net/textproto: faster header canonicalization with fewer allocations

By keeping a single copy of the strings that commonly show up
in headers, we can avoid one string allocation per header.

benchmark                  old ns/op    new ns/op    delta
BenchmarkReadMIMEHeader        19590        10824  -44.75%
BenchmarkUncommon               3168         1861  -41.26%

benchmark                 old allocs   new allocs    delta
BenchmarkReadMIMEHeader           32           25  -21.88%
BenchmarkUncommon                  5            5    0.00%

R=bradfitz, golang-dev, dave, rsc, jra
CC=golang-dev
https://golang.org/cl/6721055

src/pkg/net/textproto/reader.go

@@ -486,6 +486,7 @@ func (r *Reader) ReadMIMEHeader() (MIMEHeader, error) {
 // letter and any letter following a hyphen to upper case;
 // the rest are converted to lowercase.  For example, the
 // canonical key for "accept-encoding" is "Accept-Encoding".
+// MIME header keys are assumed to be ASCII only.
 func CanonicalMIMEHeaderKey(s string) string {
 	// Quick check for canonical encoding.
 	upper := true
@@ -502,28 +503,90 @@ func CanonicalMIMEHeaderKey(s string) string {
 	return s
 }
 
+const toLower = 'a' - 'A'
+
 // canonicalMIMEHeaderKey is like CanonicalMIMEHeaderKey but is
 // allowed to mutate the provided byte slice before returning the
 // string.
 func canonicalMIMEHeaderKey(a []byte) string {
+	// Look for it in commonHeaders , so that we can avoid an
+	// allocation by sharing the strings among all users
+	// of textproto. If we don't find it, a has been canonicalized
+	// so just return string(a).
+	upper := true
+	lo := 0
+	hi := len(commonHeaders)
+	for i := 0; i < len(a); i++ {
 		// Canonicalize: first letter upper case
 		// and upper case after each dash.
 		// (Host, User-Agent, If-Modified-Since).
 		// MIME headers are ASCII only, so no Unicode issues.
-	upper := true
+		if a[i] == ' ' {
-	for i, v := range a {
-		if v == ' ' {
 			a[i] = '-'
 			upper = true
 			continue
 		}
-		if upper && 'a' <= v && v <= 'z' {
+		c := a[i]
-			a[i] = v + 'A' - 'a'
+		if upper && 'a' <= c && c <= 'z' {
+			c -= toLower
+		} else if !upper && 'A' <= c && c <= 'Z' {
+			c += toLower
 		}
-		if !upper && 'A' <= v && v <= 'Z' {
+		a[i] = c
-			a[i] = v + 'a' - 'A'
+		upper = c == '-' // for next time
+
+		if lo < hi {
+			for lo < hi && (len(commonHeaders[lo]) <= i || commonHeaders[lo][i] < c) {
+				lo++
 			}
-		upper = v == '-'
+			for hi > lo && commonHeaders[hi-1][i] > c {
+				hi--
+			}
+		}
+	}
+	if lo < hi && len(commonHeaders[lo]) == len(a) {
+		return commonHeaders[lo]
 	}
 	return string(a)
 }
+
+var commonHeaders = []string{
+	"Accept",
+	"Accept-Charset",
+	"Accept-Encoding",
+	"Accept-Language",
+	"Accept-Ranges",
+	"Cache-Control",
+	"Cc",
+	"Connection",
+	"Content-Id",
+	"Content-Language",
+	"Content-Length",
+	"Content-Transfer-Encoding",
+	"Content-Type",
+	"Date",
+	"Dkim-Signature",
+	"Etag",
+	"Expires",
+	"From",
+	"Host",
+	"If-Modified-Since",
+	"If-None-Match",
+	"In-Reply-To",
+	"Last-Modified",
+	"Location",
+	"Message-Id",
+	"Mime-Version",
+	"Pragma",
+	"Received",
+	"Return-Path",
+	"Server",
+	"Set-Cookie",
+	"Subject",
+	"To",
+	"User-Agent",
+	"Via",
+	"X-Forwarded-For",
+	"X-Imforwards",
+	"X-Powered-By",
+}

src/pkg/net/textproto/reader_test.go

@@ -24,6 +24,7 @@ var canonicalHeaderKeyTests = []canonicalHeaderKeyTest{
 	{"uSER-aGENT", "User-Agent"},
 	{"user-agent", "User-Agent"},
 	{"USER-AGENT", "User-Agent"},
+	{"üser-agenT", "üser-Agent"}, // non-ASCII unchanged
 }
 
 func TestCanonicalMIMEHeaderKey(t *testing.T) {
@@ -241,18 +242,94 @@ func TestRFC959Lines(t *testing.T) {
 	}
 }
 
+func TestCommonHeaders(t *testing.T) {
+	// need to disable the commonHeaders-based optimization
+	// during this check, or we'd not be testing anything
+	oldch := commonHeaders
+	commonHeaders = []string{}
+	defer func() { commonHeaders = oldch }()
+
+	last := ""
+	for _, h := range oldch {
+		if last > h {
+			t.Errorf("%v is out of order", h)
+		}
+		if last == h {
+			t.Errorf("%v is duplicated", h)
+		}
+		if canon := CanonicalMIMEHeaderKey(h); h != canon {
+			t.Errorf("%v is not canonical", h)
+		}
+		last = h
+	}
+}
+
+var clientHeaders = strings.Replace(`Host: golang.org
+Connection: keep-alive
+Cache-Control: max-age=0
+Accept: application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.8,image/png,*/*;q=0.5
+User-Agent: Mozilla/5.0 (X11; U; Linux x86_64; en-US) AppleWebKit/534.3 (KHTML, like Gecko) Chrome/6.0.472.63 Safari/534.3
+Accept-Encoding: gzip,deflate,sdch
+Accept-Language: en-US,en;q=0.8,fr-CH;q=0.6
+Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.3
+COOKIE: __utma=000000000.0000000000.0000000000.0000000000.0000000000.00; __utmb=000000000.0.00.0000000000; __utmc=000000000; __utmz=000000000.0000000000.00.0.utmcsr=code.google.com|utmccn=(referral)|utmcmd=referral|utmcct=/p/go/issues/detail
+Non-Interned: test
+
+`, "\n", "\r\n", -1)
+
+var serverHeaders = strings.Replace(`Content-Type: text/html; charset=utf-8
+Content-Encoding: gzip
+Date: Thu, 27 Sep 2012 09:03:33 GMT
+Server: Google Frontend
+Cache-Control: private
+Content-Length: 2298
+VIA: 1.1 proxy.example.com:80 (XXX/n.n.n-nnn)
+Connection: Close
+Non-Interned: test
+
+`, "\n", "\r\n", -1)
+
 func BenchmarkReadMIMEHeader(b *testing.B) {
 	var buf bytes.Buffer
 	br := bufio.NewReader(&buf)
 	r := NewReader(br)
 	for i := 0; i < b.N; i++ {
-		buf.WriteString("User-Agent: not mozilla\r\nContent-Length: 23452\r\nContent-Type: text/html; charset-utf8\r\nFoo-Bar: foobar\r\nfoo-bar: some more string\r\n\r\n")
+		var want int
+		var find string
+		if (i & 1) == 1 {
+			buf.WriteString(clientHeaders)
+			want = 10
+			find = "Cookie"
+		} else {
+			buf.WriteString(serverHeaders)
+			want = 9
+			find = "Via"
+		}
 		h, err := r.ReadMIMEHeader()
 		if err != nil {
 			b.Fatal(err)
 		}
-		if len(h) != 4 {
+		if len(h) != want {
-			b.Fatalf("want 4")
+			b.Fatalf("wrong number of headers: got %d, want %d", len(h), want)
+		}
+		if _, ok := h[find]; !ok {
+			b.Fatalf("did not find key %s", find)
+		}
+	}
+}
+
+func BenchmarkUncommon(b *testing.B) {
+	var buf bytes.Buffer
+	br := bufio.NewReader(&buf)
+	r := NewReader(br)
+	for i := 0; i < b.N; i++ {
+		buf.WriteString("uncommon-header-for-benchmark: foo\r\n\r\n")
+		h, err := r.ReadMIMEHeader()
+		if err != nil {
+			b.Fatal(err)
+		}
+		if _, ok := h["Uncommon-Header-For-Benchmark"]; !ok {
+			b.Fatal("Missing result header.")
 		}
 	}
 }