Make Terraform reject empty strings as invalid workspace names (#37267)

* Add tests defining what are valid workspace names

* Update workspace name validation to mark an empty string as not valid

* Add change file

* Add command-level test showing the "" workspace cannot be created or selected

* Update invalid name error text to include empty string as invalid.

.changes/v1.13/BUG FIXES-20250623-125514.yaml

@@ -0,0 +1,5 @@
+kind: BUG FIXES
+body: 'workspace: Updated validation to reject workspaces named ""'
+time: 2025-06-23T12:55:14.226554+01:00
+custom:
+    Issue: "37267"

internal/command/workspace_command.go

@@ -45,6 +45,9 @@ func (c *WorkspaceCommand) Synopsis() string {
 // Since most named states are accessed via a filesystem path or URL, check if
 // escaping the name would be required.
 func validWorkspaceName(name string) bool {
+	if name == "" {
+		return false
+	}
 	return name == url.PathEscape(name)
 }
 
@@ -100,7 +103,7 @@ to another workspace and try again.
 
 	envInvalidName = `
 The workspace name %q is not allowed. The name must contain only URL safe
-characters, and no path separators.
+characters, contain no path separators, and not be an empty string.
 `
 
 	envIsOverriddenNote = `

internal/command/workspace_command_test.go

@@ -62,6 +62,49 @@ func TestWorkspace_createAndChange(t *testing.T) {
 
 }
 
+func TestWorkspace_cannotCreateOrSelectEmptyStringWorkspace(t *testing.T) {
+	// Create a temporary working directory that is empty
+	td := t.TempDir()
+	os.MkdirAll(td, 0755)
+	defer testChdir(t, td)()
+
+	newCmd := &WorkspaceNewCommand{}
+
+	current, _ := newCmd.Workspace()
+	if current != backend.DefaultStateName {
+		t.Fatal("current workspace should be 'default'")
+	}
+
+	args := []string{""}
+	ui := cli.NewMockUi()
+	view, _ := testView(t)
+	newCmd.Meta = Meta{Ui: ui, View: view}
+	if code := newCmd.Run(args); code != 1 {
+		t.Fatalf("expected failure when trying to create the \"\" workspace.\noutput: %s", ui.OutputWriter)
+	}
+
+	gotStderr := ui.ErrorWriter.String()
+	if want, got := `The workspace name "" is not allowed`, gotStderr; !strings.Contains(got, want) {
+		t.Errorf("missing expected error message\nwant substring: %s\ngot:\n%s", want, got)
+	}
+
+	ui = cli.NewMockUi()
+	selectCmd := &WorkspaceSelectCommand{
+		Meta: Meta{
+			Ui:   ui,
+			View: view,
+		},
+	}
+	if code := selectCmd.Run(args); code != 1 {
+		t.Fatalf("expected failure when trying to select the the \"\" workspace.\noutput: %s", ui.OutputWriter)
+	}
+
+	gotStderr = ui.ErrorWriter.String()
+	if want, got := `The workspace name "" is not allowed`, gotStderr; !strings.Contains(got, want) {
+		t.Errorf("missing expected error message\nwant substring: %s\ngot:\n%s", want, got)
+	}
+}
+
 // Create some workspaces and test the list output.
 // This also ensures we switch to the correct env after each call
 func TestWorkspace_createAndList(t *testing.T) {
@@ -476,3 +519,36 @@ func TestWorkspace_selectWithOrCreate(t *testing.T) {
 	}
 
 }
+
+func TestValidWorkspaceName(t *testing.T) {
+	cases := map[string]struct {
+		input string
+		valid bool
+	}{
+		"foobar": {
+			input: "foobar",
+			valid: true,
+		},
+		"valid symbols": {
+			input: "-._~@:",
+			valid: true,
+		},
+		"includes space": {
+			input: "two words",
+			valid: false,
+		},
+		"empty string": {
+			input: "",
+			valid: false,
+		},
+	}
+
+	for tn, tc := range cases {
+		t.Run(tn, func(t *testing.T) {
+			valid := validWorkspaceName(tc.input)
+			if valid != tc.valid {
+				t.Fatalf("unexpected output when processing input %q. Wanted %v got %v", tc.input, tc.valid, valid)
+			}
+		})
+	}
+}