go/internal/gcimporter: don't waste CPU copying bytes in `io.ReadAll`

`io.ReadAll` dynamically reallocates byte slice because it doesn't know
its size in advance. We don't need to read an entire file into memory
and therefore may use `bufio.Reader` to read its contents.

Fixes #46564

Change-Id: Id504b1512662b6dea4775d523455896fa4162ab3
Reviewed-on: https://go-review.googlesource.com/c/go/+/325429
Reviewed-by: Dominik Honnef <dominik@honnef.co>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Trust: Matthew Dempsky <mdempsky@google.com>
Trust: Dominik Honnef <dominik@honnef.co>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

2 files changed, 15 insertions, 17 deletions

diff --git a/src/go/internal/gcimporter/gcimporter.go b/src/go/internal/gcimporter/gcimporter.go
index b74daca246..73cf6334fd 100644
--- a/src/go/internal/gcimporter/gcimporter.go
+++ b/src/go/internal/gcimporter/gcimporter.go
@@ -145,17 +145,14 @@ func Import(fset *token.FileSet, packages map[string]*types.Package, path, srcDi
 		err = fmt.Errorf("import %q: old textual export format no longer supported (recompile library)", path)
 
 	case "$$B\n":
-		var data []byte
-		data, err = io.ReadAll(buf)
-		if err != nil {
-			break
-		}
+		var exportFormat byte
+		exportFormat, err = buf.ReadByte()
 
 		// The indexed export format starts with an 'i'; the older
 		// binary export format starts with a 'c', 'd', or 'v'
 		// (from "version"). Select appropriate importer.
-		if len(data) > 0 && data[0] == 'i' {
-			_, pkg, err = iImportData(fset, packages, data[1:], id)
+		if err == nil && exportFormat == 'i' {
+			pkg, err = iImportData(fset, packages, buf, id)
 		} else {
 			err = fmt.Errorf("import %q: old binary export format no longer supported (recompile library)", path)
 		}
diff --git a/src/go/internal/gcimporter/iimport.go b/src/go/internal/gcimporter/iimport.go
index a3184e7641..76d47d08f1 100644
--- a/src/go/internal/gcimporter/iimport.go
+++ b/src/go/internal/gcimporter/iimport.go
@@ -8,6 +8,7 @@
 package gcimporter
 
 import (
+	"bufio"
 	"bytes"
 	"encoding/binary"
 	"fmt"
@@ -20,7 +21,7 @@ import (
 )
 
 type intReader struct {
-	*bytes.Reader
+	*bufio.Reader
 	path string
 }
 
@@ -61,7 +62,7 @@ const (
 // and returns the number of bytes consumed and a reference to the package.
 // If the export data version is not recognized or the format is otherwise
 // compromised, an error is returned.
-func iImportData(fset *token.FileSet, imports map[string]*types.Package, data []byte, path string) (_ int, pkg *types.Package, err error) {
+func iImportData(fset *token.FileSet, imports map[string]*types.Package, dataReader *bufio.Reader, path string) (pkg *types.Package, err error) {
 	const currentVersion = 1
 	version := int64(-1)
 	defer func() {
@@ -74,7 +75,7 @@ func iImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 		}
 	}()
 
-	r := &intReader{bytes.NewReader(data), path}
+	r := &intReader{dataReader, path}
 
 	version = int64(r.uint64())
 	switch version {
@@ -86,10 +87,12 @@ func iImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 	sLen := int64(r.uint64())
 	dLen := int64(r.uint64())
 
-	whence, _ := r.Seek(0, io.SeekCurrent)
-	stringData := data[whence : whence+sLen]
-	declData := data[whence+sLen : whence+sLen+dLen]
-	r.Seek(sLen+dLen, io.SeekCurrent)
+	data := make([]byte, sLen+dLen)
+	if _, err := io.ReadFull(r, data); err != nil {
+		errorf("cannot read %d bytes of stringData and declData: %s", len(data), err)
+	}
+	stringData := data[:sLen]
+	declData := data[sLen:]
 
 	p := iimporter{
 		ipath:   path,
@@ -165,9 +168,7 @@ func iImportData(fset *token.FileSet, imports map[string]*types.Package, data []
 
 	// package was imported completely and without errors
 	localpkg.MarkComplete()
-
-	consumed, _ := r.Seek(0, io.SeekCurrent)
-	return int(consumed), localpkg, nil
+	return localpkg, nil
 }
 
 type iimporter struct {