*: Fix panic for empty revisions

Signed-off-by: Paulo Gomes <pjbgf@linux.com>

repository.go

@@ -750,21 +750,20 @@ func (r *Repository) buildTagSignature(tag *object.Tag, signKey *openpgp.Entity)
 // If you want to check to see if the tag is an annotated tag, you can call
 // TagObject on the hash of the reference in ForEach:
 //
-//   ref, err := r.Tag("v0.1.0")
-//   if err != nil {
-//     // Handle error
-//   }
-//
-//   obj, err := r.TagObject(ref.Hash())
-//   switch err {
-//   case nil:
-//     // Tag object present
-//   case plumbing.ErrObjectNotFound:
-//     // Not a tag object
-//   default:
-//     // Some other error
-//   }
+//	ref, err := r.Tag("v0.1.0")
+//	if err != nil {
+//	  // Handle error
+//	}
 //
+//	obj, err := r.TagObject(ref.Hash())
+//	switch err {
+//	case nil:
+//	  // Tag object present
+//	case plumbing.ErrObjectNotFound:
+//	  // Not a tag object
+//	default:
+//	  // Some other error
+//	}
 func (r *Repository) Tag(name string) (*plumbing.Reference, error) {
 	ref, err := r.Reference(plumbing.ReferenceName(path.Join("refs", "tags", name)), false)
 	if err != nil {
@@ -1241,26 +1240,25 @@ func commitIterFunc(order LogOrder) func(c *object.Commit) object.CommitIter {
 // If you want to check to see if the tag is an annotated tag, you can call
 // TagObject on the hash Reference passed in through ForEach:
 //
-//   iter, err := r.Tags()
-//   if err != nil {
-//     // Handle error
-//   }
-//
-//   if err := iter.ForEach(func (ref *plumbing.Reference) error {
-//     obj, err := r.TagObject(ref.Hash())
-//     switch err {
-//     case nil:
-//       // Tag object present
-//     case plumbing.ErrObjectNotFound:
-//       // Not a tag object
-//     default:
-//       // Some other error
-//       return err
-//     }
-//   }); err != nil {
-//     // Handle outer iterator error
-//   }
+//	iter, err := r.Tags()
+//	if err != nil {
+//	  // Handle error
+//	}
 //
+//	if err := iter.ForEach(func (ref *plumbing.Reference) error {
+//	  obj, err := r.TagObject(ref.Hash())
+//	  switch err {
+//	  case nil:
+//	    // Tag object present
+//	  case plumbing.ErrObjectNotFound:
+//	    // Not a tag object
+//	  default:
+//	    // Some other error
+//	    return err
+//	  }
+//	}); err != nil {
+//	  // Handle outer iterator error
+//	}
 func (r *Repository) Tags() (storer.ReferenceIter, error) {
 	refIter, err := r.Storer.IterReferences()
 	if err != nil {
@@ -1424,9 +1422,13 @@ func (r *Repository) Worktree() (*Worktree, error) {
 //
 // Implemented resolvers : HEAD, branch, tag, heads/branch, refs/heads/branch,
 // refs/tags/tag, refs/remotes/origin/branch, refs/remotes/origin/HEAD, tilde and caret (HEAD~1, master~^, tag~2, ref/heads/master~1, ...), selection by text (HEAD^{/fix nasty bug}), hash (prefix and full)
-func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, error) {
-	p := revision.NewParserFromString(string(rev))
+func (r *Repository) ResolveRevision(in plumbing.Revision) (*plumbing.Hash, error) {
+	rev := in.String()
+	if rev == "" {
+		return &plumbing.ZeroHash, plumbing.ErrReferenceNotFound
+	}
 
+	p := revision.NewParserFromString(rev)
 	items, err := p.Parse()
 
 	if err != nil {
@@ -1557,6 +1559,10 @@ func (r *Repository) ResolveRevision(rev plumbing.Revision) (*plumbing.Hash, err
 		}
 	}
 
+	if commit == nil {
+		return &plumbing.ZeroHash, plumbing.ErrReferenceNotFound
+	}
+
 	return &commit.Hash, nil
 }
 

repository_test.go

@@ -2953,6 +2953,15 @@ func (s *RepositorySuite) TestDotGitToOSFilesystemsInvalidPath(c *C) {
 	c.Assert(err, NotNil)
 }
 
+func (s *RepositorySuite) TestIssue674(c *C) {
+	r, _ := Init(memory.NewStorage(), nil)
+	h, err := r.ResolveRevision(plumbing.Revision(""))
+
+	c.Assert(err, NotNil)
+	c.Assert(h, NotNil)
+	c.Check(h.IsZero(), Equals, true)
+}
+
 func BenchmarkObjects(b *testing.B) {
 	defer fixtures.Clean()