Fix wrong required size

pkg/io/io.go

@@ -4,7 +4,7 @@ package io
 // InsufficientBufferError.
 func Copy(dst, src []byte) (n int, err error) {
 	if len(dst) < len(src) {
-		return 0, &InsufficientBufferError{len(dst)}
+		return 0, &InsufficientBufferError{len(src)}
 	}
 
 	return copy(dst, src), nil

pkg/io/io_test.go

@@ -0,0 +1,45 @@
+package io
+
+import (
+	"log"
+	"testing"
+)
+
+func TestCopy(t *testing.T) {
+	var dst []byte
+	src := make([]byte, 4)
+
+	n, err := Copy(dst, src)
+	if err == nil {
+		t.Fatal("expected err to be non-nill")
+	}
+
+	if n != 0 {
+		t.Fatalf("expected n to be 0, but got %d", n)
+	}
+
+	e, ok := err.(*InsufficientBufferError)
+	if !ok {
+		t.Fatalf("expected error to be InsufficientBufferError")
+	}
+
+	if e.RequiredSize != len(src) {
+		t.Fatalf("expected required size to be %d, but got %d", len(src), e.RequiredSize)
+	}
+
+	dst = make([]byte, 2*e.RequiredSize)
+	n, err = Copy(dst, src)
+	if err != nil {
+		t.Fatalf("expected to not get an error after expanding the buffer")
+	}
+
+	if n != len(src) {
+		t.Fatalf("expected n to be %d, but got %d", len(src), n)
+	}
+
+	for i := 0; i < len(src); i++ {
+		if src[i] != dst[i] {
+			log.Fatalf("expected value at %d to be %d, but got %d", i, src[i], dst[i])
+		}
+	}
+}