Protocol: support error messages from the remote

Recognize the error messages from the remote and throw an exception
with the message if we receive an error. The code is messy, but a
networ stream doesn't support peeking, so we need to read the first
four bytes to see if they contain "ERR " and pass them to PktRef() if
they don't, because we just removed part of the commit's hash from the
stream.

References libgit2#210

Author: carlosmn

LibGit2Sharp/GitProtocolException.cs

@@ -0,0 +1,21 @@
+using System;
+namespace LibGit2Sharp
+{
+    public class GitProtocolException : Exception
+    {
+        public GitProtocolException()
+        {
+        }
+
+        public GitProtocolException(string message)
+            : base(message)
+        {
+        }
+
+        public GitProtocolException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+    }
+}
+

LibGit2Sharp/GitTransport.cs

@@ -45,6 +45,9 @@ public void Connect()
 
                 if (pkt is PktRef)
                     Refs.Add(pkt as PktRef);
+
+                if (pkt is PktError)
+                    throw new GitProtocolException((pkt as PktError).Error);
             }
         }
 

LibGit2Sharp/LibGit2Sharp.csproj

@@ -166,6 +166,7 @@
     <Compile Include="ITransport.cs" />
     <Compile Include="GitTransport.cs" />
     <Compile Include="Refspec.cs" />
+    <Compile Include="GitProtocolException.cs" />
   </ItemGroup>
   <ItemGroup>
     <CodeAnalysisDictionary Include="CustomDictionary.xml" />

LibGit2Sharp/Pkt.cs

@@ -15,22 +15,39 @@ static public Pkt Parse(BinaryReader reader)
 
             len -= 4;
 
-            return new PktRef(reader, len);
+            return NewPkt(reader, len);
+        }
+
+        /*
+         * We can't seek on a NetworkStream, which means that we can't use
+         * reader.PeekChar() to see if we got an error (which starts with
+         * "ERR " instead of a hash) so we need this new function that chooses
+         * which type to create
+         */
+        public static Pkt NewPkt(BinaryReader reader, int len)
+        {
+            var mayerr = Encoding.ASCII.GetString(reader.ReadBytes(4));
+            len -= 4;
+            if (mayerr.Equals("ERR "))
+                return new PktError(reader, len);
+
+            return new PktRef(reader, len, mayerr);
         }
     }
 
+
     public class PktRef : Pkt
     {
         public readonly ObjectId Id;
         public readonly string Name;
         public readonly string CapString;
 
-        public PktRef(BinaryReader reader, int len)
+        public PktRef(BinaryReader reader, int len, string pre)
         {
-            var buf = reader.ReadBytes(40);
-            len -= 40; /* The hash */
-            var hash = Encoding.ASCII.GetString(buf);
-            ObjectId.TryParse(hash, out Id);
+            int toread = 40 - pre.Length;
+            var hash = Encoding.ASCII.GetString(reader.ReadBytes(toread));
+            len -= toread; /* The hash */
+            ObjectId.TryParse(pre + hash, out Id);
             reader.ReadByte(); /* Skip the SP */
             len -= 1; /* The SP */
 
@@ -55,6 +72,17 @@ public PktRef(BinaryReader reader, int len)
         }
     }
 
+    public class PktError : Pkt
+    {
+        public readonly string Error;
+
+        public PktError(BinaryReader reader, int len)
+        {
+            /* NewPkt() has already removed "ERR " from the stream */
+            Error = Encoding.UTF8.GetString(reader.ReadBytes(len));
+        }
+    }
+
     public class PktFlush : Pkt
     {
     }