Improve IsFQDN performance (#1453)

* Improve IsFQDN performance

While this code may be slightly less clear, it's significantly faster
and this function seems to be a hot path for certain workloads.

name                 old time/op    new time/op    delta
IsFQDN/no_dot-12       5.86ns ± 2%    1.48ns ± 3%  -74.71%  (p=0.000 n=10+10)
IsFQDN/unescaped-12    8.73ns ± 2%    1.57ns ± 1%  -81.98%  (p=0.000 n=9+8)
IsFQDN/escaped-12      27.4ns ± 2%    23.8ns ± 2%  -13.19%  (p=0.000 n=10+10)
FQDN/is_fqdn-12        8.36ns ± 1%    1.80ns ± 2%  -78.50%  (p=0.000 n=9+10)
FQDN/not_fqdn-12       36.8ns ±15%    33.4ns ±12%   -9.25%  (p=0.035 n=10+10)

* Fixup IsFQDN comment

defaults.go

@@ -272,18 +272,24 @@ func IsMsg(buf []byte) error {
 
 // IsFqdn checks if a domain name is fully qualified.
 func IsFqdn(s string) bool {
-	s2 := strings.TrimSuffix(s, ".")
-	if s == s2 {
+	// Check for (and remove) a trailing dot, returning if there isn't one.
+	if s == "" || s[len(s)-1] != '.' {
 		return false
 	}
+	s = s[:len(s)-1]
 
-	i := strings.LastIndexFunc(s2, func(r rune) bool {
+	// If we don't have an escape sequence before the final dot, we know it's
+	// fully qualified and can return here.
+	if s == "" || s[len(s)-1] != '\\' {
+		return true
+	}
+
+	// Otherwise we have to check if the dot is escaped or not by checking if
+	// there are an odd or even number of escape sequences before the dot.
+	i := strings.LastIndexFunc(s, func(r rune) bool {
 		return r != '\\'
 	})
-
-	// Test whether we have an even number of escape sequences before
-	// the dot or none.
-	return (len(s2)-i)%2 != 0
+	return (len(s)-i)%2 != 0
 }
 
 // IsRRset checks if a set of RRs is a valid RRset as defined by RFC 2181.

dns_test.go

@@ -314,3 +314,49 @@ func TestTKEY(t *testing.T) {
 		t.Fatalf("unable to parse TKEY string: %s", newError)
 	}
 }
+
+var (
+	sinkBool   bool
+	sinkString string
+)
+
+func BenchmarkIsFQDN(b *testing.B) {
+	b.Run("no_dot", func(b *testing.B) {
+		var r bool
+		for n := 0; n < b.N; n++ {
+			r = IsFqdn("www.google.com")
+		}
+		sinkBool = r
+	})
+	b.Run("unescaped", func(b *testing.B) {
+		var r bool
+		for n := 0; n < b.N; n++ {
+			r = IsFqdn("www.google.com.")
+		}
+		sinkBool = r
+	})
+	b.Run("escaped", func(b *testing.B) {
+		var r bool
+		for n := 0; n < b.N; n++ {
+			r = IsFqdn(`www.google.com\\\\\\\\.`)
+		}
+		sinkBool = r
+	})
+}
+
+func BenchmarkFQDN(b *testing.B) {
+	b.Run("is_fqdn", func(b *testing.B) {
+		var r string
+		for n := 0; n < b.N; n++ {
+			r = Fqdn("www.google.com.")
+		}
+		sinkString = r
+	})
+	b.Run("not_fqdn", func(b *testing.B) {
+		var r string
+		for n := 0; n < b.N; n++ {
+			r = Fqdn("www.google.com")
+		}
+		sinkString = r
+	})
+}