archive/tar: reuse temporary buffer in writeHeader

A temporary 512 bytes buffer is allocated for every call to
writeHeader. This buffer could be reused the lower the number
of memory allocations.

benchmark                   old ns/op     new ns/op     delta
BenchmarkWriteFiles100k     634622051     583810847     -8.01%

benchmark                   old allocs     new allocs     delta
BenchmarkWriteFiles100k     2701920        2602621        -3.68%

benchmark                   old bytes     new bytes     delta
BenchmarkWriteFiles100k     115383884     64349922      -44.23%

This change is very important if your code has to write a lot of
tarballs with a lot of files.

LGTM=dsymonds
R=golang-codereviews, dave, dsymonds
CC=golang-codereviews
https://golang.org/cl/107440043

src/pkg/archive/tar/writer.go

@@ -39,6 +39,7 @@ type Writer struct {
 	closed     bool
 	usedBinary bool            // whether the binary numeric field extension was used
 	preferPax  bool            // use pax header instead of binary numeric header
+	hdrBuff    [blockSize]byte // buffer to use in writeHeader
 }
 
 // NewWriter creates a new Writer writing to w.
@@ -160,7 +161,8 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
 	// subsecond time resolution, but for now let's just capture
 	// too long fields or non ascii characters
 
-	header := make([]byte, blockSize)
+	header := tw.hdrBuff[:]
+	copy(header, zeroBlock)
 	s := slicer(header)
 
 	// keep a reference to the filename to allow to overwrite it later if we detect that we can use ustar longnames instead of pax