Use ETag for conditional requests. Fixes #2.

The URIs and ETags of all GitHub API responses that are used to construct the feed are hashed together to make the ETag for the feed itself.

This doesn't save any processing time on the server, but benefits the client by allowing it to easily reuse its own cached data.

src/GitHubFeeds/Controllers/CommentsController.cs

@@ -5,6 +5,7 @@
 using System.IO;
 using System.Linq;
 using System.Net;
+using System.Security.Cryptography;
 using System.ServiceModel.Syndication;
 using System.Text;
 using System.Text.RegularExpressions;
@@ -23,14 +24,15 @@ public void ListAsync(ListParameters p)
 		{
 			AsyncManager.OutstandingOperations.Increment();
 
+			ListData data = new ListData { RequestETag = Request.Headers["If-None-Match"] };
 			Uri uri = CreateUri(p, 1, 1);
 			HttpWebRequest request = CreateRequest(p, uri);
 			request.GetHttpResponseAsync()
-				.ContinueWith(t => GetCommentCount(p, t))
+				.ContinueWith(t => GetCommentCount(t))
 				.ContinueWith(t => GetCommentPages(p, t.Result))
-				.ContinueWith(t => Task.Factory.ContinueWhenAll(t.Result, ts => GetComments(p, ts))).Unwrap()
+				.ContinueWith(t => Task.Factory.ContinueWhenAll(t.Result, ts => GetComments(data, ts))).Unwrap()
 				.ContinueWith(t => CreateFeed(p, t.Result))
-				.ContinueWith(t => CreateResult(p, t))
+				.ContinueWith(t => CreateResult(data, t))
 				.ContinueWith(t =>
 				{
 					AsyncManager.Parameters["result"] = t.Result;
@@ -66,7 +68,7 @@ private static HttpWebRequest CreateRequest(ListParameters p, Uri uri)
 		}
 
 		// Gets the number of comments for a repo.
-		private static int GetCommentCount(ListParameters p, Task<HttpWebResponse> responseTask)
+		private static int GetCommentCount(Task<HttpWebResponse> responseTask)
 		{
 			string linkHeader;
 			using (HttpWebResponse response = responseTask.Result)
@@ -120,8 +122,11 @@ private static Task<HttpWebResponse>[] GetCommentPages(ListParameters p, int com
 		}
 
 		// Merges the comments returned from multiple HTTP requests and returns the last 50.
-		private static List<GitHubComment> GetComments(ListParameters p, Task<HttpWebResponse>[] tasks)
+		private static List<GitHubComment> GetComments(ListData data, Task<HttpWebResponse>[] tasks)
 		{
+			// concatenate all the response URIs and ETags; we will use this to build our own ETag
+			var responseETags = new List<string>();
+
 			// download comments as JSON and deserialize them
 			List<GitHubComment> comments = new List<GitHubComment>();
 			foreach (Task<HttpWebResponse> task in tasks)
@@ -131,6 +136,11 @@ private static List<GitHubComment> GetComments(ListParameters p, Task<HttpWebRes
 					if (response.StatusCode != HttpStatusCode.OK)
 						throw new ApplicationException("GitHub server returned " + response.StatusCode);
 
+					// if the response has an ETag, add it to the list of all ETags
+					string eTag = response.Headers[HttpResponseHeader.ETag];
+					if (!string.IsNullOrEmpty(eTag))
+						responseETags.Add(response.ResponseUri.AbsoluteUri + ":" + eTag);
+
 					// TODO: Use asynchronous reads on this asynchronous stream
 					// TODO: Read encoding from Content-Type header; don't assume UTF-8
 					using (Stream stream = response.GetResponseStream())
@@ -139,6 +149,21 @@ private static List<GitHubComment> GetComments(ListParameters p, Task<HttpWebRes
 				}
 			}
 
+			// if each response had an ETag, build our own ETag from that data
+			if (responseETags.Count == tasks.Length)
+			{
+				// concatenate all the ETag data
+				string eTagData = responseETags.Join("\n");
+
+				// hash it
+				byte[] md5;
+				using (MD5 hash = MD5.Create())
+					md5 = hash.ComputeHash(Encoding.UTF8.GetBytes(eTagData));
+
+				// the ETag is the quoted MD5 hash
+				data.ResponseETag = "\"" + string.Join("", md5.Select(by => by.ToString("x2", CultureInfo.InvariantCulture))) + "\"";
+			}
+
 			return comments
 				.OrderByDescending(c => c.created_at)
 				.Take(50)
@@ -187,19 +212,25 @@ private static string CreateCommentHtml(GitHubComment comment)
 				HttpUtility.HtmlEncode(comment.commit_id.Substring(0, 8)), comment.body_html);
 		}
 
-		private ActionResult CreateResult(ListParameters p, Task<SyndicationFeed> feedTask)
+		private ActionResult CreateResult(ListData data, Task<SyndicationFeed> feedTask)
 		{
 			if (feedTask.IsFaulted)
 				return new HttpStatusCodeResult((int) HttpStatusCode.InternalServerError, feedTask.Exception.Message);
 
-			SyndicationFeed feed = feedTask.Result;
-			Response.AppendHeader("Last-Modified", feed.LastUpdatedTime.ToString("r"));
+			if (data.ResponseETag != null)
+				Response.AppendHeader("ETag", data.ResponseETag);
 
-			DateTime ifModifiedSince;
-			if (DateTime.TryParse(Request.Headers["If-Modified-Since"], out ifModifiedSince) && ifModifiedSince >= feed.LastUpdatedTime)
+			if (data.RequestETag == data.ResponseETag)
 				return new HttpStatusCodeResult((int) HttpStatusCode.NotModified);
 
 			return new SyndicationFeedAtomResult(feedTask.Result);			
 		}
+
+		// ListData contains private state that needs to be communicated between the various async methods invoked by List.
+		private sealed class ListData
+		{
+			public string RequestETag { get; set; }
+			public string ResponseETag { get; set; }
+		}
 	}
 }