Checkout branch as snapshot

3 files changed, 80 insertions, 11 deletions

diff --git a/LibGit2Sharp.Tests/CheckoutFixture.cs b/LibGit2Sharp.Tests/CheckoutFixture.cs
index 6df94fe6..3b219f3e 100644
--- a/LibGit2Sharp.Tests/CheckoutFixture.cs
+++ b/LibGit2Sharp.Tests/CheckoutFixture.cs
@@ -233,10 +233,10 @@ namespace LibGit2Sharp.Tests
 
                 // Assert that normal checkout throws exception
                 // for the conflict.
-                Assert.Throws<LibGit2SharpException>(() => master.Checkout());
+                Assert.Throws<LibGit2SharpException>(() => repo.Checkout(master.CanonicalName));
 
                 // Checkout with force option should succeed.
-                master.Checkout(CheckoutOptions.Force, null);
+                repo.Checkout(master.CanonicalName, CheckoutOptions.Force, null);
 
                 // Assert that master branch is checked out.
                 Assert.True(repo.Branches["master"].IsCurrentRepositoryHead);
@@ -504,6 +504,56 @@ namespace LibGit2Sharp.Tests
             }
         }
 
+        [Fact]
+        public void CheckoutBranchSnapshot()
+        {
+            SelfCleaningDirectory scd = BuildSelfCleaningDirectory();
+
+            using (var repo = Repository.Init(scd.DirectoryPath))
+            {
+                PopulateBasicRepository(repo);
+
+                // Get the current status of master
+                // and the current tip.
+                Branch initial = repo.Branches["master"];
+                Commit initialCommit = initial.Tip;
+
+                // Add commit to master
+                string fullPath = Path.Combine(repo.Info.WorkingDirectory, originalFilePath);
+                File.WriteAllText(fullPath, "Update : hello from master branch!\n");
+                repo.Index.Stage(fullPath);
+                repo.Commit("2nd commit", Constants.Signature, Constants.Signature);
+
+                initial.Checkout();
+
+                // Head should point at initial commit.
+                Assert.Equal(repo.Head.Tip, initialCommit);
+                Assert.False(repo.Index.RetrieveStatus().IsDirty);
+                // Verify that HEAD is detached.
+                Assert.Equal(repo.Refs["HEAD"].TargetIdentifier, initial.Tip.Sha);
+            }
+        }
+
+        [Fact]
+        public void CheckingOutRemoteBranchResultsInDetachedHead()
+        {
+            TemporaryCloneOfTestRepo path = BuildTemporaryCloneOfTestRepo(StandardTestRepoWorkingDirPath);
+            using (var repo = new Repository(path.RepositoryPath))
+            {
+                Branch master = repo.Branches["master"];
+                Assert.True(master.IsCurrentRepositoryHead);
+
+                // Set the working directory to the current head
+                ResetAndCleanWorkingDirectory(repo);
+
+                repo.Checkout("refs/remotes/origin/master");
+
+                // Verify that HEAD is detached.
+                Assert.Equal(repo.Refs["HEAD"].TargetIdentifier, repo.Branches["origin/master"].Tip.Sha);
+
+            }
+        }
+
         /// <summary>
         ///   Helper method to populate a simple repository with
         ///   a single file and two branches.
diff --git a/LibGit2Sharp/Branch.cs b/LibGit2Sharp/Branch.cs
index 8744b2ff..da7e8dd7 100644
--- a/LibGit2Sharp/Branch.cs
+++ b/LibGit2Sharp/Branch.cs
@@ -189,7 +189,10 @@ namespace LibGit2Sharp
         }
 
         /// <summary>
-        ///   Checkout this branch.
+        ///   Checkout the tip commit of this <see cref = "Branch" /> object.
+        ///   If this commit is the current tip of the branch, will checkout
+        ///   the named branch. Otherwise, will checkout the tip commit as a
+        ///   detached HEAD.
         /// </summary>
         public virtual void Checkout()
         {
@@ -197,7 +200,10 @@ namespace LibGit2Sharp
         }
 
         /// <summary>
-        ///   Checkout this branch with a callback for progress reporting.
+        ///   Checkout the tip commit of this <see cref = "Branch" /> object
+        ///   with a callback for progress reporting. If this commit is the
+        ///   current tip of the branch, will checkout the named branch. Otherwise,
+        ///   will checkout the tip commit as a detached HEAD.
         /// </summary>
         /// <param name="checkoutOptions">Options controlling checkout behavior.</param>
         /// <param name="onCheckoutProgress">Callback method to report checkout progress updates through.</param>
diff --git a/LibGit2Sharp/Repository.cs b/LibGit2Sharp/Repository.cs
index 668da5bd..08452cf3 100644
--- a/LibGit2Sharp/Repository.cs
+++ b/LibGit2Sharp/Repository.cs
@@ -517,7 +517,9 @@ namespace LibGit2Sharp
         }
 
         /// <summary>
-        ///   Checkout the specified <see cref = "Branch" />.
+        ///   Checkout the tip commit of the specified <see cref = "Branch" /> object. If this commit is the
+        ///   current tip of the branch, will checkout the named branch. Otherwise, will checkout the tip commit
+        ///   as a detached HEAD.
         /// </summary>
         /// <param name="branch">The <see cref = "Branch" /> to check out. </param>
         /// <param name="checkoutOptions"><see cref = "CheckoutOptions" /> controlling checkout behavior.</param>
@@ -527,14 +529,25 @@ namespace LibGit2Sharp
         {
             Ensure.ArgumentNotNull(branch, "branch");
 
-            // Get the current tip commit of the branch, instead of
-            // relying on branch.Tip, which references the commit at
-            // the time the passed in branch object was created.
-            Commit commit = LookupCommit(branch.CanonicalName);
-            CheckoutTree(commit.Tree, checkoutOptions, onCheckoutProgress);
+            // Make sure this is not an unborn branch.
+            if (branch.Tip.Tree == null)
+            {
+                throw new Exception("branch tip is null, nothing to checkout.");
+            }
+
+            CheckoutTree(branch.Tip.Tree, checkoutOptions, onCheckoutProgress);
 
             // Update HEAD.
-            Refs.UpdateTarget("HEAD", branch.CanonicalName);
+            if (!branch.IsRemote &&
+                string.Equals(Refs[branch.CanonicalName].TargetIdentifier, branch.Tip.Id.Sha,
+                StringComparison.OrdinalIgnoreCase))
+            {
+                Refs.UpdateTarget("HEAD", branch.CanonicalName);
+            }
+            else
+            {
+                Refs.UpdateTarget("HEAD", branch.Tip.Id.Sha);
+            }
 
             return Head;
         }