Merge pull request lima-vm#2723 from nirs/parallel-downloads

Fix parallel downloads of the same image

pkg/downloader/downloader.go

@@ -12,6 +12,7 @@ import (
 	"os/exec"
 	"path"
 	"path/filepath"
+	"strconv"
 	"strings"
 	"time"
 
@@ -262,14 +263,11 @@ func Download(ctx context.Context, local, remote string, opts ...Opt) (*Result,
 			return res, nil
 		}
 	}
-	if err := os.RemoveAll(shad); err != nil {
-		return nil, err
-	}
 	if err := os.MkdirAll(shad, 0o700); err != nil {
 		return nil, err
 	}
 	shadURL := filepath.Join(shad, "url")
-	if err := os.WriteFile(shadURL, []byte(remote), 0o644); err != nil {
+	if err := atomicWrite(shadURL, []byte(remote), 0o644); err != nil {
 		return nil, err
 	}
 	if err := downloadHTTP(ctx, shadData, shadTime, shadType, remote, o.description, o.expectedDigest); err != nil {
@@ -280,7 +278,7 @@ func Download(ctx context.Context, local, remote string, opts ...Opt) (*Result,
 		return nil, err
 	}
 	if shadDigest != "" && o.expectedDigest != "" {
-		if err := os.WriteFile(shadDigest, []byte(o.expectedDigest.String()), 0o644); err != nil {
+		if err := atomicWrite(shadDigest, []byte(o.expectedDigest.String()), 0o644); err != nil {
 			return nil, err
 		}
 	}
@@ -600,10 +598,8 @@ func downloadHTTP(ctx context.Context, localPath, lastModified, contentType, url
 		return fmt.Errorf("downloadHTTP: got empty localPath")
 	}
 	logrus.Debugf("downloading %q into %q", url, localPath)
-	localPathTmp := localPath + ".tmp"
-	if err := os.RemoveAll(localPathTmp); err != nil {
-		return err
-	}
+
+	localPathTmp := perProcessTempfile(localPath)
 	fileWriter, err := os.Create(localPathTmp)
 	if err != nil {
 		return err
@@ -616,13 +612,13 @@ func downloadHTTP(ctx context.Context, localPath, lastModified, contentType, url
 	}
 	if lastModified != "" {
 		lm := resp.Header.Get("Last-Modified")
-		if err := os.WriteFile(lastModified, []byte(lm), 0o644); err != nil {
+		if err := atomicWrite(lastModified, []byte(lm), 0o644); err != nil {
 			return err
 		}
 	}
 	if contentType != "" {
 		ct := resp.Header.Get("Content-Type")
-		if err := os.WriteFile(contentType, []byte(ct), 0o644); err != nil {
+		if err := atomicWrite(contentType, []byte(ct), 0o644); err != nil {
 			return err
 		}
 	}
@@ -674,10 +670,44 @@ func downloadHTTP(ctx context.Context, localPath, lastModified, contentType, url
 	if err := fileWriter.Close(); err != nil {
 		return err
 	}
-	if err := os.RemoveAll(localPath); err != nil {
+
+	return os.Rename(localPathTmp, localPath)
+}
+
+// To allow parallel download we use a per-process unique suffix for tempoary
+// files. Renaming the temporary file to the final file is safe without
+// synchronization on posix.
+// https://github.com/lima-vm/lima/issues/2722
+func perProcessTempfile(path string) string {
+	return path + ".tmp." + strconv.FormatInt(int64(os.Getpid()), 10)
+}
+
+// atomicWrite writes data to path, creating a new file or replacing existing
+// one. Multiple processess can write to the same path safely. Safe on posix and
+// likely safe on windows when using NTFS.
+func atomicWrite(path string, data []byte, perm os.FileMode) error {
+	tmpPath := perProcessTempfile(path)
+	tmp, err := os.OpenFile(tmpPath, os.O_WRONLY|os.O_CREATE|os.O_TRUNC, perm)
+	if err != nil {
+		return err
+	}
+	defer func() {
+		if err != nil {
+			tmp.Close()
+			os.RemoveAll(tmpPath)
+		}
+	}()
+	if _, err = tmp.Write(data); err != nil {
 		return err
 	}
-	return os.Rename(localPathTmp, localPath)
+	if err = tmp.Sync(); err != nil {
+		return err
+	}
+	if err = tmp.Close(); err != nil {
+		return err
+	}
+	err = os.Rename(tmpPath, path)
+	return err
 }
 
 // CacheEntries returns a map of cache entries.