RuntimeTarget refactor - Add this-scoped async executors to all Tar…

…gets

Summary:
Changelog: [Internal]

# This diff

1. Provides all Targets with an `executorFromThis()` method, which can be used from within a Target to access a *`this`-scoped main thread executor* = a `std::function` that will execute a callback asynchronously iff the current Target isn't destroyed first.
2. Refactors how (all) Target objects are constructed and retained, from a plain constructor to `static shared_ptr create()`. This is because `executorFromThis()` relies internally on `enable_shared_from_this` plus two-phase construction to populate the executor.
3. Creates utilities for deriving scoped executors from other executors and `shared_ptr`s.

The concept is very much like `RuntimeExecutor` in reverse: the facebook#1 use case is moving from the JS thread back to the main thread - where "main thread" is defined loosely as "anywhere it's legal to call methods on Target/Agent objects, access session state, etc". The actual dispatching mechanism is left up to the owner of each `PageTarget` object; for now we only have an iOS integration, where we use `RCTExecuteOnMainQueue`.

Coupling the ownership/lifetime semantics with task scheduling is helpful, because it avoids the footgun of accidentally/nondeterministically moving `shared_ptr`s (and destructors!) to a different thread/queue .

# This stack
I'm refactoring the way the Runtime concept works in the modern CDP backend to bring it in line with the Page/Instance concepts. 

Overall, this will let us:

* Integrate with engines that require us to instantiate a shared Target-like object (e.g. Hermes AsyncDebuggingAPI) in addition to an per-session Agent-like object.
* Access JSI in a CDP context (both at target setup/teardown time and during a CDP session) to implement our own engine-agnostic functionality (`console` interception, `Runtime.addBinding`, etc).
* Manage CDP execution contexts natively in RN, and (down the line) enable first-class debugging support for multiple Runtimes in an Instance.

The core diffs in this stack:

* ~~Introduce a `RuntimeTarget` class similar to `{Page,Instance}Target`. ~~
* ~~Make runtime registration explicit (`InstanceTarget::registerRuntime` similar to `PageTarget::registerInstance`). ~~
* ~~Rename the existing `RuntimeAgent` interface to `RuntimeAgentDelegate`.~~
* ~~Create a new concrete `RuntimeAgent` class similar to `{Page,Instance}Agent`.~~
* ~~Provide `RuntimeTarget` and `RuntimeAgent` with primitives for safe JSI access, namely a `RuntimeExecutor` for scheduling work on the JS thread.~~ 
  * Provide RuntimeTarget with mechanism for scheduling work on the "main" thread from the JS thread, for when we need to do more than just send a CDP message (which we can already do with the thread-safe `FrontendChannel`) in response to a JS event. *← This diff*

## Architecture diagrams

Before this stack:
https://pxl.cl/4h7m0

After this stack:
https://pxl.cl/4h7m7

Reviewed By: hoxyq

Differential Revision: D53356953

packages/react-native/React/Base/RCTBridge.mm

@@ -209,7 +209,7 @@ @implementation RCTBridge {
   NSURL *_delegateBundleURL;
 
   std::unique_ptr<RCTBridgePageTargetDelegate> _inspectorPageDelegate;
-  std::unique_ptr<facebook::react::jsinspector_modern::PageTarget> _inspectorTarget;
+  std::shared_ptr<facebook::react::jsinspector_modern::PageTarget> _inspectorTarget;
   std::optional<int> _inspectorPageId;
 }
 
@@ -412,7 +412,12 @@ - (void)setUp
 
   auto &inspectorFlags = facebook::react::jsinspector_modern::InspectorFlags::getInstance();
   if (inspectorFlags.getEnableModernCDPRegistry() && !_inspectorPageId.has_value()) {
-    _inspectorTarget = std::make_unique<facebook::react::jsinspector_modern::PageTarget>(*_inspectorPageDelegate);
+    _inspectorTarget =
+        facebook::react::jsinspector_modern::PageTarget::create(*_inspectorPageDelegate, [](auto callback) {
+          RCTExecuteOnMainQueue(^{
+            callback();
+          });
+        });
     __weak RCTBridge *weakSelf = self;
     _inspectorPageId = facebook::react::jsinspector_modern::getInspectorInstance().addPage(
         "React Native Bridge (Experimental)",

packages/react-native/ReactCommon/jsinspector-modern/InstanceTarget.cpp

@@ -12,6 +12,14 @@
 
 namespace facebook::react::jsinspector_modern {
 
+std::shared_ptr<InstanceTarget> InstanceTarget::create(
+    InstanceTargetDelegate& delegate,
+    VoidExecutor executor) {
+  std::shared_ptr<InstanceTarget> instanceTarget{new InstanceTarget(delegate)};
+  instanceTarget->setExecutor(executor);
+  return instanceTarget;
+}
+
 InstanceTarget::InstanceTarget(InstanceTargetDelegate& delegate)
     : delegate_(delegate) {
   (void)delegate_;
@@ -24,8 +32,7 @@ std::shared_ptr<InstanceAgent> InstanceTarget::createAgent(
     SessionState& sessionState) {
   auto instanceAgent =
       std::make_shared<InstanceAgent>(channel, *this, sessionState);
-  instanceAgent->setCurrentRuntime(
-      currentRuntime_.has_value() ? &*currentRuntime_ : nullptr);
+  instanceAgent->setCurrentRuntime(currentRuntime_.get());
   agents_.push_back(instanceAgent);
   return instanceAgent;
 }
@@ -47,9 +54,11 @@ InstanceTarget::~InstanceTarget() {
 
 RuntimeTarget& InstanceTarget::registerRuntime(
     RuntimeTargetDelegate& delegate,
-    RuntimeExecutor executor) {
+    RuntimeExecutor jsExecutor) {
   assert(!currentRuntime_ && "Only one Runtime allowed");
-  currentRuntime_.emplace(delegate, executor);
+  currentRuntime_ = RuntimeTarget::create(
+      delegate, jsExecutor, makeVoidExecutor(executorFromThis()));
+
   forEachAgent([currentRuntime = &*currentRuntime_](InstanceAgent& agent) {
     agent.setCurrentRuntime(currentRuntime);
   });
@@ -58,7 +67,7 @@ RuntimeTarget& InstanceTarget::registerRuntime(
 
 void InstanceTarget::unregisterRuntime(RuntimeTarget& Runtime) {
   assert(
-      currentRuntime_.has_value() && &currentRuntime_.value() == &Runtime &&
+      currentRuntime_ && currentRuntime_.get() == &Runtime &&
       "Invalid unregistration");
   forEachAgent([](InstanceAgent& agent) { agent.setCurrentRuntime(nullptr); });
   currentRuntime_.reset();

packages/react-native/ReactCommon/jsinspector-modern/InstanceTarget.h

@@ -8,6 +8,7 @@
 #pragma once
 
 #include "RuntimeTarget.h"
+#include "ScopedExecutor.h"
 #include "SessionState.h"
 
 #include <jsinspector-modern/InspectorInterfaces.h>
@@ -40,14 +41,20 @@ class InstanceTargetDelegate {
 /**
  * A Target that represents a single instance of React Native.
  */
-class InstanceTarget final {
+class InstanceTarget : public EnableExecutorFromThis<InstanceTarget> {
  public:
   /**
+   * Constructs a new InstanceTarget.
    * \param delegate The object that will receive events from this target.
    * The caller is responsible for ensuring that the delegate outlives this
    * object.
+   * \param executor An executor that may be used to call methods on this
+   * InstanceTarget while it exists. \c create additionally guarantees that the
+   * executor will not be called after the InstanceTarget is destroyed.
    */
-  explicit InstanceTarget(InstanceTargetDelegate& delegate);
+  static std::shared_ptr<InstanceTarget> create(
+      InstanceTargetDelegate& delegate,
+      VoidExecutor executor);
 
   InstanceTarget(const InstanceTarget&) = delete;
   InstanceTarget(InstanceTarget&&) = delete;
@@ -65,8 +72,17 @@ class InstanceTarget final {
   void unregisterRuntime(RuntimeTarget& runtime);
 
  private:
+  /**
+   * Constructs a new InstanceTarget. The caller must call setExecutor
+   * immediately afterwards.
+   * \param delegate The object that will receive events from this target.
+   * The caller is responsible for ensuring that the delegate outlives this
+   * object.
+   */
+  InstanceTarget(InstanceTargetDelegate& delegate);
+
   InstanceTargetDelegate& delegate_;
-  std::optional<RuntimeTarget> currentRuntime_{std::nullopt};
+  std::shared_ptr<RuntimeTarget> currentRuntime_{nullptr};
   std::list<std::weak_ptr<InstanceAgent>> agents_;
 
   /**

packages/react-native/ReactCommon/jsinspector-modern/PageTarget.cpp

@@ -99,14 +99,22 @@ class PageTargetSession {
   SessionState state_;
 };
 
+std::shared_ptr<PageTarget> PageTarget::create(
+    PageTargetDelegate& delegate,
+    VoidExecutor executor) {
+  std::shared_ptr<PageTarget> pageTarget{new PageTarget(delegate)};
+  pageTarget->setExecutor(executor);
+  return pageTarget;
+}
+
 PageTarget::PageTarget(PageTargetDelegate& delegate) : delegate_(delegate) {}
 
 std::unique_ptr<ILocalConnection> PageTarget::connect(
     std::unique_ptr<IRemoteConnection> connectionToFrontend,
     SessionMetadata sessionMetadata) {
   auto session = std::make_shared<PageTargetSession>(
       std::move(connectionToFrontend), controller_, std::move(sessionMetadata));
-  session->setCurrentInstance(currentInstance_ ? &*currentInstance_ : nullptr);
+  session->setCurrentInstance(currentInstance_.get());
   sessions_.push_back(std::weak_ptr(session));
   return std::make_unique<CallbackLocalConnection>(
       [session](std::string message) { (*session)(message); });
@@ -131,7 +139,8 @@ PageTargetDelegate::~PageTargetDelegate() {}
 
 InstanceTarget& PageTarget::registerInstance(InstanceTargetDelegate& delegate) {
   assert(!currentInstance_ && "Only one instance allowed");
-  currentInstance_.emplace(delegate);
+  currentInstance_ =
+      InstanceTarget::create(delegate, makeVoidExecutor(executorFromThis()));
   forEachSession(
       [currentInstance = &*currentInstance_](PageTargetSession& session) {
         session.setCurrentInstance(currentInstance);
@@ -141,7 +150,7 @@ InstanceTarget& PageTarget::registerInstance(InstanceTargetDelegate& delegate) {
 
 void PageTarget::unregisterInstance(InstanceTarget& instance) {
   assert(
-      currentInstance_.has_value() && &currentInstance_.value() == &instance &&
+      currentInstance_ && currentInstance_.get() == &instance &&
       "Invalid unregistration");
   forEachSession(
       [](PageTargetSession& session) { session.setCurrentInstance(nullptr); });

packages/react-native/ReactCommon/jsinspector-modern/PageTarget.h

@@ -7,6 +7,8 @@
 
 #pragma once
 
+#include "ScopedExecutor.h"
+
 #include <jsinspector-modern/InspectorInterfaces.h>
 #include <jsinspector-modern/InstanceTarget.h>
 
@@ -95,19 +97,25 @@ class PageTargetController final {
  * "Host" in React Native's architecture - the entity that manages the
  * lifecycle of a React Instance.
  */
-class JSINSPECTOR_EXPORT PageTarget final {
+class JSINSPECTOR_EXPORT PageTarget
+    : public EnableExecutorFromThis<PageTarget> {
  public:
   struct SessionMetadata {
     std::optional<std::string> integrationName;
   };
 
   /**
    * Constructs a new PageTarget.
-   * \param delegate The PageTargetDelegate that will receive events from
-   * this PageTarget. The caller is responsible for ensuring that the
-   * PageTargetDelegate outlives this object.
+   * \param delegate The PageTargetDelegate that will
+   * receive events from this PageTarget. The caller is responsible for ensuring
+   * that the PageTargetDelegate outlives this object.
+   * \param executor An executor that may be used to call methods on this
+   * PageTarget while it exists. \c create additionally guarantees that the
+   * executor will not be called after the PageTarget is destroyed.
    */
-  explicit PageTarget(PageTargetDelegate& delegate);
+  static std::shared_ptr<PageTarget> create(
+      PageTargetDelegate& delegate,
+      VoidExecutor executor);
 
   PageTarget(const PageTarget&) = delete;
   PageTarget(PageTarget&&) = delete;
@@ -146,11 +154,19 @@ class JSINSPECTOR_EXPORT PageTarget final {
   void unregisterInstance(InstanceTarget& instance);
 
  private:
-  std::list<std::weak_ptr<PageTargetSession>> sessions_;
+  /**
+   * Constructs a new PageTarget.
+   * The caller must call setExecutor immediately afterwards.
+   * \param delegate The PageTargetDelegate that will
+   * receive events from this PageTarget. The caller is responsible for ensuring
+   * that the PageTargetDelegate outlives this object.
+   */
+  PageTarget(PageTargetDelegate& delegate);
 
   PageTargetDelegate& delegate_;
+  std::list<std::weak_ptr<PageTargetSession>> sessions_;
   PageTargetController controller_{*this};
-  std::optional<InstanceTarget> currentInstance_{std::nullopt};
+  std::shared_ptr<InstanceTarget> currentInstance_{nullptr};
 
   /**
    * Call the given function for every active session, and clean up any
@@ -175,7 +191,7 @@ class JSINSPECTOR_EXPORT PageTarget final {
   }
 
   inline bool hasInstance() const {
-    return currentInstance_.has_value();
+    return currentInstance_ != nullptr;
   }
 
   // Necessary to allow PageAgent to access PageTarget's internals in a

packages/react-native/ReactCommon/jsinspector-modern/ReactCdp.h

@@ -11,4 +11,5 @@
 #include <jsinspector-modern/InstanceTarget.h>
 #include <jsinspector-modern/PageTarget.h>
 #include <jsinspector-modern/RuntimeTarget.h>
+#include <jsinspector-modern/ScopedExecutor.h>
 #include <jsinspector-modern/SessionState.h>

packages/react-native/ReactCommon/jsinspector-modern/RuntimeTarget.cpp

@@ -9,10 +9,20 @@
 
 namespace facebook::react::jsinspector_modern {
 
+std::shared_ptr<RuntimeTarget> RuntimeTarget::create(
+    RuntimeTargetDelegate& delegate,
+    RuntimeExecutor jsExecutor,
+    VoidExecutor selfExecutor) {
+  std::shared_ptr<RuntimeTarget> runtimeTarget{
+      new RuntimeTarget(delegate, jsExecutor)};
+  runtimeTarget->setExecutor(selfExecutor);
+  return runtimeTarget;
+}
+
 RuntimeTarget::RuntimeTarget(
     RuntimeTargetDelegate& delegate,
-    RuntimeExecutor executor)
-    : delegate_(delegate), executor_(executor) {}
+    RuntimeExecutor jsExecutor)
+    : delegate_(delegate), jsExecutor_(jsExecutor) {}
 
 std::shared_ptr<RuntimeAgent> RuntimeTarget::createAgent(
     FrontendChannel channel,

packages/react-native/ReactCommon/jsinspector-modern/RuntimeTarget.h

@@ -10,6 +10,7 @@
 #include <ReactCommon/RuntimeExecutor.h>
 #include "InspectorInterfaces.h"
 #include "RuntimeAgent.h"
+#include "ScopedExecutor.h"
 #include "SessionState.h"
 
 #include <list>
@@ -48,18 +49,27 @@ class RuntimeTargetDelegate {
 /**
  * A Target corresponding to a JavaScript runtime.
  */
-class JSINSPECTOR_EXPORT RuntimeTarget final {
+class JSINSPECTOR_EXPORT RuntimeTarget
+    : public EnableExecutorFromThis<RuntimeTarget> {
  public:
   /**
+   * Constructs a new RuntimeTarget. The caller must call setExecutor
+   * immediately afterwards.
    * \param delegate The object that will receive events from this target.
    * The caller is responsible for ensuring that the delegate outlives this
    * object.
-   * \param executor A RuntimeExecutor that can be used to schedule work on
+   * \param jsExecutor A RuntimeExecutor that can be used to schedule work on
    * the JS runtime's thread. The executor's queue should be empty when
    * RuntimeTarget is constructed (i.e. anything scheduled during the
    * constructor should be executed before any user code is run).
+   * \param selfExecutor An executor that may be used to call methods on this
+   * RuntimeTarget while it exists. \c create additionally guarantees that the
+   * executor will not be called after the RuntimeTarget is destroyed.
    */
-  RuntimeTarget(RuntimeTargetDelegate& delegate, RuntimeExecutor executor);
+  static std::shared_ptr<RuntimeTarget> create(
+      RuntimeTargetDelegate& delegate,
+      RuntimeExecutor jsExecutor,
+      VoidExecutor selfExecutor);
 
   RuntimeTarget(const RuntimeTarget&) = delete;
   RuntimeTarget(RuntimeTarget&&) = delete;
@@ -81,8 +91,21 @@ class JSINSPECTOR_EXPORT RuntimeTarget final {
       SessionState& sessionState);
 
  private:
+  /**
+   * Constructs a new RuntimeTarget. The caller must call setExecutor
+   * immediately afterwards.
+   * \param delegate The object that will receive events from this target.
+   * The caller is responsible for ensuring that the delegate outlives this
+   * object.
+   * \param jsExecutor A RuntimeExecutor that can be used to schedule work on
+   * the JS runtime's thread. The executor's queue should be empty when
+   * RuntimeTarget is constructed (i.e. anything scheduled during the
+   * constructor should be executed before any user code is run).
+   */
+  RuntimeTarget(RuntimeTargetDelegate& delegate, RuntimeExecutor jsExecutor);
+
   RuntimeTargetDelegate& delegate_;
-  RuntimeExecutor executor_;
+  RuntimeExecutor jsExecutor_;
   std::list<std::weak_ptr<RuntimeAgent>> agents_;
 
   /**