Prevent concurrency issues when completing tasks (#5324)

* Prevent concurrency issues when completing tasks

CHANGELOG.md

@@ -30,6 +30,9 @@ _(none)_
 - feat(autoapi): Add support for non default secret providers in local workspaces
   [#5320](https://github.com/pulumi/pulumi/pull/5320)
 
+- .NET SDK: Prevent a task completion race condition
+  [#5324](https://github.com/pulumi/pulumi/pull/5324)
+
 ## 2.9.2 (2020-08-31)
 
 - Alpha version of the Automation API for Go

sdk/dotnet/Pulumi/Deployment/Deployment.Runner.cs

@@ -117,7 +117,11 @@ namespace Pulumi
                             await task.ConfigureAwait(false);
 
                             // Log the descriptions of completed tasks.
-                            var descriptions = _inFlightTasks[task];
+                            List<string> descriptions;
+                            lock (_inFlightTasks)
+                            {
+                                descriptions = _inFlightTasks[task];
+                            }
                             foreach (var description in descriptions)
                             {
                                 Serilog.Log.Information($"Completed task: {description}");