Be more defensive in HtmlDirectoryFormatter (#9755)

Fixes aspnet/AspNetCore-Internal#2245

Our tests were flaking out due to this possible race where the directory browser middleware enumerates the directory but a file is deleted while we're formatting the response. This change just catches the FNF/DNF exceptions and suppresses the file from the directory listing

src/Middleware/StaticFiles/src/HtmlDirectoryFormatter.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System;
+using System.IO;
 using System.Collections.Generic;
 using System.Globalization;
 using System.Linq;
@@ -131,6 +132,11 @@ namespace Microsoft.AspNetCore.StaticFiles
             HtmlEncode(Resources.HtmlDir_LastModified));
 
             foreach (var subdir in contents.Where(info => info.IsDirectory))
+            {
+                // Collect directory metadata in a try...catch in case the file is deleted while we're getting the data.
+                // The metadata is retrieved prior to calling AppendFormat so if it throws, we won't have written a row
+                // to the table.
+                try
                 {
                     builder.AppendFormat(@"
       <tr class=""directory"">
@@ -141,8 +147,30 @@ namespace Microsoft.AspNetCore.StaticFiles
                         HtmlEncode(subdir.Name),
                         HtmlEncode(subdir.LastModified.ToString(CultureInfo.CurrentCulture)));
                 }
+                catch (DirectoryNotFoundException)
+                {
+                    // The physical DirectoryInfo class doesn't appear to throw for either
+                    // of Name or LastWriteTimeUtc (which backs LastModified in the physical provider)
+                    // if the directory doesn't exist. However, we don't know what other providers might do.
+
+                    // Just skip this directory. It was deleted while we were enumerating.
+                }
+                catch (FileNotFoundException)
+                {
+                    // The physical DirectoryInfo class doesn't appear to throw for either
+                    // of Name or LastWriteTimeUtc (which backs LastModified in the physical provider)
+                    // if the directory doesn't exist. However, we don't know what other providers might do.
+
+                    // Just skip this directory. It was deleted while we were enumerating.
+                }
+            }
 
             foreach (var file in contents.Where(info => !info.IsDirectory))
+            {
+                // Collect file metadata in a try...catch in case the file is deleted while we're getting the data.
+                // The metadata is retrieved prior to calling AppendFormat so if it throws, we won't have written a row
+                // to the table.
+                try
                 {
                     builder.AppendFormat(@"
       <tr class=""file"">
@@ -154,6 +182,18 @@ namespace Microsoft.AspNetCore.StaticFiles
                         HtmlEncode(file.Length.ToString("n0", CultureInfo.CurrentCulture)),
                         HtmlEncode(file.LastModified.ToString(CultureInfo.CurrentCulture)));
                 }
+                catch (DirectoryNotFoundException)
+                {
+                    // There doesn't appear to be a case where DirectoryNotFound is thrown in the physical provider,
+                    // but we don't know what other providers might do.
+
+                    // Just skip this file. It was deleted while we were enumerating.
+                }
+                catch (FileNotFoundException)
+                {
+                    // Just skip this file. It was deleted while we were enumerating.
+                }
+            }
 
             builder.Append(@"
     </tbody>