Add parsing for workspace already exists error

When calling `terraform workspace new <name>` to create a workspace that already exists, an error is returned.

tfexec/errors.go

@@ -21,6 +21,8 @@ var (
 	noConfigErrRegexp = regexp.MustCompile(`Error: No configuration files`)
 
 	workspaceDoesNotExistRegexp = regexp.MustCompile(`Workspace "(.+)" doesn't exist.`)
+
+	workspaceAlreadyExistsRegexp = regexp.MustCompile(`Workspace "(.+)" already exists`)
 )
 
 func parseError(err error, stderr string) error {
@@ -51,6 +53,11 @@ func parseError(err error, stderr string) error {
 		if len(submatches) == 2 {
 			return &ErrNoWorkspace{submatches[1]}
 		}
+	case workspaceAlreadyExistsRegexp.MatchString(stderr):
+		submatches := workspaceAlreadyExistsRegexp.FindStringSubmatch(stderr)
+		if len(submatches) == 2 {
+			return &ErrWorkspaceExists{submatches[1]}
+		}
 	}
 	return errors.New(stderr)
 }
@@ -132,3 +139,12 @@ type ErrNoWorkspace struct {
 func (err *ErrNoWorkspace) Error() string {
 	return fmt.Sprintf("workspace %q does not exist", err.Name)
 }
+
+// ErrWorkspaceExists is returned when creating a workspace that already exists
+type ErrWorkspaceExists struct {
+	Name string
+}
+
+func (err *ErrWorkspaceExists) Error() string {
+	return fmt.Sprintf("workspace %q already exists", err.Name)
+}

tfexec/internal/e2etest/workspace_test.go

@@ -52,6 +52,32 @@ func TestWorkspace_does_not_exist(t *testing.T) {
 	})
 }
 
+func TestWorkspace_already_exists(t *testing.T) {
+	runTest(t, "basic", func(t *testing.T, tfv *version.Version, tf *tfexec.Terraform) {
+		const newWorkspace = "existing-workspace"
+		t.Run("create new workspace", func(t *testing.T) {
+			err := tf.WorkspaceNew(context.Background(), newWorkspace)
+			if err != nil {
+				t.Fatalf("got error creating new workspace: %s", err)
+			}
+
+			assertWorkspaceList(t, tf, newWorkspace, newWorkspace)
+		})
+
+		t.Run("create existing workspace", func(t *testing.T) {
+			err := tf.WorkspaceNew(context.Background(), newWorkspace)
+
+			var wsErr *tfexec.ErrWorkspaceExists
+			if !errors.As(err, &wsErr) {
+				t.Fatalf("expected ErrWorkspaceExists, %T returned: %s", err, err)
+			}
+			if wsErr.Name != newWorkspace {
+				t.Fatalf("expected %q, got %q", newWorkspace, wsErr.Name)
+			}
+		})
+	})
+}
+
 func TestWorkspace_multiple(t *testing.T) {
 	runTest(t, "workspaces", func(t *testing.T, tfv *version.Version, tf *tfexec.Terraform) {
 		assertWorkspaceList(t, tf, "foo", "foo")