make io.ReadFile use Stat.Size as a hint for preallocation

R=rsc
CC=golang-dev
https://golang.org/cl/163069

src/pkg/io/utils.go

@@ -26,7 +26,23 @@ func ReadFile(filename string) ([]byte, os.Error) {
 		return nil, err
 	}
 	defer f.Close();
-	return ReadAll(f);
+	// It's a good but not certain bet that Stat will tell us exactly how much to
+	// read, so let's try it but be prepared for the answer to be wrong.
+	dir, err := f.Stat();
+	var n uint64;
+	if err != nil && dir.Size < 2e9 {	// Don't preallocate a huge buffer, just in case.
+		n = dir.Size
+	}
+	if n == 0 {
+		n = 1024	// No idea what's right, but zero isn't.
+	}
+	// 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
+	// in the overwhelmingly common case we'll get it just right.
+	buf := bytes.NewBuffer(make([]byte, n)[0:0]);
+	_, err = Copy(buf, f);
+	return buf.Bytes(), err;
 }
 
 // WriteFile writes data to a file named by filename.