internal/gcimporter: reuse archive.ReadHeader implementation

Adds an internal copy of the GOROOT/src/cmd/internal/archive.ReadHeader
to x/tools. This replaces readGopackHeader. The result is one less
different implementation of reading an archive header between GOROOT
and x/tools.

Updates golang/go#70651

Change-Id: I69028da6472d6b06a613b554158301894326e735
Reviewed-on: https://go-review.googlesource.com/c/tools/+/633657
Commit-Queue: Tim King <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Robert Findley <[email protected]>

Author: timothy-king

internal/gcimporter/exportdata.go

@@ -11,33 +11,9 @@ package gcimporter
 import (
 	"bufio"
 	"fmt"
-	"io"
-	"strconv"
 	"strings"
 )
 
-func readGopackHeader(r *bufio.Reader) (name string, size int64, err error) {
-	// See $GOROOT/include/ar.h.
-	hdr := make([]byte, 16+12+6+6+8+10+2)
-	_, err = io.ReadFull(r, hdr)
-	if err != nil {
-		return
-	}
-	// leave for debugging
-	if false {
-		fmt.Printf("header: %s", hdr)
-	}
-	s := strings.TrimSpace(string(hdr[16+12+6+6+8:][:10]))
-	length, err := strconv.Atoi(s)
-	size = int64(length)
-	if err != nil || hdr[len(hdr)-2] != '`' || hdr[len(hdr)-1] != '\n' {
-		err = fmt.Errorf("invalid archive header")
-		return
-	}
-	name = strings.TrimSpace(string(hdr[:16]))
-	return
-}
-
 // FindExportData positions the reader r at the beginning of the
 // export data section of an underlying cmd/compile created archive
 // file by reading from it. The reader must be positioned at the
@@ -61,19 +37,13 @@ func FindExportData(r *bufio.Reader) (size int64, err error) {
 		return
 	}
 
-	// Archive file. Scan to __.PKGDEF.
-	var arsize int64
-	var name string
-	if name, arsize, err = readGopackHeader(r); err != nil {
-		return
-	}
-	size = arsize
-
-	// First entry should be __.PKGDEF.
-	if name != "__.PKGDEF" {
-		err = fmt.Errorf("go archive is missing __.PKGDEF")
+	// Archive file with the first file being __.PKGDEF.
+	arsize := readArchiveHeader(r, "__.PKGDEF")
+	if arsize <= 0 {
+		err = fmt.Errorf("not a package file")
 		return
 	}
+	size = int64(arsize)
 
 	// Read first line of __.PKGDEF data, so that line
 	// is once again the first line of the input.

internal/gcimporter/support.go

@@ -0,0 +1,30 @@
+// Copyright 2024 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package gcimporter
+
+import (
+	"bufio"
+	"io"
+	"strconv"
+	"strings"
+)
+
+// Copy of $GOROOT/src/cmd/internal/archive.ReadHeader.
+func readArchiveHeader(b *bufio.Reader, name string) int {
+	// architecture-independent object file output
+	const HeaderSize = 60
+
+	var buf [HeaderSize]byte
+	if _, err := io.ReadFull(b, buf[:]); err != nil {
+		return -1
+	}
+	aname := strings.Trim(string(buf[0:16]), " ")
+	if !strings.HasPrefix(aname, name) {
+		return -1
+	}
+	asize := strings.Trim(string(buf[48:58]), " ")
+	i, _ := strconv.Atoi(asize)
+	return i
+}