recv_when(false) does not detect pdata channel closure when buffer has messages

[lalitb]

Pre-filing checklist

• I searched existing issues and didn't find a duplicate

Component(s)

Rust OTAP dataflow (rust/otap-dataflow/)

Bug Description

recv_when(false) only checks if the pdata channel is closed when the buffer is empty (is_empty() == true). If the upstream sends messages and then shuts down, the channel is closed but has buffered messages. is_empty() returns false, the closure check is skipped, and recv_when blocks on the control channel waiting for a Shutdown that never arrives - because the engine only sends Shutdown via control to receivers, not to processors or exporters.

This means any node calling recv_when(false) can hang if the pdata channel closes while there are still buffered messages.

Known callers affected:

• Azure Monitor exporter - accepting_pdata is false during auth outages (see comment in azure monitor exporter: move auth to event loop only - non-blocking auth. #2312)
• Fanout processor - accept_pdata() returns false when inflight count hits max_inflight

Steps to Reproduce

1. Have a node that calls recv_when(false) for an extended period (e.g., Azure Monitor exporter during auth failure)
2. Upstream sends a few pdata messages
3. Upstream shuts down (drops its pdata sender)
4. The node's pdata channel is now closed with buffered messages
5. recv_when(false) never detects the closure - node hangs

Expected Behavior

recv_when(false) should detect pdata channel closure regardless of whether the buffer is empty, so the node can shut down cleanly.

Actual Behavior

recv_when(false) skips the closure check when the buffer is not empty, and the node hangs forever.

OTel-Arrow Version

latest

Environment

All env

Configuration

Log Output

Additional Context

No response