Remote: don't download a pack if all objects are available locally

Add a Lookup 'event' for asking the plastic DB whether a commit is
available locally, letting us know whether we should even download a
packfile.

References libgit2#212, libgit2#217

LibGit2Sharp/Remote.cs

@@ -23,6 +23,7 @@ public ProgressEventArgs(PktDemux pkt)
 
     public delegate void ProgressHandler(object sender, ProgressEventArgs args);
     public delegate List<ObjectId> CommitRequestHandler(object sender);
+    public delegate bool LookupHandler(object sender, ObjectId id);
 
     /// <summary>
     ///   A remote repository whose branches are tracked.
@@ -34,8 +35,11 @@ public class Remote : IEquatable<Remote>
         public List<PktRef> Refs { get; private set; }
         public event ProgressHandler Progress;
         public event CommitRequestHandler RequestCommits;
+        public event LookupHandler Lookup;
+        public bool NeedsPack;
         private Lazy<IEnumerable<string>> refnames;
         private Lazy<IEnumerable<string>> matching_refs;
+        private IEnumerable<ObjectId> interesting;
         private Indexer indexer;
         private ProtocolCapabilities caps;
 
@@ -93,13 +97,25 @@ public void Connect()
                     throw new GitProtocolException((pkt as PktError).Error);
             }
 
+            /* See what we actually need to download */
+            if (Lookup != null)
+                interesting = Refs.Where(s => Refspec.Matches(s.Name) && !Lookup(this, s.Id)).Select(s => s.Id);
+            else
+                interesting = Refs.Select(s => s.Id);
+
+            NeedsPack = interesting.Count() != 0;
+
             /* Figure out what capabilities we have in common */
             caps = new ProtocolCapabilities(Refs[0].CapString);
         }
 
         public IndexerStats Stats
         {
             get {
+                /* In case somebody asks at the wrong time */
+                if (indexer == null)
+                    return new IndexerStats();
+
                 return indexer.Stats;
             }
         }
@@ -108,15 +124,12 @@ public IndexerStats Stats
         private void SendWants(StreamWriter writer)
         {
             bool first = true;
-            foreach (PktRef p in Refs) {
-                if (!Refspec.Matches(p.Name))
-                    continue;
-
+            foreach (ObjectId id in interesting) {
                 if (first) {
-                    writer.Write(Pkt.Line("want " + p.Id.Sha + " " + caps.Common));
+                    writer.Write(Pkt.Line("want " + id.Sha + " " + caps.Common));
                     first = false;
                 } else {
-                    writer.Write(Pkt.Line("want " + p.Id.Sha));
+                    writer.Write(Pkt.Line("want " + id.Sha));
                 }
             }
             /* Flush after the want lines */
@@ -133,9 +146,23 @@ private int SendHaves(StreamWriter writer)
             return haves.Count;
         }
 
+        /// <summary>
+        /// Download a packfile and store it in the dir specified by the path.
+        /// </summary>
+        /// <param name='path'>
+        /// The directory where the packfile and its index should be stored
+        /// </param>
+        /// <returns>
+        /// Whether a packfile was downloaded. A return value of false
+        /// means that the local repository already has all the required objects
+        /// </returns>
         public void Download(string path)
         {
             Ensure.ArgumentNotNullOrEmptyString(path, "path");
+
+            if (!NeedsPack)
+                return;
+
             if (!Directory.Exists(path))
                 Directory.CreateDirectory(path);
 
@@ -148,6 +175,7 @@ public void Download(string path)
             var writer = new StreamWriter(new MemoryStream());
 
             SendWants(writer);
+
             if (RequestCommits != null) {
                 do {
                     var n = SendHaves(writer);