Ensure Tags can be created in detached Head state

Fix #791

Author: nulltoken

LibGit2Sharp.Tests/ResetHeadFixture.cs

@@ -18,7 +18,7 @@ public void ResetANewlyInitializedRepositoryThrows(bool isBare)
 
             using (var repo = new Repository(repoPath))
             {
-                Assert.Throws<LibGit2SharpException>(() => repo.Reset(ResetMode.Soft));
+                Assert.Throws<UnbornBranchException>(() => repo.Reset(ResetMode.Soft));
             }
         }
 

LibGit2Sharp.Tests/ResetIndexFixture.cs

@@ -25,7 +25,7 @@ public void ResetANewlyInitializedNonBareRepositoryThrows()
 
             using (var repo = new Repository(repoPath))
             {
-                Assert.Throws<LibGit2SharpException>(() => repo.Reset());
+                Assert.Throws<UnbornBranchException>(() => repo.Reset());
             }
         }
 

LibGit2Sharp.Tests/TagFixture.cs

@@ -234,7 +234,7 @@ public void CreatingATagInAEmptyRepositoryThrows()
 
             using (var repo = new Repository(repoPath))
             {
-                Assert.Throws<LibGit2SharpException>(() => repo.ApplyTag("mynotag"));
+                Assert.Throws<UnbornBranchException>(() => repo.ApplyTag("mynotag"));
             }
         }
 
@@ -246,7 +246,8 @@ public void CreatingATagForHeadInAEmptyRepositoryThrows()
 
             using (var repo = new Repository(repoPath))
             {
-                Assert.Throws<LibGit2SharpException>(() => repo.ApplyTag("mytaghead", "HEAD"));
+                Assert.Throws<UnbornBranchException>(() => repo.ApplyTag("mytaghead", "HEAD"));
+                Assert.Throws<UnbornBranchException>(() => repo.ApplyTag("mytaghead"));
             }
         }
 
@@ -287,6 +288,26 @@ public void CanAddATagForImplicitHead()
             }
         }
 
+        [Fact]
+        public void CanAddATagForImplicitHeadInDetachedState()
+        {
+            string path = CloneStandardTestRepo();
+            using (var repo = new Repository(path))
+            {
+                repo.Checkout(repo.Head.Tip);
+
+                Assert.True(repo.Info.IsHeadDetached);
+
+                Tag tag = repo.ApplyTag("mytag");
+                Assert.NotNull(tag);
+
+                Assert.Equal(repo.Head.Tip.Id, tag.Target.Id);
+
+                Tag retrievedTag = repo.Tags[tag.CanonicalName];
+                Assert.Equal(retrievedTag, tag);
+            }
+        }
+
         [Fact]
         // Ported from cgit (https://github.com/git/git/blob/1c08bf50cfcf924094eca56c2486a90e2bf1e6e2/t/t7004-tag.sh#L87)
         public void CreatingADuplicateTagThrows()

LibGit2Sharp/Core/Ensure.cs

@@ -209,13 +209,32 @@ public static void ArgumentConformsTo<T>(T argumentValue, Func<T, bool> checker,
         }
 
         public static void GitObjectIsNotNull(GitObject gitObject, string identifier)
+        {
+            Func<string, LibGit2SharpException> exceptionBuilder;
+
+            if (string.Equals("HEAD", identifier, StringComparison.Ordinal))
+            {
+                exceptionBuilder = m => new UnbornBranchException(m);
+            }
+            else
+            {
+                exceptionBuilder = m => new LibGit2SharpException(m);
+            }
+
+            GitObjectIsNotNull(gitObject, identifier, exceptionBuilder);
+        }
+
+        public static void GitObjectIsNotNull(
+            GitObject gitObject,
+            string identifier,
+            Func<string, LibGit2SharpException> exceptionBuilder)
         {
             if (gitObject != null)
             {
                 return;
             }
 
-            throw new LibGit2SharpException(string.Format(CultureInfo.InvariantCulture,
+            throw exceptionBuilder(string.Format(CultureInfo.InvariantCulture,
                                                      "No valid git object identified by '{0}' exists in the repository.",
                                                      identifier));
         }

LibGit2Sharp/RepositoryExtensions.cs

@@ -68,7 +68,7 @@ private static void EnsureNoGitLink<T>() where T : GitObject
         /// <param name="tagName">The name of the tag to create.</param>
         public static Tag ApplyTag(this IRepository repository, string tagName)
         {
-            return ApplyTag(repository, tagName, repository.Head.CanonicalName);
+            return repository.Tags.Add(tagName, RetrieveHeadCommit(repository));
         }
 
         /// <summary>
@@ -91,7 +91,16 @@ public static Tag ApplyTag(this IRepository repository, string tagName, string o
         /// <param name="message">The annotation message.</param>
         public static Tag ApplyTag(this IRepository repository, string tagName, Signature tagger, string message)
         {
-            return ApplyTag(repository, tagName, repository.Head.CanonicalName, tagger, message);
+            return repository.Tags.Add(tagName, RetrieveHeadCommit(repository), tagger, message);
+        }
+
+        private static Commit RetrieveHeadCommit(IRepository repository)
+        {
+            Commit commit = repository.Head.Tip;
+
+            Ensure.GitObjectIsNotNull(commit, "HEAD", m => new UnbornBranchException(m));
+
+            return commit;
         }
 
         /// <summary>