diff --git a/pkg/atomicfile/file.go b/pkg/atomicfile/file.go new file mode 100644 index 000000000..7b870f7a7 --- /dev/null +++ b/pkg/atomicfile/file.go @@ -0,0 +1,148 @@ +/* + 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 atomicfile provides a mechanism (on Unix-like platforms) to present a consistent view of a file to separate +processes even while the file is being written. This is accomplished by writing a temporary file, syncing to disk, and +renaming over the destination file name. + +Partial/inconsistent reads can occur due to: + 1. A process attempting to read the file while it is being written to (both in the case of a new file with a + short/incomplete write or in the case of an existing, updated file where new bytes may be written at the beginning + but old bytes may still be present after). + 2. Concurrent goroutines leading to multiple active writers of the same file. + +The above mechanism explicitly protects against (1) as all writes are to a file with a temporary name. + +There is no explicit protection against multiple, concurrent goroutines attempting to write the same file. However, +atomically writing the file should mean only one writer will "win" and a consistent file will be visible. + +Note: atomicfile is partially implemented for Windows. The Windows codepath performs the same operations, however +Windows does not guarantee that a rename operation is atomic; a crash in the middle may leave the destination file +truncated rather than with the expected content. +*/ +package atomicfile + +import ( + "errors" + "fmt" + "io" + "os" + "path/filepath" + "sync" +) + +// File is an io.ReadWriteCloser that can also be Canceled if a change needs to be abandoned. +type File interface { + io.ReadWriteCloser + // Cancel abandons a change to a file. This can be called if a write fails or another error occurs. + Cancel() error +} + +// ErrClosed is returned if Read or Write are called on a closed File. +var ErrClosed = errors.New("file is closed") + +// New returns a new atomic file. On Unix-like platforms, the writer (an io.ReadWriteCloser) is backed by a temporary +// file placed into the same directory as the destination file (using filepath.Dir to split the directory from the +// name). On a call to Close the temporary file is synced to disk and renamed to its final name, hiding any previous +// file by the same name. +// +// Note: Take care to call Close and handle any errors that are returned. Errors returned from Close may indicate that +// the file was not written with its final name. +func New(name string, mode os.FileMode) (File, error) { + return newFile(name, mode) +} + +type atomicFile struct { + name string + f *os.File + closed bool + closedMu sync.RWMutex +} + +func newFile(name string, mode os.FileMode) (File, error) { + dir := filepath.Dir(name) + f, err := os.CreateTemp(dir, "") + if err != nil { + return nil, fmt.Errorf("failed to create temp file: %w", err) + } + if err := f.Chmod(mode); err != nil { + return nil, fmt.Errorf("failed to change temp file permissions: %w", err) + } + return &atomicFile{name: name, f: f}, nil +} + +func (a *atomicFile) Close() (err error) { + a.closedMu.Lock() + defer a.closedMu.Unlock() + + if a.closed { + return nil + } + a.closed = true + + defer func() { + if err != nil { + _ = os.Remove(a.f.Name()) // ignore errors + } + }() + // The order of operations here is: + // 1. sync + // 2. close + // 3. rename + // While the ordering of 2 and 3 is not important on Unix-like operating systems, Windows cannot rename an open + // file. By closing first, we allow the rename operation to succeed. + if err = a.f.Sync(); err != nil { + return fmt.Errorf("failed to sync temp file %q: %w", a.f.Name(), err) + } + if err = a.f.Close(); err != nil { + return fmt.Errorf("failed to close temp file %q: %w", a.f.Name(), err) + } + if err = os.Rename(a.f.Name(), a.name); err != nil { + return fmt.Errorf("failed to rename %q to %q: %w", a.f.Name(), a.name, err) + } + return nil +} + +func (a *atomicFile) Cancel() error { + a.closedMu.Lock() + defer a.closedMu.Unlock() + + if a.closed { + return nil + } + a.closed = true + _ = a.f.Close() // ignore error + return os.Remove(a.f.Name()) +} + +func (a *atomicFile) Read(p []byte) (n int, err error) { + a.closedMu.RLock() + defer a.closedMu.RUnlock() + if a.closed { + return 0, ErrClosed + } + return a.f.Read(p) +} + +func (a *atomicFile) Write(p []byte) (n int, err error) { + a.closedMu.RLock() + defer a.closedMu.RUnlock() + if a.closed { + return 0, ErrClosed + } + return a.f.Write(p) +} diff --git a/pkg/atomicfile/file_test.go b/pkg/atomicfile/file_test.go new file mode 100644 index 000000000..4c86d335c --- /dev/null +++ b/pkg/atomicfile/file_test.go @@ -0,0 +1,77 @@ +/* + 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 atomicfile + +import ( + "fmt" + "os" + "path/filepath" + "testing" + + "github.com/stretchr/testify/assert" + "github.com/stretchr/testify/require" +) + +func TestFile(t *testing.T) { + const content = "this is some test content for a file" + dir := t.TempDir() + path := filepath.Join(dir, "test-file") + + f, err := New(path, 0o644) + require.NoError(t, err, "failed to create file") + n, err := fmt.Fprint(f, content) + assert.NoError(t, err, "failed to write content") + assert.Equal(t, len(content), n, "written bytes should be equal") + err = f.Close() + require.NoError(t, err, "failed to close file") + + actual, err := os.ReadFile(path) + assert.NoError(t, err, "failed to read file") + assert.Equal(t, content, string(actual)) +} + +func TestConcurrentWrites(t *testing.T) { + const content1 = "this is the first content of the file. there should be none other." + const content2 = "the second content of the file should win!" + dir := t.TempDir() + path := filepath.Join(dir, "test-file") + + file1, err := New(path, 0o600) + require.NoError(t, err, "failed to create file1") + file2, err := New(path, 0o644) + require.NoError(t, err, "failed to create file2") + + n, err := fmt.Fprint(file1, content1) + assert.NoError(t, err, "failed to write content1") + assert.Equal(t, len(content1), n, "written bytes should be equal") + + n, err = fmt.Fprint(file2, content2) + assert.NoError(t, err, "failed to write content2") + assert.Equal(t, len(content2), n, "written bytes should be equal") + + err = file1.Close() + require.NoError(t, err, "failed to close file1") + actual, err := os.ReadFile(path) + assert.NoError(t, err, "failed to read file") + assert.Equal(t, content1, string(actual)) + + err = file2.Close() + require.NoError(t, err, "failed to close file2") + actual, err = os.ReadFile(path) + assert.NoError(t, err, "failed to read file") + assert.Equal(t, content2, string(actual)) +}