io/ioutil: clean-up of ReadAll and ReadFile

Make ReadAll use the same mechanism as ReadFile.

R=r
CC=golang-dev
https://golang.org/cl/4279041

src/pkg/io/ioutil/ioutil.go

@@ -13,11 +13,17 @@ import (
 	"sort"
 )
 
+// readAll reads from r until an error or EOF and returns the data it read
+// from the internal buffer allocated with a specified capacity.
+func readAll(r io.Reader, capacity int64) ([]byte, os.Error) {
+	buf := bytes.NewBuffer(make([]byte, 0, capacity))
+	_, err := buf.ReadFrom(r)
+	return buf.Bytes(), err
+}
+
 // ReadAll reads from r until an error or EOF and returns the data it read.
 func ReadAll(r io.Reader) ([]byte, os.Error) {
-	var buf bytes.Buffer
-	_, err := io.Copy(&buf, r)
-	return buf.Bytes(), err
+	return readAll(r, bytes.MinRead)
 }
 
 // ReadFile reads the file named by filename and returns the contents.
@@ -34,16 +40,12 @@ func ReadFile(filename string) ([]byte, os.Error) {
 	if err == nil && fi.Size < 2e9 { // Don't preallocate a huge buffer, just in case.
 		n = fi.Size
 	}
-	// Add a little extra in case Size is zero, and to avoid another allocation after
-	// Read has filled the buffer.
-	n += bytes.MinRead
-	// Pre-allocate the correct size of buffer, then set its size to zero.  The
-	// Buffer will read into the allocated space cheaply.  If the size was wrong,
-	// we'll either waste some space off the end or reallocate as needed, but
+	// As initial capacity for readAll, use n + a little extra in case Size is zero,
+	// and to avoid another allocation after Read has filled the buffer.  The readAll
+	// call will read into its allocated internal buffer cheaply.  If the size was
+	// wrong, we'll either waste some space off the end or reallocate as needed, but
 	// in the overwhelmingly common case we'll get it just right.
-	buf := bytes.NewBuffer(make([]byte, 0, n))
-	_, err = buf.ReadFrom(f)
-	return buf.Bytes(), err
+	return readAll(f, n+bytes.MinRead)
 }
 
 // WriteFile writes data to a file named by filename.