gopack: change archive file name length back to 16

This CL grew the archive file name length from 16 to 64:

        changeset:   909:58574851d792
        user:        Russ Cox <rsc@golang.org>
        date:        Mon Oct 20 13:53:56 2008 -0700

Back then, every x.go file in a package became an x.6 file
in the archive.  It was important to be able to allow the
use of long Go source file names, hence the increase in size.

Today, all Go source files compile into a single _go_.6 file
regardless of their names, so the archive file name length
no longer needs to be long.  The longer name causes some
problems on Plan 9, where the native archive format is the
same but with 16-byte names, so revert back to 16.

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5333050

include/ar.h

@@ -32,7 +32,7 @@
 #define	SARMAG	8
 
 #define	ARFMAG	"`\n"
-#define SARNAME	64
+#define	SARNAME	16
 
 struct	ar_hdr
 {

src/cmd/ld/lib.c

@@ -313,15 +313,9 @@ nextar(Biobuf *bp, int off, struct ar_hdr *a)
 			return 0;
 		return -1;
 	}
-	if(r == SAR_HDR) {
-		memmove(a, buf, SAR_HDR);
-	} else if (r == SAR_HDR-SARNAME+16) {	// old Plan 9
-		memset(a->name, ' ', sizeof a->name);
-		memmove(a, buf, 16);
-		memmove((char*)a+SARNAME, buf+16, SAR_HDR-SARNAME);
-	} else {	// unexpected
+	if(r != SAR_HDR)
 		return -1;
-	}
+	memmove(a, buf, SAR_HDR);
 	if(strncmp(a->fmag, ARFMAG, sizeof a->fmag))
 		return -1;
 	arsize = strtol(a->size, 0, 0);

src/pkg/exp/types/exportdata.go

@@ -17,7 +17,7 @@ import (
 
 func readGopackHeader(buf *bufio.Reader) (name string, size int, err os.Error) {
 	// See $GOROOT/include/ar.h.
-	hdr := make([]byte, 64+12+6+6+8+10+2)
+	hdr := make([]byte, 16+12+6+6+8+10+2)
 	_, err = io.ReadFull(buf, hdr)
 	if err != nil {
 		return
@@ -25,13 +25,13 @@ func readGopackHeader(buf *bufio.Reader) (name string, size int, err os.Error) {
 	if trace {
 		fmt.Printf("header: %s", hdr)
 	}
-	s := strings.TrimSpace(string(hdr[64+12+6+6+8:][:10]))
+	s := strings.TrimSpace(string(hdr[16+12+6+6+8:][:10]))
 	size, err = strconv.Atoi(s)
 	if err != nil || hdr[len(hdr)-2] != '`' || hdr[len(hdr)-1] != '\n' {
 		err = os.NewError("invalid archive header")
 		return
 	}
-	name = strings.TrimSpace(string(hdr[:64]))
+	name = strings.TrimSpace(string(hdr[:16]))
 	return
 }