Formula cycle detection: show full extends chain in error message (gt-8tmz.15)

When bd cook encounters a circular extends chain (A extends B extends A),
the error message now shows the full chain: "cycle-a -> cycle-b -> cycle-a"
instead of just "circular extends detected: cycle-a".

This makes debugging circular dependencies much easier by showing exactly
which formulas are involved in the cycle.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/formula/parser.go

@@ -24,8 +24,11 @@ type Parser struct {
 	// cache stores loaded formulas by name.
 	cache map[string]*Formula
 
-	// resolving tracks formulas currently being resolved (for cycle detection).
+	// resolvingSet tracks formulas currently being resolved (for cycle detection).
-	resolving map[string]bool
+	resolvingSet map[string]bool
+
+	// resolvingChain tracks the order of formulas being resolved (for error messages).
+	resolvingChain []string
 }
 
 // NewParser creates a new formula parser.
@@ -37,9 +40,10 @@ func NewParser(searchPaths ...string) *Parser {
 		paths = defaultSearchPaths()
 	}
 	return &Parser{
-		searchPaths: paths,
+		searchPaths:    paths,
-		cache:       make(map[string]*Formula),
+		cache:          make(map[string]*Formula),
-		resolving:   make(map[string]bool),
+		resolvingSet:   make(map[string]bool),
+		resolvingChain: nil,
 	}
 }
 
@@ -118,11 +122,17 @@ func (p *Parser) Parse(data []byte) (*Formula, error) {
 // Returns a new formula with all inheritance applied.
 func (p *Parser) Resolve(formula *Formula) (*Formula, error) {
 	// Check for cycles
-	if p.resolving[formula.Formula] {
+	if p.resolvingSet[formula.Formula] {
-		return nil, fmt.Errorf("circular extends detected: %s", formula.Formula)
+		// Build the cycle chain for a clear error message
+		chain := append(p.resolvingChain, formula.Formula)
+		return nil, fmt.Errorf("circular extends detected: %s", strings.Join(chain, " -> "))
 	}
-	p.resolving[formula.Formula] = true
+	p.resolvingSet[formula.Formula] = true
-	defer delete(p.resolving, formula.Formula)
+	p.resolvingChain = append(p.resolvingChain, formula.Formula)
+	defer func() {
+		delete(p.resolvingSet, formula.Formula)
+		p.resolvingChain = p.resolvingChain[:len(p.resolvingChain)-1]
+	}()
 
 	// If no extends, just validate and return
 	if len(formula.Extends) == 0 {

internal/formula/parser_test.go

@@ -3,6 +3,7 @@ package formula
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -533,6 +534,18 @@ func TestResolve_CircularExtends(t *testing.T) {
 	if err == nil {
 		t.Error("Resolve should fail for circular extends")
 	}
+
+	// Verify the error message shows the full cycle chain
+	errStr := err.Error()
+	if !strings.Contains(errStr, "cycle-a") {
+		t.Errorf("error should mention cycle-a: %v", err)
+	}
+	if !strings.Contains(errStr, "cycle-b") {
+		t.Errorf("error should mention cycle-b: %v", err)
+	}
+	if !strings.Contains(errStr, "->") {
+		t.Errorf("error should show cycle chain with '->': %v", err)
+	}
 }
 
 func TestGetStepByID(t *testing.T) {