Fix deadlock when getting parameters in an event

When getting dynamic parameters from a thread other than the pipeline
thread, the call to CommandInfo.Parameters internally tries to get
back to it using the PowerShell EventManager. If dynamic parameters
are being obtained from an event that is being processed by the
EventManager then a deadlock will occur.

This change routes the call to CommandInfo.Parameters to the main
thread using AsyncCmdlet.ExecuteOnMainThread, bypassing the
EventManager.

src/Microsoft.PowerShell.PackageManagement/Cmdlets/CmdletWithProvider.cs

@@ -51,7 +51,7 @@ public string[] ProviderName {
                     //meaning a user specifies -ProviderName, we will use it
                     return _providerName;
                 }
-                //need to call it so that cases like get-packagesource | find-package -name Jquery will work 
+                //need to call it so that cases like get-packagesource | find-package -name Jquery will work
                 return GetDynamicParameterValue<string[]>("ProviderName");
             }
             set {
@@ -103,7 +103,7 @@ protected virtual IEnumerable<PackageProvider> SelectedProviders {
 
                     // save the resolved package source name and location
                     potentialSources.SerialForEach(src => UserSpecifiedSourcesList.AddOrSet(src.Name, src.Location));
-                
+
                     // prefer registered sources
                     registeredSources = potentialSources.Where(source => source.IsRegistered).ReEnumerable();
 
@@ -116,7 +116,7 @@ protected virtual IEnumerable<PackageProvider> SelectedProviders {
                         if (!didUserSpecifyProviders) {
                             // if cmdlet is generating parameter, we have to log error that source is wrong
                             if (IsInvocation && CmdletState >= AsyncCmdletState.GenerateParameters ) {
-                                
+
                                 // user didn't actually specify provider(s), the sources can't be tied to any particular provider
                                 QueueHeldMessage(() => Error(Constants.Errors.SourceNotFound, userSpecifiedSources.JoinWithComma()));
                                 IsFailingEarly = true;
@@ -438,19 +438,30 @@ public override bool GenerateDynamicParameters() {
                         //      any access to MyInvocation.MyCommand.*
                         //      modifying parameter validation sets
                         //
+                        Dictionary<string, ParameterMetadata> parameters = null;
+
+                        // If MyInvocation.MyCommand.Parameters is not called from the pipeline thread
+                        // then it is rerouted back to the pipeline thread via the EventManager. This
+                        // can cause a deadlock in the rare case that the EventManager is already busy.
+                        ExecuteOnMainThread(
+                            () =>
+                            {
+                                parameters = MyInvocation?.MyCommand?.Parameters;
+                                return true;
+                            }).GetAwaiter().GetResult();
 
-                        if (MyInvocation != null && MyInvocation.MyCommand != null && MyInvocation.MyCommand.Parameters != null) {
-                            GetType().AddOrSet(MyInvocation.MyCommand.Parameters, "MyInvocation.MyCommand.Parameters");
+                        if (parameters != null) {
+                            GetType().AddOrSet(parameters, "MyInvocation.MyCommand.Parameters");
                         }
 #if DEEP_DEBUG
                         else {
                             if (MyInvocation == null) {
-                                Console.WriteLine("»»» Attempt to get parameters MyInvocation == NULL");
+                                Console.WriteLine("��� Attempt to get parameters MyInvocation == NULL");
                             } else {
                                 if (MyInvocation.MyCommand == null) {
-                                    Console.WriteLine("»»» Attempt to get parameters MyCommand == NULL");
+                                    Console.WriteLine("��� Attempt to get parameters MyCommand == NULL");
                                 } else {
-                                    Console.WriteLine("»»» Attempt to get parameters Parameters == NULL");
+                                    Console.WriteLine("��� Attempt to get parameters Parameters == NULL");
                                 }
                             }
                         }
@@ -477,10 +488,10 @@ public override bool GenerateDynamicParameters() {
                                 {
                                     ProviderName = new[] { pName.ToString() };
                                 }
-                             
+
                                 // a user specifies -providerName
                                 _isUserSpecifyOneProviderName = (ProviderName.Count() == 1);
-                                
+
                             }
                         }
 
@@ -490,7 +501,7 @@ public override bool GenerateDynamicParameters() {
 
                         if (null== CachedSelectedProviders || IsFailingEarly || IsCanceled ) {
 #if DEEP_DEBUG
-                            Console.WriteLine("»»» Cancelled before we got finished doing dynamic parameters");
+                            Console.WriteLine("��� Cancelled before we got finished doing dynamic parameters");
 #endif
                             // this happens if there is a serious failure early in the cmdlet
                             // i.e. - if the SelectedProviders comes back empty (due to aggressive filtering)
@@ -574,7 +585,7 @@ private string GetSoftwareIdentityTypeName(SoftwareIdentity package)
                 return _softwareIdentityTypeName;
             }
 
-            //check if a user specifies -source with a long source name such as http://www.powershellgallery.com/api/v2, 
+            //check if a user specifies -source with a long source name such as http://www.powershellgallery.com/api/v2,
             // if so we will choose the longsource column format.
             if (!UseDefaultSourceFormat)
             {