78 lines
3.3 KiB
Markdown
78 lines
3.3 KiB
Markdown
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# Post-to and post-back in posting invokers
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## Two targets
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Posting coroutines (`TaggedPostingPromise` / `*ViralPostingInvoker` / `*NonViralPostingInvoker`) use two different `io_context` targets:
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| Direction | When | Target |
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|---|---|---|
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| **Post-TO** | `initial_suspend` | Where the callee coroutine **starts running** |
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| **Post-BACK** | `final_suspend` | Where the caller resumes after `co_await` |
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Post-back always uses `callerIoContext`, initialized from `ComponentThread::getSelf()->getIoContext()` at the `co_await` site. That behavior is unchanged.
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Post-to defaults to the static `ThreadTag::io_context()` for the invoker alias (e.g. `MrnttThreadTag`, `BodyThreadTag`).
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## Static post-TO (default)
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Existing coroutines need no signature change. `TaggedPostingPromise::initial_suspend()` posts the callee handle to `ThreadTag::io_context()`.
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## Dynamic post-TO (`ExplicitPostTarget`)
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Opt in by adding `sscl::co::ExplicitPostTarget` to the coroutine parameter list. The promise constructor extracts it into `std::optional<ExplicitPostTarget> calleePostTarget`; `initial_suspend()` posts to `calleePostTarget->ioContext` instead of `ThreadTag::io_context()`.
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```cpp
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// Viral member example: run on body thread, post back to mrntt caller
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BodyViralPostingInvoker<bool> Body::initializeCReq(
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sscl::co::ExplicitPostTarget postTarget)
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{
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co_await ...;
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}
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// Non-viral example
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reattachKnownListCReq(
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sscl::co::ExplicitPostTarget{someIoContext},
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exceptionPtr,
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callback);
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```
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The caller must ensure the referenced `io_context` outlives the callee frame.
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## DeviceManager attach/detach (mrntt orchestrator, library on body/world)
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`DeviceManager::attachStimBuffDeviceCReq` and `detachStimBuffDeviceCReq` stay
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`MrnttViralPostingInvoker` on the marionette thread. They `co_await` library
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`attachDeviceCReq` / `detachDeviceCReq` (`DynamicViralPostingInvoker`) with
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`ExplicitPostTarget` first:
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- **edev** (`sensorType == 'e'`) → world thread
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- **otherwise** → body thread
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Post-TO runs the library coroutine on that target; post-back resumes the
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DeviceManager coroutine on mrntt. Attach also passes `componentThread` for
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device/producer binding inside the library.
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```cpp
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auto targetThread = threadForDeviceOp(*spec);
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co_return co_await lib.stimBuffApiDesc.sal_mgmt_libOps.attachDeviceCReq(
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sscl::co::ExplicitPostTarget{targetThread->getIoContext()},
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spec,
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targetThread);
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```
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Detach uses the same `threadForDeviceOp` rule (not producer lookup).
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## Constructor discrimination
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`PostingPromise` mirrors the existing implicit-`this` peel:
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- Member coroutines pass the object parameter first; it is discarded.
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- When present, `ExplicitPostTarget` is the next parameter (before non-viral `exception_ptr&` and completion callback).
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- Free non-viral dynamic: `ExplicitPostTarget`, then `exception_ptr&`, then callback.
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- `ExplicitPostTarget` is recognized by type (`is_explicit_post_target_v`) and stored in `calleePostTarget`.
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- `ObjectArg` requires `!is_explicit_post_target_v` so the wrapper is never mistaken for `thisptr`.
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## Timing invariant
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Post-TO and post-BACK work must run from `InitialSuspendPostingInvoker::await_suspend` and `FinalSuspendPostingInvoker::await_suspend`, not synchronously inside `initial_suspend()` / `final_suspend()` bodies. See comments in `postingPromise.h`.
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