feat: dependency cycle detection

Signed-off-by: Brian McGee <brian@bmcgee.ie>

internal/cli/format.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"os"
 	"os/signal"
+	"strings"
 	"syscall"
 	"time"
 
@@ -66,8 +67,7 @@ func (f *Format) Run() error {
 
 	formatters := make(map[string]*format.Formatter)
 
-	// detect dependency cycles and broken dependencies
-	// todo dependency cycle detection
+	// detect broken dependencies
 	for name, config := range cfg.Formatters {
 		before := config.Before
 		if before != "" {
@@ -79,6 +79,30 @@ func (f *Format) Run() error {
 		}
 	}
 
+	// dependency cycle detection
+	for name, config := range cfg.Formatters {
+		var ok bool
+		var history []string
+		childName := name
+		for {
+			// add to history
+			history = append(history, childName)
+
+			if config.Before == "" {
+				break
+			} else if config.Before == name {
+				return fmt.Errorf("formatter cycle detected %v", strings.Join(history, " -> "))
+			}
+
+			// load child config
+			childName = config.Before
+			config, ok = cfg.Formatters[config.Before]
+			if !ok {
+				return fmt.Errorf("formatter not found: %v", config.Before)
+			}
+		}
+	}
+
 	// init formatters
 	for name, config := range cfg.Formatters {
 		if !includeFormatter(name) {

internal/cli/format_test.go

@@ -39,6 +39,27 @@ func TestAllowMissingFormatter(t *testing.T) {
 	as.NoError(err)
 }
 
+func TestDependencyCycle(t *testing.T) {
+	as := require.New(t)
+
+	tempDir := t.TempDir()
+	configPath := tempDir + "/treefmt.toml"
+
+	test.WriteConfig(t, configPath, format.Config{
+		Formatters: map[string]*format.FormatterConfig{
+			"a": {Command: "echo", Before: "b"},
+			"b": {Command: "echo", Before: "c"},
+			"c": {Command: "echo", Before: "a"},
+			"d": {Command: "echo", Before: "e"},
+			"e": {Command: "echo", Before: "f"},
+			"f": {Command: "echo"},
+		},
+	})
+
+	_, err := cmd(t, "--config-file", configPath, "--tree-root", tempDir)
+	as.ErrorContains(err, "formatter cycle detected a -> b -> c")
+}
+
 func TestSpecifyingFormatters(t *testing.T) {
 	as := require.New(t)