Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Jump to bottom

Fix races during parallel downloads #2903

Open
wants to merge 5 commits into
base: master
Choose a base branch
from
Open

Fix races during parallel downloads #2903

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
5 commits
Select commit Hold shift + click to select a range
92537e5
Add failing redownload test
nirs Nov 13, 2024
8b90a0a
Split Download() to smaller functions
nirs Nov 14, 2024
76d5c86
Lock cache directory during download
nirs Nov 14, 2024
c1b4d72
Make parallel downloads test more strict
nirs Nov 14, 2024
5071535
Fix race in dowloader.Cached()
nirs Nov 14, 2024
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
159 changes: 92 additions & 67 deletions pkg/downloader/downloader.go
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -229,6 +229,31 @@ func Download(ctx context.Context, local, remote string, opts ...Opt) (*Result,
return res, nil
}

shad := cacheDirectoryPath(o.cacheDir, remote)
if err := os.MkdirAll(shad, 0o700); err != nil {
return nil, err
}

var res *Result
err := lockutil.WithDirLock(shad, func() error {
var err error
res, err = getCached(ctx, localPath, remote, o)
if err != nil {
return err
}
if res != nil {
return nil
}
res, err = fetch(ctx, localPath, remote, o)
return err
})
return res, err
}

// getCached tries to copy the file from the cache to local path. Return result,
// nil if the file was copied, nil, nil if the file is not in the cache or the
// cache needs update, or nil, error on fatal error.
func getCached(ctx context.Context, localPath, remote string, o options) (*Result, error) {
shad := cacheDirectoryPath(o.cacheDir, remote)
shadData := filepath.Join(shad, "data")
shadTime := filepath.Join(shad, "time")
Expand All @@ -237,53 +262,62 @@ func Download(ctx context.Context, local, remote string, opts ...Opt) (*Result,
if err != nil {
return nil, err
}
if _, err := os.Stat(shadData); err == nil {
logrus.Debugf("file %q is cached as %q", localPath, shadData)
useCache := true
if _, err := os.Stat(shadDigest); err == nil {
logrus.Debugf("Comparing digest %q with the cached digest file %q, not computing the actual digest of %q",
o.expectedDigest, shadDigest, shadData)
if err := validateCachedDigest(shadDigest, o.expectedDigest); err != nil {
return nil, err
}
if err := copyLocal(ctx, localPath, shadData, ext, o.decompress, "", ""); err != nil {
if _, err := os.Stat(shadData); err != nil {
return nil, nil
}
ext := path.Ext(remote)
logrus.Debugf("file %q is cached as %q", localPath, shadData)
if _, err := os.Stat(shadDigest); err == nil {
logrus.Debugf("Comparing digest %q with the cached digest file %q, not computing the actual digest of %q",
o.expectedDigest, shadDigest, shadData)
if err := validateCachedDigest(shadDigest, o.expectedDigest); err != nil {
return nil, err
}
if err := copyLocal(ctx, localPath, shadData, ext, o.decompress, "", ""); err != nil {
return nil, err
}
} else {
if match, lmCached, lmRemote, err := matchLastModified(ctx, shadTime, remote); err != nil {
logrus.WithError(err).Info("Failed to retrieve last-modified for cached digest-less image; using cached image.")
} else if match {
if err := copyLocal(ctx, localPath, shadData, ext, o.decompress, o.description, o.expectedDigest); err != nil {
return nil, err
}
} else {
if match, lmCached, lmRemote, err := matchLastModified(ctx, shadTime, remote); err != nil {
logrus.WithError(err).Info("Failed to retrieve last-modified for cached digest-less image; using cached image.")
} else if match {
if err := copyLocal(ctx, localPath, shadData, ext, o.decompress, o.description, o.expectedDigest); err != nil {
return nil, err
}
} else {
logrus.Infof("Re-downloading digest-less image: last-modified mismatch (cached: %q, remote: %q)", lmCached, lmRemote)
useCache = false
}
}
if useCache {
res := &Result{
Status: StatusUsedCache,
CachePath: shadData,
LastModified: readTime(shadTime),
ContentType: readFile(shadType),
ValidatedDigest: o.expectedDigest != "",
}
return res, nil
logrus.Infof("Re-downloading digest-less image: last-modified mismatch (cached: %q, remote: %q)", lmCached, lmRemote)
return nil, nil
}
}
if err := os.MkdirAll(shad, 0o700); err != nil {
res := &Result{
Status: StatusUsedCache,
CachePath: shadData,
LastModified: readTime(shadTime),
ContentType: readFile(shadType),
ValidatedDigest: o.expectedDigest != "",
}
return res, nil
}

// fetch downloads remote to the cache and copy the cached file to local path.
func fetch(ctx context.Context, localPath, remote string, o options) (*Result, error) {
shad := cacheDirectoryPath(o.cacheDir, remote)
shadData := filepath.Join(shad, "data")
shadTime := filepath.Join(shad, "time")
shadType := filepath.Join(shad, "type")
shadDigest, err := cacheDigestPath(shad, o.expectedDigest)
if err != nil {
return nil, err
}
ext := path.Ext(remote)
shadURL := filepath.Join(shad, "url")
if err := writeFirst(shadURL, []byte(remote), 0o644); err != nil {
if err := os.WriteFile(shadURL, []byte(remote), 0o644); err != nil {
return nil, err
}
if err := downloadHTTP(ctx, shadData, shadTime, shadType, remote, o.description, o.expectedDigest); err != nil {
return nil, err
}
if shadDigest != "" && o.expectedDigest != "" {
if err := writeFirst(shadDigest, []byte(o.expectedDigest.String()), 0o644); err != nil {
if err := os.WriteFile(shadDigest, []byte(o.expectedDigest.String()), 0o644); err != nil {
return nil, err
}
}
Expand Down Expand Up @@ -327,18 +361,33 @@ func Cached(remote string, opts ...Opt) (*Result, error) {
if err != nil {
return nil, err
}

// Checking if data file exists is safe without locking.
if _, err := os.Stat(shadData); err != nil {
return nil, err
}
if _, err := os.Stat(shadDigest); err != nil {
if err := validateCachedDigest(shadDigest, o.expectedDigest); err != nil {
return nil, err
}
} else {
if err := validateLocalFileDigest(shadData, o.expectedDigest); err != nil {
return nil, err

// But validating the digest or the data file must take the lock to avoid races
// with parallel downloads.
if err := os.MkdirAll(shad, 0o700); err != nil {
return nil, err
}
err = lockutil.WithDirLock(shad, func() error {
if _, err := os.Stat(shadDigest); err != nil {
if err := validateCachedDigest(shadDigest, o.expectedDigest); err != nil {
return err
}
} else {
if err := validateLocalFileDigest(shadData, o.expectedDigest); err != nil {
return err
}
}
return nil
})
if err != nil {
return nil, err
}

res := &Result{
Status: StatusUsedCache,
CachePath: shadData,
Expand Down Expand Up @@ -612,13 +661,13 @@ func downloadHTTP(ctx context.Context, localPath, lastModified, contentType, url
}
if lastModified != "" {
lm := resp.Header.Get("Last-Modified")
if err := writeFirst(lastModified, []byte(lm), 0o644); err != nil {
if err := os.WriteFile(lastModified, []byte(lm), 0o644); err != nil {
return err
}
}
if contentType != "" {
ct := resp.Header.Get("Content-Type")
if err := writeFirst(contentType, []byte(ct), 0o644); err != nil {
if err := os.WriteFile(contentType, []byte(ct), 0o644); err != nil {
return err
}
}
Expand Down Expand Up @@ -679,19 +728,7 @@ func downloadHTTP(ctx context.Context, localPath, lastModified, contentType, url
return err
}

// If localPath was created by a parallel download keep it. Replacing it
// while another process is copying it to the destination may fail the
// clonefile syscall. We use a lock to ensure that only one process updates
// data, and when we return data file exists.

return lockutil.WithDirLock(filepath.Dir(localPath), func() error {
if _, err := os.Stat(localPath); err == nil {
return nil
} else if !errors.Is(err, os.ErrNotExist) {
return err
}
return os.Rename(localPathTmp, localPath)
})
return os.Rename(localPathTmp, localPath)
}

var tempfileCount atomic.Uint64
Expand All @@ -706,18 +743,6 @@ func perProcessTempfile(path string) string {
return fmt.Sprintf("%s.tmp.%d.%d", path, os.Getpid(), tempfileCount.Add(1))
}

// writeFirst writes data to path unless path already exists.
func writeFirst(path string, data []byte, perm os.FileMode) error {
return lockutil.WithDirLock(filepath.Dir(path), func() error {
if _, err := os.Stat(path); err == nil {
return nil
} else if !errors.Is(err, os.ErrNotExist) {
return err
}
return os.WriteFile(path, data, perm)
})
}

// CacheEntries returns a map of cache entries.
// The key is the SHA256 of the URL.
// The value is the path to the cache entry.
Expand Down
68 changes: 40 additions & 28 deletions pkg/downloader/downloader_test.go
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -8,7 +8,6 @@ import (
"os/exec"
"path/filepath"
"runtime"
"slices"
"strings"
"testing"
"time"
Expand All @@ -31,11 +30,6 @@ type downloadResult struct {
// races quicker. 20 parallel downloads take about 120 milliseconds on M1 Pro.
const parallelDownloads = 20

// When downloading in parallel usually all downloads completed with
// StatusDownload, but some may be delayed and find the data file when they
// start. Can be reproduced locally using 100 parallel downloads.
var parallelStatus = []Status{StatusDownloaded, StatusUsedCache}

func TestDownloadRemote(t *testing.T) {
ts := httptest.NewServer(http.FileServer(http.Dir("testdata")))
t.Cleanup(ts.Close)
Expand Down Expand Up @@ -103,15 +97,10 @@ func TestDownloadRemote(t *testing.T) {
results <- downloadResult{r, err}
}()
}
// We must process all results before cleanup.
for i := 0; i < parallelDownloads; i++ {
result := <-results
if result.err != nil {
t.Errorf("Download failed: %s", result.err)
} else if !slices.Contains(parallelStatus, result.r.Status) {
t.Errorf("Expected download status %s, got %s", parallelStatus, result.r.Status)
}
}
// Only one thread should download, the rest should use the cache.
downloaded, cached := countResults(t, results)
assert.Equal(t, downloaded, 1)
assert.Equal(t, cached, parallelDownloads-1)
})
})
t.Run("caching-only mode", func(t *testing.T) {
Expand Down Expand Up @@ -146,15 +135,10 @@ func TestDownloadRemote(t *testing.T) {
results <- downloadResult{r, err}
}()
}
// We must process all results before cleanup.
for i := 0; i < parallelDownloads; i++ {
result := <-results
if result.err != nil {
t.Errorf("Download failed: %s", result.err)
} else if !slices.Contains(parallelStatus, result.r.Status) {
t.Errorf("Expected download status %s, got %s", parallelStatus, result.r.Status)
}
}
// Only one thread should download, the rest should use the cache.
downloaded, cached := countResults(t, results)
assert.Equal(t, downloaded, 1)
assert.Equal(t, cached, parallelDownloads-1)
})
})
t.Run("cached", func(t *testing.T) {
Expand Down Expand Up @@ -188,6 +172,26 @@ func TestDownloadRemote(t *testing.T) {
})
}

func countResults(t *testing.T, results chan downloadResult) (downloaded, cached int) {
t.Helper()
for i := 0; i < parallelDownloads; i++ {
result := <-results
if result.err != nil {
t.Errorf("Download failed: %s", result.err)
} else {
switch result.r.Status {
case StatusDownloaded:
downloaded++
case StatusUsedCache:
cached++
default:
t.Errorf("Unexpected download status %q", result.r.Status)
}
}
}
return downloaded, cached
}

func TestRedownloadRemote(t *testing.T) {
remoteDir := t.TempDir()
ts := httptest.NewServer(http.FileServer(http.Dir(remoteDir)))
Expand All @@ -203,18 +207,26 @@ func TestRedownloadRemote(t *testing.T) {
assert.NilError(t, os.Chtimes(remoteFile, time.Now(), time.Now().Add(-time.Hour)))
opt := []Opt{cacheOpt}

r, err := Download(context.Background(), filepath.Join(downloadDir, "digest-less1.txt"), ts.URL+"/digest-less.txt", opt...)
// Download on the first call
r, err := Download(context.Background(), filepath.Join(downloadDir, "1"), ts.URL+"/digest-less.txt", opt...)
assert.NilError(t, err)
assert.Equal(t, StatusDownloaded, r.Status)
r, err = Download(context.Background(), filepath.Join(downloadDir, "digest-less2.txt"), ts.URL+"/digest-less.txt", opt...)

// Next download will use the cached download
r, err = Download(context.Background(), filepath.Join(downloadDir, "2"), ts.URL+"/digest-less.txt", opt...)
assert.NilError(t, err)
assert.Equal(t, StatusUsedCache, r.Status)

// modifying remote file will cause redownload
// Modifying remote file will cause redownload
assert.NilError(t, os.Chtimes(remoteFile, time.Now(), time.Now()))
r, err = Download(context.Background(), filepath.Join(downloadDir, "digest-less3.txt"), ts.URL+"/digest-less.txt", opt...)
r, err = Download(context.Background(), filepath.Join(downloadDir, "3"), ts.URL+"/digest-less.txt", opt...)
assert.NilError(t, err)
assert.Equal(t, StatusDownloaded, r.Status)

// Next download will use the cached download
r, err = Download(context.Background(), filepath.Join(downloadDir, "4"), ts.URL+"/digest-less.txt", opt...)
assert.NilError(t, err)
assert.Equal(t, StatusUsedCache, r.Status)
})

t.Run("has-digest", func(t *testing.T) {
Expand Down
Loading