 100% CPU if the engine closes the connection mid-response

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@rominf is already working on this.

Since Oct 5, 2026.

  • #527 by @rominf — open

Assessment

Difficulty
2/5
Estimated time
1-3 hours
Newbie friendliness
85/100
Issue type
Bug
Clarity
Clearly specified
Activity status
Active
Tech stack
rust
Domain
cli

Research direction

Start in apps/rocm/src/providers.rs at read_chunked_sse_body and trace how the smoke test reaches it through serve_summary::run_smoke_test. Reproduce a truncated chunked response using the stream-reader test setup described in the issue, then verify the reader returns an error naming the premature connection close instead of spinning; the regression test should complete promptly.

Written by the indexing model from the issue text.

Description

bug

Current behavior

read_chunked_sse_body (apps/rocm/src/providers.rs) reads each chunk-size line with read_line. At end-of-stream read_line returns Ok(0) and leaves the line empty — and the loop treats an empty line as a blank separator:

if size_line.trim().is_empty() {
    continue;
}

So when the connection closes before the terminating 0 chunk, the loop re-reads EOF forever. Each read returns immediately, so this is a busy loop, not a wait: one core at 100%, and the call never returns.

Expected behavior

A connection that closes before the final chunk ends the stream with an error naming what happened.

Steps to reproduce

Serve a chunked streaming response that sends headers and at least one complete chunk, then closes. Reproduced end-to-end over a real TCP socket: provider_stream_chat had not returned after 40 s — longer than the 30 s socket read timeout, which confirms it is spinning rather than waiting for data.

How a user hits it

Correction: an earlier version of this issue said rocm chat hangs. That was wrong — rocm chat and the dash chat use the non-streaming provider_chat, whose read is already time-limited. The only production caller of the streaming reader is the smoke test rocm serve runs after launching an engine (serve_summary::run_smoke_test), and that only runs when stdout is a terminal.

So the symptom is rocm serve stuck forever on "Running smoke test…" with one core at 100%, if the engine dies, is OOM-killed, or is stopped while it is streaming its first answer. vLLM streams with chunked encoding, which is the path that spins.

Possible solution

Treat Ok(0) from read_line as end-of-stream, distinct from a blank line, and return an error ("connection closed before the final chunk"). The same check belongs anywhere else this reader distinguishes "empty line" from "no more input".

Related

The close-delimited reader in the same file uses ? on ErrorKind::Interrupted instead of retrying, so a single interrupted read ends the stream. Chunked and fixed-length bodies are unaffected because std's read_line/read_exact already retry. Lower reach, because that path only runs when a response has neither chunked encoding nor a Content-Length.

Environment

  • Built from main. Platform-independent.

How it was found

Property-testing the stream readers: re-chunking real fixture bodies at every boundary showed parsing is chunking-invariant (that part holds), and truncating them showed this reader never terminates. The reproducer uses a reader that panics after 10,000 consecutive empty reads, so the test fails immediately instead of hanging.

Dominant language
Rust
Stars
41
Forks
10
Avg merge
5d 3h
Merged PRs (30d)
87

Getting set up

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

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