net/textproto: more efficient header parsing

A co creation with bradfitz

* add fast path for header lines which are not continuations
* pass hint to better size initial mime header map

lucky(~/go/src/pkg/net/http) % ~/go/misc/benchcmp {golden,new}.txt
benchmark                          old ns/op    new ns/op    delta
BenchmarkReadRequestChrome             10073         8348  -17.12%
BenchmarkReadRequestCurl                4368         4350   -0.41%
BenchmarkReadRequestApachebench         4412         4397   -0.34%
BenchmarkReadRequestSiege               6431         5924   -7.88%
BenchmarkReadRequestWrk                 2820         3146  +11.56%

benchmark                           old MB/s     new MB/s  speedup
BenchmarkReadRequestChrome             60.66        73.18    1.21x
BenchmarkReadRequestCurl               17.85        17.93    1.00x
BenchmarkReadRequestApachebench        18.58        18.65    1.00x
BenchmarkReadRequestSiege              23.48        25.49    1.09x
BenchmarkReadRequestWrk                14.18        12.71    0.90x

benchmark                         old allocs   new allocs    delta
BenchmarkReadRequestChrome                32           26  -18.75%
BenchmarkReadRequestCurl                  15           15    0.00%
BenchmarkReadRequestApachebench           16           15   -6.25%
BenchmarkReadRequestSiege                 22           19  -13.64%
BenchmarkReadRequestWrk                   11           11    0.00%

benchmark                          old bytes    new bytes    delta
BenchmarkReadRequestChrome              3148         2216  -29.61%
BenchmarkReadRequestCurl                 905         1413   56.13%
BenchmarkReadRequestApachebench          956         1413   47.80%
BenchmarkReadRequestSiege               1397         1522    8.95%
BenchmarkReadRequestWrk                  757         1369   80.85%

R=bradfitz
CC=golang-dev
https://golang.org/cl/7300098

src/pkg/net/textproto/reader.go

@@ -128,6 +128,17 @@ func (r *Reader) readContinuedLineSlice() ([]byte, error) {
 		return line, nil
 	}
 
+	// Optimistically assume that we have started to buffer the next line
+	// and it starts with an ASCII letter (the next header key), so we can
+	// avoid copying that buffered data around in memory and skipping over
+	// non-existent whitespace.
+	if r.R.Buffered() > 1 {
+		peek, err := r.R.Peek(1)
+		if err == nil && isASCIILetter(peek[0]) {
+			return trim(line), nil
+		}
+	}
+
 	// ReadByte or the next readLineSlice will flush the read buffer;
 	// copy the slice into buf.
 	r.buf = append(r.buf[:0], trim(line)...)
@@ -445,7 +456,7 @@ func (r *Reader) ReadDotLines() ([]string, error) {
 //	}
 //
 func (r *Reader) ReadMIMEHeader() (MIMEHeader, error) {
-	m := make(MIMEHeader)
+	m := make(MIMEHeader, 4)
 	for {
 		kv, err := r.readContinuedLineSlice()
 		if len(kv) == 0 {

src/pkg/net/textproto/textproto.go

@@ -147,3 +147,8 @@ func TrimBytes(b []byte) []byte {
 func isASCIISpace(b byte) bool {
 	return b == ' ' || b == '\t' || b == '\n' || b == '\r'
 }
+
+func isASCIILetter(b byte) bool {
+	b |= 0x20 // make lower case
+	return 'a' <= b && b <= 'z'
+}