Fixed a bug in reading reviews where we would not discount commented-…

…upon commits that had been garbage collected, and as a result the tool would exit due to a git command failing unexpectedly

repository/git.go

@@ -111,6 +111,19 @@ func (repo *GitRepo) HasUncommittedChanges() bool {
 	return false
 }
 
+// VerifyCommit verifies that the supplied hash points to a known commit.
+func (repo *GitRepo) VerifyCommit(hash string) error {
+	out, err := repo.runGitCommand("cat-file", "-t", hash)
+	if err != nil {
+		return err
+	}
+	objectType := strings.TrimSpace(string(out))
+	if objectType != "commit" {
+		return fmt.Errorf("Hash %q points to a non-commit object of type %q", hash, objectType)
+	}
+	return nil
+}
+
 // VerifyGitRef verifies that the supplied ref points to a known commit.
 func (repo *GitRepo) VerifyGitRef(ref string) error {
 	_, err := repo.runGitCommand("show-ref", "--verify", ref)

repository/mock_repo.go

@@ -186,6 +186,14 @@ func (r mockRepoForTest) resolveLocalRef(ref string) (string, error) {
 	return "", fmt.Errorf("The ref %q does not exist", ref)
 }
 
+// VerifyCommit verifies that the supplied hash points to a known commit.
+func (r mockRepoForTest) VerifyCommit(hash string) error {
+	if _, ok := r.Commits[hash]; !ok {
+		return fmt.Errorf("The given hash %q is not a known commit", hash)
+	}
+	return nil
+}
+
 // VerifyGitRef verifies that the supplied ref points to a known commit.
 func (r mockRepoForTest) VerifyGitRef(ref string) error {
 	_, err := r.resolveLocalRef(ref)

repository/repo.go

@@ -43,6 +43,9 @@ type Repo interface {
 	// HasUncommittedChanges returns true if there are local, uncommitted changes.
 	HasUncommittedChanges() bool
 
+	// VerifyCommit verifies that the supplied hash points to a known commit.
+	VerifyCommit(hash string) error
+
 	// VerifyGitRef verifies that the supplied ref points to a known commit.
 	VerifyGitRef(ref string) error
 

review/review.go

@@ -300,6 +300,9 @@ func (r *Review) GetJson() (string, error) {
 // and all of the commits that are referenced in the given comment threads.
 func (r *Review) findLastCommit(latestCommit string, commentThreads []CommentThread) string {
 	isLater := func(commit string) bool {
+		if err := r.Repo.VerifyCommit(commit); err != nil {
+			return false
+		}
 		if r.Repo.IsAncestor(latestCommit, commit) {
 			return true
 		}