rtpengine: async support for rtpengine_start_recording()

[zbyslb]

Summary

Exports rtpengine_start_recording() as an async command, making it usable with both async styles, exactly like the existing rtpengine_offer/answer/delete:

async(rtpengine_start_recording(), resume_route [, timeout]);
launch(rtpengine_start_recording() [, report_route]);

Typical use case: start call recording without blocking the SIP transaction, yet still learn in the resume/report route whether rtpengine actually accepted it — a middle ground between the fully synchronous call (blocks up to rtpengine_tout × rtpengine_retr) and fire-and-forget (no feedback at all).

Implementation

A thin wrapper, rtpengine_start_recording_af(), feeds OP_START_RECORDING into the existing rtpe_function_call_async() machinery — per-command socket, cookie, timeout handling and reply processing all come for free. Two small adjustments were required:

1. Skip the mandatory SDP extraction. rtpe_function_call_async() unconditionally calls extract_body() for any op != OP_DELETE, but recording commands carry no SDP and are typically fired on in-dialog requests without a body (the extraction would fail). The wrapper passes an empty str as the body, which turns the extraction into a no-op (and rtpe_function_call_prepare() only attaches "sdp" for offer/answer anyway).

2. Tighten the reply-body condition in resume_async_send_rtpe_command(). It treated every non-delete reply as carrying an "sdp" field to substitute into the message, failing the whole command when the field is absent. A recording reply only carries the result. The condition is now exactly the set of ops that do return a new SDP (offer/answer/subscribe-answer) — a no-change for the three already-exported async commands.

The synchronous rtpengine_start_recording() stays available and unchanged; the async variant is only picked up when the function is wrapped in async()/launch().

Note

This touches the same function as #4258 (resume_async_send_rtpe_command, different hunk) — trivially rebaseable in either order.