test: add basic integration tests

Signed-off-by: Justin Chadwell <me@jedevc.com>

vendor/github.com/containerd/continuity/ioutils.go

@@ -0,0 +1,62 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package continuity
+
+import (
+	"bytes"
+	"io"
+	"os"
+	"path/filepath"
+)
+
+// AtomicWriteFile atomically writes data to a file by first writing to a
+// temp file and calling rename.
+func AtomicWriteFile(filename string, data []byte, perm os.FileMode) error {
+	buf := bytes.NewBuffer(data)
+	return atomicWriteFile(filename, buf, int64(len(data)), perm)
+}
+
+// atomicWriteFile writes data to a file by first writing to a temp
+// file and calling rename.
+func atomicWriteFile(filename string, r io.Reader, dataSize int64, perm os.FileMode) error {
+	f, err := os.CreateTemp(filepath.Dir(filename), ".tmp-"+filepath.Base(filename))
+	if err != nil {
+		return err
+	}
+	err = os.Chmod(f.Name(), perm)
+	if err != nil {
+		f.Close()
+		return err
+	}
+	n, err := io.Copy(f, r)
+	if err == nil && n < dataSize {
+		f.Close()
+		return io.ErrShortWrite
+	}
+	if err != nil {
+		f.Close()
+		return err
+	}
+	if err := f.Sync(); err != nil {
+		f.Close()
+		return err
+	}
+	if err := f.Close(); err != nil {
+		return err
+	}
+	return os.Rename(f.Name(), filename)
+}