Add function to expose allowed methods for use in custom 405-handlers

Fixes go-chi#870

Author: flimzy

context.go

@@ -34,10 +34,23 @@ func NewRouteContext() *Context {
 	return &Context{}
 }
 
-var (
-	// RouteCtxKey is the context.Context key to store the request context.
-	RouteCtxKey = &contextKey{"RouteContext"}
-)
+// WithRouteContext returns the list of methods allowed for the current
+// request, based on the current routing context.
+func AllowedMethods(ctx context.Context) []string {
+	if rctx := RouteContext(ctx); rctx != nil {
+		result := make([]string, 0, len(rctx.methodsAllowed))
+		for _, method := range rctx.methodsAllowed {
+			if method := methodTypString(method); method != "" {
+				result = append(result, method)
+			}
+		}
+		return result
+	}
+	return nil
+}
+
+// RouteCtxKey is the context.Context key to store the request context.
+var RouteCtxKey = &contextKey{"RouteContext"}
 
 // Context is the default routing context set on the root node of a
 // request context to track route patterns, URL parameters and

context_test.go

@@ -1,6 +1,10 @@
 package chi
 
-import "testing"
+import (
+	"context"
+	"strings"
+	"testing"
+)
 
 // TestRoutePattern tests correct in-the-middle wildcard removals.
 // If user organizes a router like this:
@@ -85,3 +89,32 @@ func TestRoutePattern(t *testing.T) {
 		t.Fatal("unexpected route pattern for root: " + p)
 	}
 }
+
+func TestAllowedMethods(t *testing.T) {
+	t.Run("no chi context", func(t *testing.T) {
+		got := AllowedMethods(context.Background())
+		if got != nil {
+			t.Errorf("Unexpected allowed methods: %v", got)
+		}
+	})
+	t.Run("expected methods", func(t *testing.T) {
+		want := "GET HEAD"
+		ctx := context.WithValue(context.Background(), RouteCtxKey, &Context{
+			methodsAllowed: []methodTyp{mGET, mHEAD},
+		})
+		got := strings.Join(AllowedMethods(ctx), " ")
+		if want != got {
+			t.Errorf("Unexpected allowed methods: %s, want: %s", got, want)
+		}
+	})
+	t.Run("unexpected methods", func(t *testing.T) {
+		want := "GET HEAD"
+		ctx := context.WithValue(context.Background(), RouteCtxKey, &Context{
+			methodsAllowed: []methodTyp{mGET, mHEAD, 9000},
+		})
+		got := strings.Join(AllowedMethods(ctx), " ")
+		if want != got {
+			t.Errorf("Unexpected allowed methods: %s, want: %s", got, want)
+		}
+	})
+}