AUv3: parameter changes stop syncing in both directions when audio processing pauses

[singularpoint]

With a CLAP plugin wrapped as AUv3, parameter changes stop synchronizing in both directions while audio processing is suspended, even though parameters remain editable. Resuming audio processing delivers the pending changes.

Reproduced with AUv3 in both hosts:

• REAPER 7.79
• Audio Hijack 4.6.0

Tested on macOS 26.6.2 / Apple Silicon, using next at 9501dad0, matching upstream next when checked.

Expected behavior

Parameter exchange should continue without requiring audio processing to resume. CLAP explicitly provides params.flush() for this purpose. Its parameter contract says host-control changes are delivered through process() or flush(), and host_params.request_flush() requires the host to schedule one of those calls. Here, the wrapper is the CLAP host. CLAP parameter contract, at the SDK revision used.

flush() must not run concurrently with process() and must follow CLAP's active/inactive thread rules. Those constraints require serialized delivery; they do not make audio processing a prerequisite for parameter exchange.

Quick reproduction — no debugger

Attached: passive-parameters-repro.zip. It contains a small CLAP plugin, AUv3 build/install instructions, and detailed reproduction steps. The plugin has one parameter, silent audio output, exact-value buttons, and an event log.

The following steps use Audio Hijack's block off/on control as a concrete example. Build/install the plugin, load Example: Passive Parameters, and start the session with the block on. Locate both the plugin editor and the host's own Value control.

1. Click Set 0.25 in the plugin editor; confirm both sides show 0.25.
2. Turn the block off, click Set 0.75, and wait five seconds.
   Plugin: 0.75; host: still 0.25.
3. Turn the block on, without another edit. The host catches up to 0.75.
4. Reset both sides to 0.25 while on, then turn the block off again.
5. Change Value through the host's control to approximately 0.6 and wait.
   Host: new value; plugin: still 0.25.
6. Turn the block on. The plugin catches up.

The expected result in steps 2 and 5 is agreement while the block stays off. The log identifies events delivered via process or flush; compare both value displays, since acceptance of a CLAP output event alone does not prove AUv3 delivery.

Investigation

The following runtime observations were collected in Audio Hijack.

• Plugin -> Host: the request reaches
  param_request_flush(),
  which only logs it. The outgoing change remains pending until processing resumes.
• Host -> Plugin: the
  AU parameter observer
  sees _renderResourcesAllocated == YES even with the block off, queues the change for audio processing, and skips flush(). We observed pending changes and stale plugin state; enabling the block drained them through process().

Apple defines renderResourcesAllocated as resource-allocation state, not a guarantee of subsequent processing callbacks. Relying on it to defer all delivery leaves parameter exchange stalled in this state.

passive-parameters-repro.zip

[singularpoint]

Confirmed on iOS as well, using GarageBand on an iPhone 15 Pro Max.

Attached is a minimal CLAP plugin with a shared SwiftUI editor for macOS/iOS, built as AUv3 through clap-wrapper. Build/install instructions are in the README.

To reproduce in GarageBand:

1. Add the AUv3 effect to an Audio Recorder track with a recorded region. Play it once, then stop and wait for the plugin’s process counter to stop increasing.
2. Plugin → Host: click Set 0.25 or Set 0.75 in the plugin editor. Only LOCAL EDIT is logged; flush stays at zero. Resume playback: the pending change is accepted via process.
3. Host → Plugin: while stopped, use Edit below the plugin editor to access GarageBand’s Value slider. Change it, then return to the plugin editor: no incoming parameter event is logged. Resume playback: the queued changes arrive via process, updating the plugin value. The CLAP instance remains activated during the pause. Both directions stall until audio processing resumes.

passive-parameters-swiftui.zip