Fix concurrency/dir for Windows (#135)

Signed-off-by: joshvanl <me@joshvanl.dev>

Author: JoshVanL

concurrency/dir/dir.go

@@ -27,14 +27,17 @@ type Options struct {
 	Target string
 }
 
-// Dir atomically writes files to a given directory.
+// Dir atomically (best-effort on Windows) writes files to a given directory.
 type Dir struct {
 	log logger.Logger
 
 	base      string
 	target    string
 	targetDir string
 
+	// prev holds a path we should delete on the *next* successful Write.
+	// On Unix: the previously active versioned dir.
+	// On Windows: the last backup directory (target renamed aside).
 	prev *string
 }
 
@@ -50,41 +53,45 @@ func New(opts Options) *Dir {
 func (d *Dir) Write(files map[string][]byte) error {
 	newDir := filepath.Join(d.base, fmt.Sprintf("%d-%s", time.Now().UTC().UnixNano(), d.targetDir))
 
+	// Ensure base exists
 	if err := os.MkdirAll(d.base, 0o700); err != nil {
 		return err
 	}
-
+	// Create the new versioned directory
 	if err := os.MkdirAll(newDir, 0o700); err != nil {
 		return err
 	}
 
+	// Write all files into the new versioned directory
 	for file, b := range files {
 		path := filepath.Join(newDir, file)
+		// Ensure parent directories exist for nested files
+		if err := os.MkdirAll(filepath.Dir(path), 0o700); err != nil {
+			return err
+		}
 		if err := os.WriteFile(path, b, 0o600); err != nil {
 			return err
 		}
 		d.log.Infof("Written file %s", file)
 	}
 
-	if err := os.Symlink(newDir, d.target+".new"); err != nil {
-		return err
-	}
-
-	d.log.Infof("Syslink %s to %s.new", newDir, d.target)
-
-	if err := os.Rename(d.target+".new", d.target); err != nil {
+	// Platform-specific switch into place. It returns what we should delete on the NEXT run.
+	nextPrev, err := d.switchTo(newDir)
+	if err != nil {
 		return err
 	}
 
 	d.log.Infof("Atomic write to %s", d.target)
 
+	// Best-effort cleanup from the *previous* run
 	if d.prev != nil {
 		if err := os.RemoveAll(*d.prev); err != nil {
 			return err
 		}
 	}
 
-	d.prev = &newDir
+	// Set what to delete on the *next* run.
+	d.prev = nextPrev
 
 	return nil
 }

concurrency/dir/switch_unix.go

@@ -0,0 +1,46 @@
+//go:build !windows && !plan9
+
+/*
+Copyright 2025 The Dapr 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 dir
+
+import (
+	"os"
+)
+
+func (d *Dir) switchTo(newDir string) (*string, error) {
+	// Create a symlink and atomically rename it into place.
+	tmpLink := d.target + ".new"
+
+	// Remove any stale temp link
+	_ = os.Remove(tmpLink)
+
+	if err := os.Symlink(newDir, tmpLink); err != nil {
+		return nil, err
+	}
+	d.log.Debugf("Symlink %s -> %s", tmpLink, newDir)
+
+	// Atomically replace the target symlink (or create it if missing)
+	// On POSIX, rename on the same filesystem is atomic.
+	if err := os.Rename(tmpLink, d.target); err != nil {
+		// Clean up temp link if rename fails
+		_ = os.Remove(tmpLink)
+		return nil, err
+	}
+
+	d.log.Debugf("Atomic write to %s", d.target)
+
+	// On Unix we keep versioned dirs and delete the *previous* version on next run.
+	return &newDir, nil
+}

concurrency/dir/switch_windows.go

@@ -0,0 +1,61 @@
+//go:build windows
+
+/*
+Copyright 2025 The Dapr 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 dir
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+)
+
+func (d *Dir) switchTo(newDir string) (*string, error) {
+	// Windows notes:
+	// - os.Rename does NOT replace existing directories.
+	// - Directory symlinks/junctions are unreliable without privileges.
+	// Strategy:
+	//   1) If target exists, rename it to a timestamped backup alongside base.
+	//   2) Rename newDir -> target.
+	//   3) Return backup path so we delete it on the *next* run (avoids data loss if step 2 fails).
+	var backup *string
+
+	// If target exists, rename it aside
+	if fi, err := os.Lstat(d.target); err == nil && fi.IsDir() {
+		bak := filepath.Join(d.base, fmt.Sprintf("backup-%d-%s", time.Now().UTC().UnixNano(), d.targetDir))
+		// Be defensive: remove any stale leftover
+		_ = os.RemoveAll(bak)
+
+		if err := os.Rename(d.target, bak); err != nil {
+			return nil, err
+		}
+		d.log.Debugf("Renamed existing %s to backup %s", d.target, bak)
+		backup = &bak
+	}
+
+	// Move the freshly written versioned dir into place as the new target
+	if err := os.Rename(newDir, d.target); err != nil {
+		// Try to restore the backup if we created one
+		if backup != nil {
+			_ = os.Rename(*backup, d.target)
+		}
+		return nil, err
+	}
+
+	d.log.Debugf("Replaced directory at %s (Windows best-effort atomicity)", d.target)
+
+	// On Windows we delete the backup on the *next* run (so we don't risk losing data if a crash happens now).
+	return backup, nil
+}