Volatile memory singleton for memory "migration" (#476)

### Motivation and Context

<!-- Thank you for your contribution to the chat-copilot repo!
Please help reviewers and future users, providing the following
information:
  1. Why is this change required?
  2. What problem does it solve?
  3. What scenario does it contribute to?
  4. If it fixes an open issue, please link to the issue here.
-->
Muliple instances of VolatileMemory do not share state.

### Contribution Checklist

<!-- Before submitting this PR, please make sure: -->

- [x] The code builds clean without any errors or warnings
- [x] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [x] All unit tests pass, and I have added new tests where possible
- [x] I didn't break anyone 😄

---------

Co-authored-by: Gil LaHaye <[email protected]>

webapi/Controllers/MaintenanceController.cs

@@ -49,7 +49,7 @@ public MaintenanceController(
 
         var migrationStatus = await migrationMonitor.GetCurrentStatusAsync(cancellationToken);
 
-        if (migrationStatus == ChatMigrationStatus.Upgrading)
+        if (migrationStatus != ChatMigrationStatus.None)
         {
             result =
                 new MaintenanceResult

webapi/Services/MemoryMigration/ChatMigrationMonitor.cs

@@ -114,15 +114,11 @@ async Task<ChatMigrationStatus> QueryStatusAsync()
             try
             {
                 var result =
-                    await this._memory.SearchAsync(
+                    await this._memory.GetAsync(
                         this._indexNameGlobalDocs,
                         MigrationKey,
-                        limit: 1,
-                        minRelevanceScore: -1,
-                        withEmbeddings: false,
-                        cancellationToken)
-                    .SingleOrDefaultAsync(cancellationToken)
-                    ;
+                        withEmbedding: false,
+                        cancellationToken);
 
                 if (result == null)
                 {

webapi/Services/SemanticKernelProvider.cs

@@ -2,6 +2,7 @@
 
 using System;
 using System.Net.Http;
+using System.Threading;
 using CopilotChat.WebApi.Options;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
@@ -22,6 +23,8 @@ namespace CopilotChat.WebApi.Services;
 /// </summary>
 public sealed class SemanticKernelProvider
 {
+    private static IMemoryStore? _volatileMemoryStore;
+
     private readonly IServiceProvider _serviceProvider;
     private readonly IConfiguration _configuration;
 
@@ -159,7 +162,9 @@ IMemoryStore CreateMemoryStore()
             switch (memoryOptions.Retrieval.VectorDbType)
             {
                 case string x when x.Equals("SimpleVectorDb", StringComparison.OrdinalIgnoreCase):
-                    return new VolatileMemoryStore();
+                    // Maintain single instance of volatile memory.
+                    Interlocked.CompareExchange(ref _volatileMemoryStore, new VolatileMemoryStore(), null);
+                    return _volatileMemoryStore;
 
                 case string x when x.Equals("Qdrant", StringComparison.OrdinalIgnoreCase):
                     var qdrantConfig = memoryOptions.GetServiceConfig<QdrantConfig>(this._configuration, "Qdrant");

webapp/src/components/views/BackendProbe.tsx

@@ -55,8 +55,7 @@ export const BackendProbe: FC<IData> = ({ onBackendFound }) => {
                     .then((data) => {
                         // Body has payload. This means the app is in maintenance
                         setModel(data as IMaintenance);
-                        clearInterval(timer);
-                        return false;
+                        return true;
                     })
                     .catch((e: any) => {
                         if (e instanceof TypeError) {