net/http: speed up parsing of Cookie headers

Parse the headers without splitting them upfront to reduce
memory allocations.

For non-pathological Cookie headers we can make
a good estimate of the number of cookies in there and preallocate
the slice of cookies

name              old time/op    new time/op    delta
CookieString-4      1.73µs ± 2%    1.70µs ± 5%     ~     (p=0.841 n=5+5)
ReadSetCookies-4    6.09µs ± 3%    5.93µs ± 3%     ~     (p=0.095 n=5+5)
ReadCookies-4       7.63µs ± 1%    6.41µs ± 4%  -15.99%  (p=0.008 n=5+5)

name              old alloc/op   new alloc/op   delta
CookieString-4        360B ± 0%      360B ± 0%     ~     (all equal)
ReadSetCookies-4      976B ± 0%      976B ± 0%     ~     (all equal)
ReadCookies-4       2.17kB ± 0%    1.84kB ± 0%  -15.13%  (p=0.008 n=5+5)

name              old allocs/op  new allocs/op  delta
CookieString-4        5.00 ± 0%      5.00 ± 0%     ~     (all equal)
ReadSetCookies-4      15.0 ± 0%      15.0 ± 0%     ~     (all equal)
ReadCookies-4         16.0 ± 0%      11.0 ± 0%  -31.25%  (p=0.008 n=5+5)

Change-Id: Ica1ca0d40c0d8d275134d1dfafb73f1082115826
Reviewed-on: https://go-review.googlesource.com/c/go/+/163617Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/cookie.go

@@ -230,25 +230,28 @@ func (c *Cookie) String() string {
 //
 // if filter isn't empty, only cookies of that name are returned
 func readCookies(h Header, filter string) []*Cookie {
-	lines, ok := h["Cookie"]
-	if !ok {
+	lines := h["Cookie"]
+	if len(lines) == 0 {
 		return []*Cookie{}
 	}
 
-	cookies := []*Cookie{}
+	cookies := make([]*Cookie, 0, len(lines)+strings.Count(lines[0], ";"))
 	for _, line := range lines {
-		parts := strings.Split(strings.TrimSpace(line), ";")
-		if len(parts) == 1 && parts[0] == "" {
-			continue
-		}
-		// Per-line attributes
-		for i := 0; i < len(parts); i++ {
-			parts[i] = strings.TrimSpace(parts[i])
-			if len(parts[i]) == 0 {
+		line = strings.TrimSpace(line)
+
+		var part string
+		for len(line) > 0 { // continue since we have rest
+			if splitIndex := strings.Index(line, ";"); splitIndex > 0 {
+				part, line = line[:splitIndex], line[splitIndex+1:]
+			} else {
+				part, line = line, ""
+			}
+			part = strings.TrimSpace(part)
+			if len(part) == 0 {
 				continue
 			}
-			name, val := parts[i], ""
-			if j := strings.Index(name, "="); j >= 0 {
+			name, val := part, ""
+			if j := strings.Index(part, "="); j >= 0 {
 				name, val = name[:j], name[j+1:]
 			}
 			if !isCookieNameValid(name) {