Bug: ai-load-balancer plugin hangs on requests without body (GET/HEAD/DELETE), causing 504 timeout

[Thomas-Eliot]

Environment

• Higress Gateway version: 2.1.11 (also reproducible on latest code in main branch)
• Plugin: ai-load-balancer
• Affected lb_policy: global_least_request, endpoint_metrics, prefix_cache
• Not affected: cluster_metrics

Description

When ai-load-balancer plugin is enabled on a route, any request without a body (e.g. GET, HEAD, DELETE) will hang indefinitely until idle timeout, resulting in a 504 or downstream disconnect (DC).

The root cause is that 3 out of 4 lb_policy implementations return types.HeaderStopIteration in HandleHttpRequestHeaders, deferring all load-balancing logic (upstream host selection + proxywasm.ResumeHttpRequest()) to HandleHttpRequestBody. However, for requests with no body, the OnHttpRequestBody callback is never invoked by Envoy, so the request is paused forever.

Affected Code

• global_least_request/lb_policy.go → HandleHttpRequestHeaders returns HeaderStopIteration
• endpoint_metrics/lb_policy.go → HandleHttpRequestHeaders returns HeaderStopIteration
• prefix_cache/lb_policy.go → HandleHttpRequestHeaders returns HeaderStopIteration

All three rely on HandleHttpRequestBody to call proxywasm.ResumeHttpRequest(), which never happens for bodiless requests.

cluster_metrics/lb_policy.go is not affected because it completes host selection in HandleHttpRequestHeaders and returns ActionContinue.

Steps to Reproduce

1. Configure a route with ai-load-balancer plugin enabled (any of the 3 affected policies)
2. Send a GET request through Higress to that route:

   curl -i "http://<higress-gateway>/some-path?param=value" -H "host: example.com"

3. The request hangs until timeout

Evidence from Envoy Debug Logs

# Request arrives with end_stream=true in headers (no body)
request headers complete (end_stream=true):
':path', '/sink?messageID=8c13992ee64e4dcaaf8f496c1387a9b5'
':method', 'GET'

# ~10 seconds later, stream reset due to client disconnect
stream reset: reset reason: connection termination

# ai-load-balancer reports error because HandleHttpRequestBody was never called
wasm log higress-system.ai-load-balancer-1.0.0: [ai-load-balancer] get host_selected failed

Access log confirms:

{
  "method": "GET",
  "path": "/sink?messageID=8c13992ee64e4dcaaf8f496c1387a9b5",
  "response_code": "0",
  "response_flags": "DC",
  "upstream_host": "-",
  "response_code_details": "downstream_remote_disconnect",
  "duration": "10320"
}

Suggested Fix

In HandleHttpRequestHeaders, detect whether the request has a body. If not, perform the load-balancing logic directly in the headers phase and return ActionContinue instead of HeaderStopIteration.

For example, check the :method header or use the endOfStream flag (if available via the wrapper framework):

func (lb GlobalLeastRequestLoadBalancer) HandleHttpRequestHeaders(ctx wrapper.HttpContext) types.Action {
    method, _ := proxywasm.GetHttpRequestHeader(":method")
    if method == "GET" || method == "HEAD" || method == "DELETE" || method == "OPTIONS" {
        // No body expected, perform LB logic here and continue
        // ... (same logic as HandleHttpRequestBody)
        return types.ActionContinue
    }
    return types.HeaderStopIteration
}

Workaround

Disable the ai-load-balancer plugin for routes that receive bodiless requests (GET/HEAD/DELETE).