go/internal/gcimporter: use saferio.ReadData instead of io.ReadAll when possible

saferio.ReadData avoids unnecessary allocations because the buffer can be
preallocated with the right size (up to a limit) instead of having to resize
and copy it step by step.

Change-Id: Id70f6908971d4f126c601a9571ac3c67ea0accdc
Reviewed-on: https://go-review.googlesource.com/c/go/+/481616
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
Auto-Submit: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Matthew Dempsky <mdempsky@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

diff --git a/src/go/internal/gcimporter/gcimporter.go b/src/go/internal/gcimporter/gcimporter.go
index 2140a9f98c2c4a2f855f130c92e4aa151aaadd97..0b784de58407eb2c2b3cec51b310ffb1da981c67 100644 (file)
--- a/src/go/internal/gcimporter/gcimporter.go
+++ b/src/go/internal/gcimporter/gcimporter.go
@@ -13,6 +13,7 @@ import (
        "go/token"
        "go/types"
        "internal/pkgbits"
+       "internal/saferio"
        "io"
        "os"
        "os/exec"
@@ -204,6 +205,7 @@ func Import(fset *token.FileSet, packages map[string]*types.Package, path, srcDi
                if exportFormat, err = buf.ReadByte(); err != nil {
                        return
                }
+               size--
 
                // The unified export format starts with a 'u'; the indexed export
                // format starts with an 'i'; and the older binary export format
@@ -214,9 +216,10 @@ func Import(fset *token.FileSet, packages map[string]*types.Package, path, srcDi
                        var data []byte
                        var r io.Reader = buf
                        if size >= 0 {
-                               r = io.LimitReader(r, int64(size))
-                       }
-                       if data, err = io.ReadAll(r); err != nil {
+                               if data, err = saferio.ReadData(r, uint64(size)); err != nil {
+                                       return
+                               }
+                       } else if data, err = io.ReadAll(r); err != nil {
                                return
                        }
                        s := string(data)