Add function that returns if PR is locked

This function returns whether a specific pull request for a project is
locked or not.

server/events/working_dir_locker.go

@@ -38,6 +38,7 @@ type WorkingDirLocker interface {
 	// an error if the workspace is already locked. The error is expected to
 	// be printed to the pull request.
 	TryLockPull(repoFullName string, pullNum int) (func(), error)
+	IsPullLocked(repoFullName string, pullNum int) (bool, error)
 }
 
 // DefaultWorkingDirLocker implements WorkingDirLocker.
@@ -128,3 +129,13 @@ func (d *DefaultWorkingDirLocker) workspaceKey(repo string, pull int, workspace
 func (d *DefaultWorkingDirLocker) pullKey(repo string, pull int) string {
 	return fmt.Sprintf("%s/%d", repo, pull)
 }
+
+func (d *DefaultWorkingDirLocker) IsPullLocked(repoFullName string, pullNum int) (bool, error) {
+	pullKey := d.pullKey(repoFullName, pullNum)
+	for _, l := range d.locks {
+		if l == pullKey || strings.HasPrefix(l, pullKey+"/") {
+			return true, nil
+		}
+	}
+	return false, nil
+}

server/events/working_dir_locker_test.go

@@ -196,3 +196,21 @@ func TestLockPull_WorkspaceFirst(t *testing.T) {
 	_, err = locker.TryLockPull("owner/repo", 1)
 	Ok(t, err)
 }
+
+func TestIsPullLocked(t *testing.T) {
+	locker := events.NewDefaultWorkingDirLocker()
+
+	isLocked, err := locker.IsPullLocked("owner/repo", 1)
+	Assert(t, isLocked == false, "exp unlocked")
+
+	unlock, err := locker.TryLockPull("owner/repo", 1)
+	Ok(t, err)
+
+	isLocked, err = locker.IsPullLocked("owner/repo", 1)
+	Assert(t, isLocked == true, "exp locked")
+
+	// After unlocking, should be able to get a pull lock.
+	unlock()
+	isLocked, err = locker.IsPullLocked("owner/repo", 1)
+	Assert(t, isLocked == false, "exp unlocked")
+}