fix(Delete index files): Retry policy

src/Kentico.Xperience.Lucene.Admin/UIPages/IndexListingPage.cs

@@ -178,30 +178,30 @@ public async Task<ICommandResponse<RowActionResult>> Rebuild(int id, Cancellatio
     }
 
     [PageCommand(Permission = SystemPermissions.DELETE)]
-    public Task<ICommandResponse> Delete(int id, CancellationToken _)
+    public async Task<ICommandResponse> Delete(int id, CancellationToken _)
     {
         var index = indexManager.GetIndex(id);
         var result = new RowActionResult(false);
 
         if (index is null)
         {
-            return Task.FromResult<ICommandResponse>(ResponseFrom(result)
+            return await Task.FromResult<ICommandResponse>(ResponseFrom(result)
                 .AddErrorMessage(string.Format("Error loading Lucene index with identifier {0}.", id)));
         }
 
-        bool indexDeleted = luceneClient.DeleteIndex(index!);
+        bool indexDeleted = await luceneClient.DeleteIndex(index!);
 
         if (!indexDeleted)
         {
-            return Task.FromResult<ICommandResponse>(ResponseFrom(result)
+            return await Task.FromResult<ICommandResponse>(ResponseFrom(result)
                .AddErrorMessage(string.Format("Errors occurred while deleting the '{0}' index. Please check the Event Log for more details.", index.IndexName)));
         }
 
         configurationStorageService.TryDeleteIndex(id);
 
         var response = NavigateTo(pageLinkGenerator.GetPath<IndexListingPage>());
 
-        return Task.FromResult<ICommandResponse>(response);
+        return await Task.FromResult<ICommandResponse>(response);
     }
 
     private static void AddMissingStatistics(ref ICollection<LuceneIndexStatisticsModel> statistics, ILuceneIndexManager indexManager)

src/Kentico.Xperience.Lucene.Core/Indexing/DefaultLuceneClient.cs

@@ -127,8 +127,8 @@ public Task<int> UpsertRecords(IEnumerable<Document> documents, string indexName
         return UpsertRecordsInternal(documents, indexName);
     }
 
-    public bool DeleteIndex(LuceneIndex luceneIndex) =>
-        luceneIndex?.StorageContext.DeleteIndex() ?? false;
+    public async Task<bool> DeleteIndex(LuceneIndex luceneIndex) =>
+        await luceneIndex.StorageContext.DeleteIndex();
 
     private Task<int> DeleteRecordsInternal(IEnumerable<string> itemGuids, string indexName)
     {

src/Kentico.Xperience.Lucene.Core/Indexing/ILuceneClient.cs

@@ -24,7 +24,7 @@ public interface ILuceneClient
     /// Removes Lucene index.
     /// </summary>
     /// <param name="luceneIndex">The index to be deleted.</param>
-    bool DeleteIndex(LuceneIndex luceneIndex);
+    Task<bool> DeleteIndex(LuceneIndex luceneIndex);
 
     /// <summary>
     /// Gets the indices of the Lucene application with basic statistics.

src/Kentico.Xperience.Lucene.Core/Indexing/ILuceneIndexStorageStrategy.cs

@@ -52,7 +52,7 @@ public interface ILuceneIndexStorageStrategy
     /// Deletes all files associated with an index.
     /// </summary>
     /// <param name="indexStoragePath">Path root of the index storage.</param>
-    bool DeleteIndex(string indexStoragePath);
+    Task<bool> DeleteIndex(string indexStoragePath);
 }
 
 internal class GenerationStorageStrategy : ILuceneIndexStorageStrategy
@@ -152,7 +152,7 @@ public bool ScheduleRemoval(IndexStorageModel storage)
         return true;
     }
 
-    public bool DeleteIndex(string indexStoragePath)
+    public async Task<bool> DeleteIndex(string indexStoragePath)
     {
         var deleteDir = new DirectoryInfo(indexStoragePath);
 
@@ -161,6 +161,25 @@ public bool DeleteIndex(string indexStoragePath)
             return true;
         }
 
+        int numberOfRetries = 10;
+        int millisecondsRetryDelay = 100;
+
+        for (int i = 0; i < numberOfRetries; i++)
+        {
+            try
+            {
+                Trace.WriteLine($"D={deleteDir.Name}: delete *.*", $"GenerationStorageStrategy.DeleteIndex");
+                deleteDir.Delete(true);
+                return true;
+            }
+            catch
+            {
+                // Do nothing with exception and retry.
+                // The directory may be locked by another process, but we can not know about it without trying to delete it.
+                // The exact exception is not known and is not written in .NET documentation.
+                await Task.Delay(millisecondsRetryDelay);
+            }
+        }
         try
         {
             Trace.WriteLine($"D={deleteDir.Name}: delete *.*", $"GenerationStorageStrategy.DeleteIndex");

src/Kentico.Xperience.Lucene.Core/Indexing/IndexStorageContext.cs

@@ -149,6 +149,6 @@ public void EnforceRetentionPolicy()
         storageStrategy.PerformCleanup(indexStoragePathRoot);
     }
 
-    public bool DeleteIndex() =>
-        storageStrategy.DeleteIndex(indexStoragePathRoot);
+    public async Task<bool> DeleteIndex() =>
+        await storageStrategy.DeleteIndex(indexStoragePathRoot);
 }