feat: Add `MessageBus` to core, add `MakeMessageHandler` to `Feature` by awforsythe · Pull Request #181 · DataDog/dd-sdk-cpp

awforsythe · 2026-03-11T23:17:33Z

refs: RUM-12206

This PR builds upon #180, hooking the new MessageBus into the Core and providing Feature implementations with the ability to respond to messages.

This change allows a Feature to react to changes in CoreContext by overriding Feature::MakeMessageHandler and providing a callback that will handle ContextChangedMessage.

A feature's message handler function is executed on a new messaging thread, which reads from the Message Bus's queue and fans out each new message to all registered handler functions. This design is consistent with the iOS SDK: messages are processed asynchronously, and handling of messages does not block other threads. It's up to each feature to synchronize access to any state that's shared between the message-handler and other threads.

Out of scope:

This PR does not include support for sending messages from a Feature to other features.
The current design uses a simple broadcast approach: Features are opted out of message handling by default, but once a feature opts in, it will receive all messages, regardless of type. (Currently, there's only one type.)

Code changes

Core now owns a MessageBus and a std::thread that runs MessagingThreadMain
CoreContextProvider now keeps a reference to the active MessageBus (valid only while the SDK is running)
When any call to FeatureScope::UpdateContext is processed on the context thread, CoreContextProvider broadcasts ContextChangedMessage with an immutable snapshot of the new context value, allowing features to respond on the messaging thread
Feature now provides a virtual MakeMessageHandler method, optionally returning a std::function
- if a feature needs to access or modify its own state in response to messages, it can return a function that captures a weak_ptr to itself
On startup, Core:
- creates a fresh MessageBus
- calls MakeMessageHandler on all features
- adds each valid function to the message bus's list of receivers
- spawns the messaging thread once all features are fully initialized
- installs the MessageBus into the CoreContextProvider
On shutdown, Core tears down messaging state

refs: RUM-12206

datadog-prod-us1-3 · 2026-03-11T23:17:39Z

✅ Code Quality ✅ Code Vulnerabilities ✅ Library Vulnerabilities ✅ Secrets

🎉 All green!

🛠️ No new code quality issues
🛡️ No new code vulnerabilities
📚 No new vulnerable libraries detected
🔑 No new secrets detected

_{This comment will be updated automatically if new data arrives.

🔗 Commit SHA: 1a908ed | Docs | Datadog PR Page | Was this helpful? React with 👍/👎 or give us feedback!}

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chatgpt-codex-connector · 2026-03-11T23:46:43Z

+  // Start the messaging thread, which will invoke relevant message handler functions
+  // for each message sent on the message bus
+  DATADOG_ASSERT(!_message_bus_thread, "_message_bus_thread already exists on Start()");
+  _message_bus_thread = std::thread(
+      MessagingThreadMain, std::ref(_diagnostic_logger), std::ref(*_message_bus)


Defer message thread until all features are started

Core::Start() launches the messaging thread before the OnCoreStarted loop, so a feature that mutates context during Start() (for example Rum::Start() dispatching UpdateContext) can enqueue ContextChangedMessage values that are delivered to handlers of later features before those features have run OnCoreStarted. In that window, handler code can observe partially initialized feature state (including unset _scope) and crash or drop required work. Please activate message delivery only after all features complete startup, or gate handlers until startup is finished.

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Startup ordering fixed with ea485aa: all features will receive Start() before the messaging thread is spawned, such that a feature is guaranteed to be fully started before it receives any messages.

awforsythe · 2026-03-13T11:55:04Z

/gitlab resync-job-status

gh-worker-devflow-routing-ef8351 · 2026-03-13T11:55:07Z

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2026-03-13 11:55:22 UTC ℹ️ Devflow: /gitlab resync-job-status

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awforsythe · 2026-03-13T12:42:54Z

@codex review

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chatgpt-codex-connector · 2026-03-13T12:49:44Z

+   * Called by `Core::RegisterFeature()` after the feature has been stored in the
+   * feature list, at which point `weak_from_this()` is safe to call. If the feature


Align MakeMessageHandler lifecycle contract with implementation

The new API contract says MakeMessageHandler() is invoked from Core::RegisterFeature(), but Core::Start() now calls it each time the SDK starts (core.cpp startup loop). This mismatch can lead feature authors to implement one-shot handler setup (e.g., moving state or assuming a single call), which breaks or changes behavior on restart when handlers are recreated per run. Please update this contract to reflect per-Start() invocation (or move the call site to registration).

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Indeed, this documentation comment is outdated. Fixed with 1a908ed.

The base branch was changed.

awforsythe added 6 commits March 11, 2026 15:40

feat: Add MessageBus to core, add MakeMessageHandler to Feature

48b0638

refs: RUM-12206

chore: Fix redeclared sentinel in test

73c793d

chore: Move messaging_thread.hpp include to core.cpp

d461f48

chore: Use IIFE for snapshot rather than mutating std::optional

90b4c8c

chore: Add a failing test to catch lost message handler on Core restart

39a53d1

fix: Move per-Feature message handler creation to Core::Start()

22d0dc5

chatgpt-codex-connector Bot reviewed Mar 11, 2026

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fix: Start message thread after Start() for consistency

ea485aa

chatgpt-codex-connector Bot reviewed Mar 13, 2026

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awforsythe mentioned this pull request Mar 13, 2026

feat: Include RUM context in crash reports #182

Merged

awforsythe added 2 commits March 13, 2026 09:01

chore: Who puts em dashes in source code, seriously

2317e65

chore: Fix outdated comment on MakeMessageHandler

1a908ed

awforsythe marked this pull request as ready for review March 13, 2026 13:29

awforsythe requested a review from a team as a code owner March 13, 2026 13:29

fuzzybinary previously approved these changes Mar 19, 2026

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Base automatically changed from aforsythe/RUM-12206/message-bus to main March 24, 2026 13:25

awforsythe requested a review from a team as a code owner March 24, 2026 13:25

awforsythe merged commit e2400fb into main Mar 24, 2026
18 checks passed

awforsythe deleted the aforsythe/RUM-12206/feature-messaging branch March 24, 2026 13:25

awforsythe mentioned this pull request Mar 26, 2026

Integrate dd-win-prof profiling feature into dd-sdk-cpp #185

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feat: Add `MessageBus` to core, add `MakeMessageHandler` to `Feature`#181

feat: Add `MessageBus` to core, add `MakeMessageHandler` to `Feature`#181
awforsythe merged 9 commits into
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		* Called by `Core::RegisterFeature()` after the feature has been stored in the
		* feature list, at which point `weak_from_this()` is safe to call. If the feature

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