Introduce MergeFetchHeadNotFoundException

LibGit2Sharp.Tests/NetworkFixture.cs

@@ -196,6 +196,38 @@ public void CanPullIntoEmptyRepo()
             }
         }
 
+        [Fact]
+        public void PullWithoutMergeBranchThrows()
+        {
+            var scd = BuildSelfCleaningDirectory();
+            string clonedRepoPath = Repository.Clone(StandardTestRepoPath, scd.DirectoryPath);
+
+            using (var repo = new Repository(clonedRepoPath))
+            {
+                Branch branch = repo.Branches["master"];
+
+                // Update the Upstream merge branch
+                repo.Branches.Update(branch,
+                    b => b.UpstreamBranch = "refs/heads/another_master");
+
+                bool didPullThrow = false;
+                MergeFetchHeadNotFoundException thrownException = null;
+
+                try
+                {
+                    repo.Network.Pull(Constants.Signature, new PullOptions());
+                }
+                catch(MergeFetchHeadNotFoundException ex)
+                {
+                    didPullThrow = true;
+                    thrownException = ex;
+                }
+
+                Assert.True(didPullThrow, "Pull did not throw.");
+                Assert.True(thrownException.Message.Contains("refs/heads/another_master"), "Exception message did not contain expected reference.");
+            }
+        }
+
         /*
         * git ls-remote https://github.com/libgit2/TestGitRepository
         * 49322bb17d3acc9146f98c97d078513228bbf3c0        HEAD

LibGit2Sharp/LibGit2Sharp.csproj

@@ -124,6 +124,7 @@
     <Compile Include="PushOptions.cs" />
     <Compile Include="Core\GitBuf.cs" />
     <Compile Include="FilteringOptions.cs" />
+    <Compile Include="MergeFetchHeadNotFoundException.cs" />
     <Compile Include="ResetMode.cs" />
     <Compile Include="NoteCollectionExtensions.cs" />
     <Compile Include="RefSpecDirection.cs" />

LibGit2Sharp/MergeFetchHeadNotFoundException.cs

@@ -0,0 +1,49 @@
+using System;
+using System.Runtime.Serialization;
+using LibGit2Sharp.Core;
+
+namespace LibGit2Sharp
+{
+    /// <summary>
+    /// The exception that is thrown when the ref to merge with was as part of a pull operation not fetched.
+    /// </summary>
+    [Serializable]
+    public class MergeFetchHeadNotFoundException : NotFoundException
+    {
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MergeFetchHeadNotFoundException"/> class.
+        /// </summary>
+        public MergeFetchHeadNotFoundException()
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MergeFetchHeadNotFoundException"/> class with a specified error message.
+        /// </summary>
+        /// <param name="message">A message that describes the error.</param>
+        public MergeFetchHeadNotFoundException(string message)
+            : base(message)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MergeFetchHeadNotFoundException"/> class with a specified error message and a reference to the inner exception that is the cause of this exception.
+        /// </summary>
+        /// <param name="message">The error message that explains the reason for the exception.</param>
+        /// <param name="innerException">The exception that is the cause of the current exception. If the <paramref name="innerException"/> parameter is not a null reference, the current exception is raised in a catch block that handles the inner exception.</param>
+        public MergeFetchHeadNotFoundException(string message, Exception innerException)
+            : base(message, innerException)
+        {
+        }
+
+        /// <summary>
+        /// Initializes a new instance of the <see cref="MergeFetchHeadNotFoundException"/> class with a serialized data.
+        /// </summary>
+        /// <param name="info">The <see cref="SerializationInfo"/> that holds the serialized object data about the exception being thrown.</param>
+        /// <param name="context">The <see cref="StreamingContext"/> that contains contextual information about the source or destination.</param>
+        protected MergeFetchHeadNotFoundException(SerializationInfo info, StreamingContext context)
+            : base(info, context)
+        {
+        }
+    }
+}

LibGit2Sharp/Repository.cs

@@ -1032,7 +1032,9 @@ internal MergeResult MergeFetchHeads(Signature merger, MergeOptions options)
 
             if (fetchHeads.Length == 0)
             {
-                throw new LibGit2SharpException("Remote ref to merge from was not fetched.");
+                var expectedRef = this.Head.UpstreamBranchCanonicalName;
+                throw new MergeFetchHeadNotFoundException(string.Format(CultureInfo.InvariantCulture,
+                    "The current branch is configured to merge with the reference '{0}' from the remote, but this reference was not fetched.", expectedRef));
             }
 
             GitMergeHeadHandle[] mergeHeadHandles = fetchHeads.Select(fetchHead =>