Reduce logging on WFTCompletedHandler.Invoke error (#7195)

## What changed?
<!-- Describe what has changed in this PR -->
Reduce logging on `WFTCompletedHandler.Invoke` error. Now it is emitted
only when effects were actually cleared.

## Why?
<!-- Tell your future self why have you made these changes -->
Remove noisy logs.

## How did you test it?
<!-- How have you verified this change? Tested locally? Added a unit
test? Checked in staging env? -->
Didn't test.

## Potential risks
<!-- Assuming the worst case, what can be broken when deploying this
change to production? -->
No risks.

## Documentation
<!-- Have you made sure this change doesn't falsify anything currently
stated in `docs/`? If significant
new behavior is added, have you described that in `docs/`? -->
No.

## Is hotfix candidate?
<!-- Is this PR a hotfix candidate or does it require a notification to
be sent to the broader community? (Yes/No) -->
No.

---------

Co-authored-by: Stephan Behnke <[email protected]>

internal/effect/buffer.go

@@ -49,20 +49,28 @@ func (b *Buffer) OnAfterRollback(effect func(context.Context)) {
 
 // Apply invokes the buffered effect functions in the order that they were added
 // to this Buffer.
-func (b *Buffer) Apply(ctx context.Context) {
+// It returns true if any effects were applied.
+func (b *Buffer) Apply(ctx context.Context) bool {
+	applied := false
 	b.cancels = nil
 	for _, effect := range b.effects {
 		effect(ctx)
+		applied = true
 	}
 	b.effects = nil
+	return applied
 }
 
 // Cancel invokes the buffered rollback functions in the reverse of the order
 // that they were added to this Buffer.
-func (b *Buffer) Cancel(ctx context.Context) {
+// It returns true if any effects were canceled.
+func (b *Buffer) Cancel(ctx context.Context) bool {
+	canceled := false
 	b.effects = nil
 	for i := len(b.cancels) - 1; i >= 0; i-- {
 		b.cancels[i](ctx)
+		canceled = true
 	}
 	b.cancels = nil
+	return canceled
 }

service/history/api/respondworkflowtaskcompleted/api.go

@@ -237,20 +237,17 @@ func (handler *WorkflowTaskCompletedHandler) Invoke(
 
 	var effects effect.Buffer
 	defer func() {
-		// code in this file and workflowTaskHandler is inconsistent in the way
-		// errors are returned - some functions which appear to return error
-		// actually return nil in all cases and instead set a member variable
-		// that should be observed by other collaborating code (e.g.
-		// workflowtaskHandler.workflowTaskFailedCause). That made me paranoid
-		// about the way this function exits so while we have this defer here
-		// there is _also_ code to call effects.Cancel at key points.
+		// `effects` are canceled immediately on WFT failure or persistence errors.
+		// This `defer` handles rare cases where an error is returned but the cancellation didn't happen.
 		if retError != nil {
-			handler.logger.Info("Cancel effects due to error.",
-				tag.Error(retError),
-				tag.WorkflowID(token.GetWorkflowId()),
-				tag.WorkflowRunID(token.GetRunId()),
-				tag.WorkflowNamespaceID(namespaceEntry.ID().String()))
-			effects.Cancel(ctx)
+			cancelled := effects.Cancel(ctx)
+			if cancelled {
+				handler.logger.Info("Canceled effects due to error.",
+					tag.Error(retError),
+					tag.WorkflowID(token.GetWorkflowId()),
+					tag.WorkflowRunID(token.GetRunId()),
+					tag.WorkflowNamespaceID(namespaceEntry.ID().String()))
+			}
 		}
 	}()
 
@@ -518,6 +515,7 @@ func (handler *WorkflowTaskCompletedHandler) Invoke(
 	}
 
 	var newWorkflowTask *workflow.WorkflowTaskInfo
+
 	// Speculative workflow task will be created after mutable state is persisted.
 	if newWorkflowTaskType == enumsspb.WORKFLOW_TASK_TYPE_NORMAL {
 		versioningStamp := request.WorkerVersionStamp