helper/backend: cache route regexps (98% speedup)

benchmark                 old ns/op     new ns/op     delta
BenchmarkBackendRoute     49144         589           -98.80%

helper/backend/backend.go

@@ -19,7 +19,8 @@ type Backend struct {
 	// backend is in use).
 	Paths []*Path
 
-	once sync.Once
+	once    sync.Once
+	pathsRe []*regexp.Regexp
 }
 
 // Path is a single path that the backend responds to.
@@ -52,12 +53,9 @@ type Path struct {
 }
 
 func (b *Backend) Route(path string) *Path {
-	regexps := make([]*regexp.Regexp, len(b.Paths))
+	b.once.Do(b.init)
-	for i, p := range b.Paths {
-		regexps[i] = regexp.MustCompile(p.Pattern)
-	}
 
-	for i, re := range regexps {
+	for i, re := range b.pathsRe {
 		if re.MatchString(path) {
 			return b.Paths[i]
 		}
@@ -66,6 +64,13 @@ func (b *Backend) Route(path string) *Path {
 	return nil
 }
 
+func (b *Backend) init() {
+	b.pathsRe = make([]*regexp.Regexp, len(b.Paths))
+	for i, p := range b.Paths {
+		b.pathsRe[i] = regexp.MustCompile(p.Pattern)
+	}
+}
+
 // FieldSchema is a basic schema to describe the format of a path field.
 type FieldSchema struct {
 	Type    FieldType

helper/backend/backend_test.go

@@ -19,10 +19,15 @@ func BenchmarkBackendRoute(b *testing.B) {
 		backend.Paths = append(backend.Paths, &Path{Pattern: p})
 	}
 
+	// Warm any caches
+	backend.Route("aws/policy/foo")
+
+	// Reset the timer since we did a lot above
 	b.ResetTimer()
+
+	// Run through and route. We do a sanity check of the return value
 	for i := 0; i < b.N; i++ {
-		p := backend.Route("aws/policy/foo")
+		if p := backend.Route("aws/policy/foo"); p == nil {
-		if p == nil {
 			b.Fatal("p should not be nil")
 		}
 	}