internal/gcimporter: update FindExportData to return a non-negative size

Updates FindExportData to return a non-negative size when there is no
error. This can only happen if the archive file is malformed.

Updates golang/go#70651

Change-Id: Ia422651cd31fba83a260cf1be600ffd134012a01
Reviewed-on: https://go-review.googlesource.com/c/tools/+/633278
Reviewed-by: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Griesemer <gri@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Commit-Queue: Tim King <taking@google.com>

Author: timothy-king

go/gcexportdata/gcexportdata.go

@@ -111,19 +111,13 @@ func NewReader(r io.Reader) (io.Reader, error) {
 		return nil, err
 	}
 
-	if size >= 0 {
-		// We were given an archive and found the __.PKGDEF in it.
-		// This tells us the size of the export data, and we don't
-		// need to return the entire file.
-		return &io.LimitedReader{
-			R: buf,
-			N: size,
-		}, nil
-	} else {
-		// We were given an object file. As such, we don't know how large
-		// the export data is and must return the entire file.
-		return buf, nil
-	}
+	// We were given an archive and found the __.PKGDEF in it.
+	// This tells us the size of the export data, and we don't
+	// need to return the entire file.
+	return &io.LimitedReader{
+		R: buf,
+		N: size,
+	}, nil
 }
 
 // readAll works the same way as io.ReadAll, but avoids allocations and copies

internal/gcimporter/exportdata.go

@@ -43,7 +43,7 @@ func readGopackHeader(r *bufio.Reader) (name string, size int64, err error) {
 // file by reading from it. The reader must be positioned at the
 // start of the file before calling this function. The hdr result
 // is the string before the export data, either "$$" or "$$B".
-// The size result is the length of the export data in bytes, or -1 if not known.
+// The size result is the length of the export data in bytes.
 //
 // This function is needed by [gcexportdata.Read], which must
 // accept inputs produced by the last two releases of cmd/compile,
@@ -67,6 +67,7 @@ func FindExportData(r *bufio.Reader) (hdr string, size int64, err error) {
 	if name, size, err = readGopackHeader(r); err != nil {
 		return
 	}
+	arsize := size
 
 	// First entry should be __.PKGDEF.
 	if name != "__.PKGDEF" {
@@ -99,8 +100,11 @@ func FindExportData(r *bufio.Reader) (hdr string, size int64, err error) {
 		size -= int64(len(line))
 	}
 	hdr = string(line)
+	// TODO(taking): Return an error when hdr != "$$B\n".
+	// TODO(taking): Remove end-of-section marker "\n$$\n" from size.
+
 	if size < 0 {
-		size = -1
+		err = fmt.Errorf("invalid size (%d) in the archive file: %d bytes remain without section headers (recompile package)", arsize, size)
 	}
 
 	return