[KoakiMiku]

Fixes #11621

Problem

WorkItemGroup passes live Task instances as [LoggerMessage] arguments: the long-running turn
warning logs task.AsyncState ?? task, and the DEBUG-only trace messages log task directly.

A provider that serializes structured parameters by reflection — for example NLog JsonLayout with
IncludeEventProperties=true — enumerates the object's public properties. Task<T> exposes Result,
and reading Result on an incomplete task blocks in Task.InternalWait/Monitor.Wait.

The warning is written inline in Execute's finally block on the activation's scheduler thread,
before _currentTask = null. When the log call blocks there, the thread never returns: the
activation stops consuming its queue, NonReentrancyQueueSize grows, every call to that activation
times out, and MaxRequestProcessingTime cannot recover it because no request is marked as
executing.

The captured stack confirms the path:

WorkItemGroup.LogWarningLongRunningTurn
  -> NLog JsonLayout.RenderJsonFormattedMessage
  -> DefaultJsonSerializer.SerializeObjectWithReflection
  -> ObjectReflectionCache.PropertyValue..ctor (reflects into Task<VoidTaskResult>.Result)
  -> Task.InternalWait
  -> Monitor.Wait

Change

Every Task-typed [LoggerMessage] parameter in WorkItemGroup becomes a string, and each call
site passes task.ToString():

• LogWarningLongRunningTurn keeps its existing argument expression: task.AsyncState ?? task is
  now rendered with .ToString(), so the warning still reports the async state object when one is
  set, and the task otherwise.
• The DEBUG-only LogTraceEnqueueWorkItem, LogTraceAddToRunQueue and LogTraceAboutToExecuteTask
  messages do the same for task.

Message templates, event IDs and log levels are unchanged; only the parameter types change
(object/Task to string). There is no behavioral change to scheduling, and no public API change.

Rationale

• Log content stays the same for text layouts. The value logged is ToString() of the same
  object as before, and text-based layouts (simple console, NLog ${message}) already rendered
  parameters through ToString(). Task does not override ToString() (checked on .NET 8 and
  .NET 10), so that text is the runtime type name; for an async turn it is the async state machine
  type, which still identifies the async method.
• No provider can reach Task.Result. The arguments are plain strings, so a reflection-based
  serializer has nothing to introspect.
• No cost on the normal path. The string is only built where the code already checked the level
  (the trace messages) or already decided to emit the warning (turn duration above
  TurnWarningLengthThreshold).

Structured sinks now record a string instead of an object graph for these parameters. That object
graph was exactly what could deadlock the scheduler.

Microsoft Reviewers: Open in CodeFlow

[KoakiMiku]

@dotnet-policy-service agree