Fix null key exception

Author: JimBobSquarePants

src/ImageSharp.Web/Caching/PhysicalFileSystemCacheResolver.cs

@@ -1,6 +1,7 @@
 // Copyright (c) Six Labors.
 // Licensed under the Apache License, Version 2.0.
 
+using System;
 using System.IO;
 using System.Threading.Tasks;
 using Microsoft.Extensions.FileProviders;
@@ -13,7 +14,7 @@ namespace SixLabors.ImageSharp.Web.Resolvers
     /// </summary>
     public class PhysicalFileSystemCacheResolver : IImageCacheResolver
     {
-        private readonly IFileInfo fileInfo;
+        private readonly IFileInfo metaFileInfo;
         private readonly FormatUtilities formatUtilities;
         private ImageCacheMetadata metadata;
 
@@ -26,14 +27,14 @@ public class PhysicalFileSystemCacheResolver : IImageCacheResolver
         /// </param>
         public PhysicalFileSystemCacheResolver(IFileInfo metaFileInfo, FormatUtilities formatUtilities)
         {
-            this.fileInfo = metaFileInfo;
+            this.metaFileInfo = metaFileInfo;
             this.formatUtilities = formatUtilities;
         }
 
         /// <inheritdoc/>
         public async Task<ImageCacheMetadata> GetMetaDataAsync()
         {
-            using Stream stream = this.fileInfo.CreateReadStream();
+            using Stream stream = this.metaFileInfo.CreateReadStream();
             this.metadata = await ImageCacheMetadata.ReadAsync(stream);
             return this.metadata;
         }
@@ -42,7 +43,7 @@ public async Task<ImageCacheMetadata> GetMetaDataAsync()
         public Task<Stream> OpenReadAsync()
         {
             string path = Path.ChangeExtension(
-                this.fileInfo.PhysicalPath,
+                this.metaFileInfo.PhysicalPath,
                 this.formatUtilities.GetExtensionFromContentType(this.metadata.ContentType));
 
             return Task.FromResult<Stream>(File.OpenRead(path));

src/ImageSharp.Web/ImageCacheMetadata.cs

@@ -123,7 +123,7 @@ public static ImageCacheMetadata FromDictionary(IDictionary<string, string> dict
             return new ImageCacheMetadata(
                 sourceLastWriteTimeUtc,
                 cacheLastWriteTimeUtc,
-                contentType ?? string.Empty,
+                contentType,
                 cacheControlMaxAge,
                 contentLength);
         }
@@ -160,7 +160,8 @@ public bool Equals(ImageCacheMetadata other)
             return this.SourceLastWriteTimeUtc == other.SourceLastWriteTimeUtc
                    && this.CacheLastWriteTimeUtc == other.CacheLastWriteTimeUtc
                    && this.ContentType == other.ContentType
-                   && this.CacheControlMaxAge == other.CacheControlMaxAge;
+                   && this.CacheControlMaxAge == other.CacheControlMaxAge
+                   && this.ContentLength == other.ContentLength;
         }
 
         /// <inheritdoc/>

src/ImageSharp.Web/Middleware/ImageSharpMiddleware.cs

@@ -323,13 +323,10 @@ private async Task ProcessRequestAsync(
                     // Save the image to the cache and send the response to the caller.
                     await this.cache.SetAsync(key, outStream, cachedImageMetadata);
 
-                    // Remove the resolver from the cache so we always resolve next request.
+                    // Remove the resolver from the cache so we always resolve next request
+                    // for the same key.
                     CacheResolverLru.TryRemove(key);
 
-                    // Replace cache metadata item value.
-                    CacheMetadataLru.TryRemove(key);
-                    CacheMetadataLru.GetOrAdd(key, _ => cachedImageMetadata);
-
                     await this.SendResponseAsync(imageContext, key, cachedImageMetadata, outStream, null);
                 }
                 catch (Exception ex)
@@ -373,9 +370,10 @@ private async Task<ValueTuple<bool, ImageMetadata>> IsNewOrUpdatedAsync(
             ImageContext imageContext,
             string key)
         {
-            ImageMetadata sourceImageMetadata = default;
             using (await AsyncLock.ReaderLockAsync(key))
             {
+                ImageMetadata sourceImageMetadata = default;
+
                 // Check to see if the cache contains this image.
                 IImageCacheResolver cachedImageResolver
                     = await CacheResolverLru.GetOrAddAsync(key, async k => await this.cache.GetAsync(k));
@@ -401,9 +399,9 @@ IImageCacheResolver cachedImageResolver
                         }
                     }
                 }
-            }
 
-            return (true, sourceImageMetadata);
+                return (true, sourceImageMetadata);
+            }
         }
 
         private async Task SendResponseAsync(