AlinsRan opened a new pull request, #13870:
URL: https://github.com/apache/apisix/pull/13870
### Description
A streaming AI request whose upstream answers with an SSE content type and
then closes without writing a single body byte crashes the gateway in
`balancer_by_lua`.
`body_reader()` returns EOF on its first call, the loop body never runs,
nothing is written downstream, and `parse_streaming_response()` returns `nil`.
`run_plugin()` only calls `core.response.exit()` when a plugin returns a status
code, so returning `nil` leaves the request neither answered nor terminated. It
falls through to `proxy_pass` and enters `balancer_by_lua`, where
`upstream_conf` is always `nil` because `handle_upstream()` skips
`set_upstream()` for `bypass_nginx_upstream` plugins:
```
failed to run balancer_by_lua*: apisix/balancer.lua:
attempt to index local 'up_conf' (a nil value)
in function 'pick_server'
in function 'run'
in function 'http_balancer_phase'
```
`header_filter` then fails as well, so the client gets no HTTP response at
all and the connection is closed.
The `output_sent` guard added in the EOF branch already covers this, but
only when a converter is active:
```lua
if converter and not output_sent then
...
return 502, msg
end
return
```
An OpenAI-format client against an OpenAI-compatible provider matches
`caps[client_protocol]`, so `converter` is `nil`, the guard is skipped, and the
bare `return` below hands `nil` back up — the passthrough case (no protocol
conversion) was never covered.
This drops the `converter` condition and words the log message by whether a
converter was involved. 502 rather than 504: nothing timed out, the upstream
closed cleanly and the request's own timeout never fired; 502 is also what this
same guard already returns for the converter case.
`t/plugin/ai-proxy-empty-stream.t` mocks an upstream that answers `200` +
SSE headers and closes with a zero-byte body, and asserts a 502 with no
`up_conf` error.
#### Which issue(s) this PR fixes:
Fixes #
### Checklist
- [x] I have explained the need for this PR and the problem it solves
- [x] I have explained the changes or the new features added to this PR
- [x] I have added tests corresponding to this change
- [ ] I have updated the documentation to reflect this change
- [x] I have verified that this change is backward compatible (If not,
please discuss on the [APISIX mailing
list](https://github.com/apache/apisix/tree/master#community) first)
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