remote: pull refactor to match default behavior of cgit

COMPATIBILITY.md

@@ -10,7 +10,7 @@ is supported by go-git.
 | config                                | ✔ | Reading and modifying per-repository configuration (`.git/config`) is supported. Global configuration (`$HOME/.gitconfig`) is not. |
 | **getting and creating repositories** |
 | init                                  | ✔ | Plain init and `--bare` are supported. Flags `--template`, `--separate-git-dir` and `--shared` are not. |
-| clone                                 | ✔ | Plain clone and equivalents to `--progress`,  `--single-branch`, `--depth`, `--origin`, `--recurse-submodules` are supported. Others are not. | 
+| clone                                 | ✔ | Plain clone and equivalents to `--progress`,  `--single-branch`, `--depth`, `--origin`, `--recurse-submodules` are supported. Others are not. |
 | **basic snapshotting** |
 | add                                   | ✔ | Plain add is supported. Any other flag aren't supported |
 | status                                | ✔ |
@@ -27,7 +27,7 @@ is supported by go-git.
 | tag                                   | ✔ |
 | **sharing and updating projects** |
 | fetch                                 | ✔ |
-| pull                                  | ✔ |
+| pull                                  | ✔ | Only supports merges where the merge can be resolved as a fast-forward. |
 | push                                  | ✔ |
 | remote                                | ✔ |
 | submodule                             | ✔ |
@@ -108,4 +108,4 @@ is supported by go-git.
 | gitattributes                         | ✖ |
 | index version                         | |
 | packfile version                      | |
-| push-certs                            | ✖ |
+| push-certs                            | ✖ |

worktree.go

@@ -15,6 +15,7 @@ import (
 	"gopkg.in/src-d/go-git.v4/plumbing/filemode"
 	"gopkg.in/src-d/go-git.v4/plumbing/format/index"
 	"gopkg.in/src-d/go-git.v4/plumbing/object"
+	"gopkg.in/src-d/go-git.v4/plumbing/storer"
 	"gopkg.in/src-d/go-git.v4/utils/ioutil"
 	"gopkg.in/src-d/go-git.v4/utils/merkletrie"
 
@@ -36,6 +37,8 @@ type Worktree struct {
 // Pull incorporates changes from a remote repository into the current branch.
 // Returns nil if the operation is successful, NoErrAlreadyUpToDate if there are
 // no changes to be fetched, or an error.
+//
+// Pull only supports merges where the can be resolved as a fast-forward.
 func (w *Worktree) Pull(o *PullOptions) error {
 	return w.PullContext(context.Background(), o)
 }
@@ -44,6 +47,8 @@ func (w *Worktree) Pull(o *PullOptions) error {
 // branch. Returns nil if the operation is successful, NoErrAlreadyUpToDate if
 // there are no changes to be fetched, or an error.
 //
+// Pull only supports merges where the can be resolved as a fast-forward.
+//
 // The provided Context must be non-nil. If the context expires before the
 // operation is complete, an error is returned. The context only affects to the
 // transport operations.
@@ -52,17 +57,55 @@ func (w *Worktree) PullContext(ctx context.Context, o *PullOptions) error {
 		return err
 	}
 
-	head, err := w.r.fetchAndUpdateReferences(ctx, &FetchOptions{
+	remote, err := w.r.Remote(o.RemoteName)
+	if err != nil {
+		return err
+	}
+
+	fetchHead, err := remote.fetch(ctx, &FetchOptions{
 		RemoteName: o.RemoteName,
 		Depth:      o.Depth,
 		Auth:       o.Auth,
 		Progress:   o.Progress,
-	}, o.ReferenceName)
+	})
+
+	updated := true
+	if err == NoErrAlreadyUpToDate {
+		updated = false
+	} else if err != nil {
+		return err
+	}
+
+	ref, err := storer.ResolveReference(fetchHead, o.ReferenceName)
 	if err != nil {
 		return err
 	}
 
-	if err := w.Reset(&ResetOptions{Commit: head.Hash()}); err != nil {
+	head, err := w.r.Head()
+	if err == nil {
+		if !updated && head.Hash() == ref.Hash() {
+			return NoErrAlreadyUpToDate
+		}
+
+		ff, err := isFastForward(w.r.Storer, head.Hash(), ref.Hash())
+		if err != nil {
+			return err
+		}
+
+		if !ff {
+			return fmt.Errorf("non-fast-forward update")
+		}
+	}
+
+	if err != nil && err != plumbing.ErrReferenceNotFound {
+		return err
+	}
+
+	if err := w.updateHEAD(ref.Hash()); err != nil {
+		return err
+	}
+
+	if err := w.Reset(&ResetOptions{Commit: ref.Hash()}); err != nil {
 		return err
 	}
 

worktree_test.go

@@ -51,6 +51,66 @@ func (s *WorktreeSuite) TestPullCheckout(c *C) {
 	c.Assert(fi, HasLen, 8)
 }
 
+func (s *WorktreeSuite) TestPullFastForward(c *C) {
+	url := c.MkDir()
+	path := fixtures.Basic().ByTag("worktree").One().Worktree().Root()
+
+	server, err := PlainClone(url, false, &CloneOptions{
+		URL: path,
+	})
+
+	r, err := PlainClone(c.MkDir(), false, &CloneOptions{
+		URL: url,
+	})
+
+	w, err := server.Worktree()
+	c.Assert(err, IsNil)
+	err = ioutil.WriteFile(filepath.Join(path, "foo"), []byte("foo"), 0755)
+	c.Assert(err, IsNil)
+	hash, err := w.Commit("foo", &CommitOptions{Author: defaultSignature()})
+	c.Assert(err, IsNil)
+
+	w, err = r.Worktree()
+	c.Assert(err, IsNil)
+
+	err = w.Pull(&PullOptions{})
+	c.Assert(err, IsNil)
+
+	head, err := r.Head()
+	c.Assert(err, IsNil)
+	c.Assert(head.Hash(), Equals, hash)
+}
+
+func (s *WorktreeSuite) TestPullNonFastForward(c *C) {
+	url := c.MkDir()
+	path := fixtures.Basic().ByTag("worktree").One().Worktree().Root()
+
+	server, err := PlainClone(url, false, &CloneOptions{
+		URL: path,
+	})
+
+	r, err := PlainClone(c.MkDir(), false, &CloneOptions{
+		URL: url,
+	})
+
+	w, err := server.Worktree()
+	c.Assert(err, IsNil)
+	err = ioutil.WriteFile(filepath.Join(path, "foo"), []byte("foo"), 0755)
+	c.Assert(err, IsNil)
+	_, err = w.Commit("foo", &CommitOptions{Author: defaultSignature()})
+	c.Assert(err, IsNil)
+
+	w, err = r.Worktree()
+	c.Assert(err, IsNil)
+	err = ioutil.WriteFile(filepath.Join(path, "bar"), []byte("bar"), 0755)
+	c.Assert(err, IsNil)
+	_, err = w.Commit("bar", &CommitOptions{Author: defaultSignature()})
+	c.Assert(err, IsNil)
+
+	err = w.Pull(&PullOptions{})
+	c.Assert(err, ErrorMatches, "non-fast-forward update")
+}
+
 func (s *WorktreeSuite) TestPullUpdateReferencesIfNeeded(c *C) {
 	r, _ := Init(memory.NewStorage(), memfs.New())
 	r.CreateRemote(&config.RemoteConfig{