internal/go/gcexportdata: read file directly when possible

internal/go/gcexportdata/gcexportdata.go

@@ -28,15 +28,32 @@ import (
 	"go/types"
 	"io"
 	"io/ioutil"
+	"os"
 
 	"honnef.co/go/tools/internal/go/gcimporter"
 )
 
+type bufferedReader struct {
+	r   io.Reader
+	buf *bufio.Reader
+}
+
+func (r *bufferedReader) Read(b []byte) (int, error) {
+	return r.buf.Read(b)
+}
+
+func (r *bufferedReader) ReadSlice(delim byte) (line []byte, err error) {
+	return r.buf.ReadSlice(delim)
+}
+
 // NewReader returns a reader for the export data section of an object
 // (.o) or archive (.a) file read from r.  The new reader may provide
 // additional trailing data beyond the end of the export data.
 func NewReader(r io.Reader) (io.Reader, error) {
-	buf := bufio.NewReader(r)
+	buf := &bufferedReader{
+		r:   r,
+		buf: bufio.NewReader(r),
+	}
 	_, err := gcimporter.FindExportData(buf)
 	// If we ever switch to a zip-like archive format with the ToC
 	// at the end, we can return the correct portion of export data,
@@ -57,9 +74,29 @@ func NewReader(r io.Reader) (io.Reader, error) {
 //
 // On return, the state of the reader is undefined.
 func Read(in io.Reader, fset *token.FileSet, imports map[string]*types.Package, path string) (*types.Package, error) {
-	data, err := ioutil.ReadAll(in)
-	if err != nil {
-		return nil, fmt.Errorf("reading export data for %q: %v", path, err)
+	var data []byte
+	if br, ok := in.(*bufferedReader); ok {
+		if f, ok := br.r.(*os.File); ok {
+			fi, err := f.Stat()
+			if err == nil {
+				// we expect to be close to the start of the file,
+				// which is why we don't bother checking with
+				// SEEK_CUR.
+				data = make([]byte, fi.Size())
+				n, err := io.ReadFull(in, data)
+				data = data[:n]
+				if err != nil && err != io.ErrUnexpectedEOF {
+					data = nil
+				}
+			}
+		}
+	}
+	if data == nil {
+		var err error
+		data, err = ioutil.ReadAll(in)
+		if err != nil {
+			return nil, fmt.Errorf("reading export data for %q: %v", path, err)
+		}
 	}
 
 	if bytes.HasPrefix(data, []byte("!<arch>")) {

internal/go/gcimporter/exportdata.go

@@ -9,14 +9,13 @@
 package gcimporter
 
 import (
-	"bufio"
 	"fmt"
 	"io"
 	"strconv"
 	"strings"
 )
 
-func readGopackHeader(r *bufio.Reader) (name string, size int, err error) {
+func readGopackHeader(r io.Reader) (name string, size int, err error) {
 	// See $GOROOT/include/ar.h.
 	hdr := make([]byte, 16+12+6+6+8+10+2)
 	_, err = io.ReadFull(r, hdr)
@@ -37,13 +36,18 @@ func readGopackHeader(r *bufio.Reader) (name string, size int, err error) {
 	return
 }
 
+type BufferedReader interface {
+	Read(b []byte) (int, error)
+	ReadSlice(delim byte) (line []byte, err error)
+}
+
 // FindExportData positions the reader r at the beginning of the
 // export data section of an underlying GC-created object/archive
 // 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".
 //
-func FindExportData(r *bufio.Reader) (hdr string, err error) {
+func FindExportData(r BufferedReader) (hdr string, err error) {
 	// Read first line to make sure this is an object file.
 	line, err := r.ReadSlice('\n')
 	if err != nil {