perf(images): support conditional ETag responses

allstarr.Tests/ImageConditionalRequestHelperTests.cs

@@ -0,0 +1,64 @@
+using Microsoft.AspNetCore.Http;
+using allstarr.Services.Common;
+
+namespace allstarr.Tests;
+
+public class ImageConditionalRequestHelperTests
+{
+    [Fact]
+    public void ComputeStrongETag_SamePayload_ReturnsStableQuotedHash()
+    {
+        var payload = new byte[] { 1, 2, 3, 4 };
+
+        var first = ImageConditionalRequestHelper.ComputeStrongETag(payload);
+        var second = ImageConditionalRequestHelper.ComputeStrongETag(payload);
+
+        Assert.Equal(first, second);
+        Assert.StartsWith("\"", first);
+        Assert.EndsWith("\"", first);
+    }
+
+    [Fact]
+    public void MatchesIfNoneMatch_WithExactMatch_ReturnsTrue()
+    {
+        var headers = new HeaderDictionary
+        {
+            ["If-None-Match"] = "\"ABC123\""
+        };
+
+        Assert.True(ImageConditionalRequestHelper.MatchesIfNoneMatch(headers, "\"ABC123\""));
+    }
+
+    [Fact]
+    public void MatchesIfNoneMatch_WithMultipleValues_ReturnsTrueForMatchingEntry()
+    {
+        var headers = new HeaderDictionary
+        {
+            ["If-None-Match"] = "\"stale\", \"fresh\""
+        };
+
+        Assert.True(ImageConditionalRequestHelper.MatchesIfNoneMatch(headers, "\"fresh\""));
+    }
+
+    [Fact]
+    public void MatchesIfNoneMatch_WithWildcard_ReturnsTrue()
+    {
+        var headers = new HeaderDictionary
+        {
+            ["If-None-Match"] = "*"
+        };
+
+        Assert.True(ImageConditionalRequestHelper.MatchesIfNoneMatch(headers, "\"anything\""));
+    }
+
+    [Fact]
+    public void MatchesIfNoneMatch_WithoutMatch_ReturnsFalse()
+    {
+        var headers = new HeaderDictionary
+        {
+            ["If-None-Match"] = "\"ABC123\""
+        };
+
+        Assert.False(ImageConditionalRequestHelper.MatchesIfNoneMatch(headers, "\"XYZ789\""));
+    }
+}

allstarr/Controllers/JellyfinController.cs

@@ -753,7 +753,7 @@ public partial class JellyfinController : ControllerBase
 
                         if (fallbackBytes != null && fallbackContentType != null)
                         {
-                            return File(fallbackBytes, fallbackContentType);
+                            return CreateConditionalImageResponse(fallbackBytes, fallbackContentType);
                         }
                     }
                 }
@@ -762,7 +762,7 @@ public partial class JellyfinController : ControllerBase
                 return await GetPlaceholderImageAsync();
             }
 
-            return File(imageBytes, contentType);
+            return CreateConditionalImageResponse(imageBytes, contentType);
         }
 
         // Check Redis cache for previously fetched external image
@@ -771,7 +771,7 @@ public partial class JellyfinController : ControllerBase
         if (cachedImageBytes != null)
         {
             _logger.LogDebug("Cache hit for external {Type} image: {Provider}/{ExternalId}", type, provider, externalId);
-            return File(cachedImageBytes, "image/jpeg");
+            return CreateConditionalImageResponse(cachedImageBytes, "image/jpeg");
         }
 
         // Get external cover art URL
@@ -842,7 +842,7 @@ public partial class JellyfinController : ControllerBase
 
             _logger.LogDebug("Successfully fetched and cached external image from host {Host}, size: {Size} bytes",
                 safeCoverUri.Host, imageBytes.Length);
-            return File(imageBytes, "image/jpeg");
+            return CreateConditionalImageResponse(imageBytes, "image/jpeg");
         }
         catch (Exception ex)
         {
@@ -864,7 +864,7 @@ public partial class JellyfinController : ControllerBase
         if (System.IO.File.Exists(placeholderPath))
         {
             var imageBytes = await System.IO.File.ReadAllBytesAsync(placeholderPath);
-            return File(imageBytes, "image/png");
+            return CreateConditionalImageResponse(imageBytes, "image/png");
         }
 
         // Fallback: Return a 1x1 transparent PNG as minimal placeholder
@@ -872,7 +872,20 @@ public partial class JellyfinController : ControllerBase
             "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg=="
         );
 
-        return File(transparentPng, "image/png");
+        return CreateConditionalImageResponse(transparentPng, "image/png");
+    }
+
+    private IActionResult CreateConditionalImageResponse(byte[] imageBytes, string contentType)
+    {
+        var etag = ImageConditionalRequestHelper.ComputeStrongETag(imageBytes);
+        Response.Headers["ETag"] = etag;
+
+        if (ImageConditionalRequestHelper.MatchesIfNoneMatch(Request.Headers, etag))
+        {
+            return StatusCode(StatusCodes.Status304NotModified);
+        }
+
+        return File(imageBytes, contentType);
     }
 
     private async Task<string?> ResolveCurrentSpotifyPlaylistImageTagAsync(string itemId, string imageType)

allstarr/Services/Common/ImageConditionalRequestHelper.cs

@@ -0,0 +1,39 @@
+using System.Security.Cryptography;
+using Microsoft.AspNetCore.Http;
+
+namespace allstarr.Services.Common;
+
+public static class ImageConditionalRequestHelper
+{
+    public static string ComputeStrongETag(byte[] payload)
+    {
+        var hash = SHA256.HashData(payload);
+        return $"\"{Convert.ToHexString(hash)}\"";
+    }
+
+    public static bool MatchesIfNoneMatch(IHeaderDictionary headers, string etag)
+    {
+        if (!headers.TryGetValue("If-None-Match", out var headerValues))
+        {
+            return false;
+        }
+
+        foreach (var headerValue in headerValues)
+        {
+            if (string.IsNullOrEmpty(headerValue))
+            {
+                continue;
+            }
+
+            foreach (var candidate in headerValue.Split(',', StringSplitOptions.RemoveEmptyEntries | StringSplitOptions.TrimEntries))
+            {
+                if (candidate == "*" || string.Equals(candidate, etag, StringComparison.Ordinal))
+                {
+                    return true;
+                }
+            }
+        }
+
+        return false;
+    }
+}