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net/dns: fix TestDNSTrampleRecovery failure under flakestress

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The test had two problems:

1. runFileWatcher passed hardcoded "/etc/" to the inotify watcher,
   but the test filesystem uses a temp directory prefix. The watcher
   was watching the real /etc/, never seeing the test's file writes.

2. The test's watchFile used gonotify.NewDirWatcher which creates
   goroutines that block on real inotify syscalls. These don't work
   inside synctest's fake-time bubble. The test only passed standalone
   by accident: gonotify walks /etc/ on startup producing fake events
   that happened to trigger trample detection at the right time.

Fix the path issue by adding ActualPath to the wholeFileFS interface,
which translates logical paths (like "/etc/resolv.conf") to real
filesystem paths (respecting any test prefix). Use it in
runFileWatcher so the inotify watch targets the correct directory.

Replace gonotify in the test with a one-shot timer that synctest can
advance through fake time, reliably triggering the trample check.

Fixes #19400

Change-Id: Idb252881ec24d0ab3b3c1d154dbdaf532db837d4
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Brad Fitzpatrick
2026-04-14 04:56:56 +00:00
committed by Brad Fitzpatrick
parent 27f1d4c15d
commit 49eb1b5d26
4 changed files with 36 additions and 31 deletions
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net/dns/direct.go
+18 -1
View File
@@ -442,7 +442,9 @@ func (m *directManager) runFileWatcher() {
if !ok {
return
}
if err := watchFile(m.ctx, "/etc/", resolvConf, m.checkForFileTrample); err != nil {
dir := m.fs.ActualPath(filepath.Dir(resolvConf))
file := m.fs.ActualPath(resolvConf)
if err := watchFile(m.ctx, dir, file, m.checkForFileTrample); err != nil {
// This is all best effort for now, so surface warnings to users.
m.logf("dns: inotify: %s", err)
}
@@ -597,6 +599,19 @@ type wholeFileFS interface {
ReadFile(name string) ([]byte, error)
Remove(name string) error
Rename(oldName, newName string) error
// ActualPath returns the real filesystem path for the given absolute
// logical path. All other methods in this interface accept logical
// paths (like "/etc/resolv.conf") and translate them internally;
// ActualPath exposes that same translation for callers that need
// the real path for use outside the interface (e.g. setting up an
// inotify watch on the correct directory).
//
// For directFS with an empty prefix (production), the input is
// returned unchanged ("/etc" → "/etc"). For directFS with a test
// prefix like "/tmp/test123", the prefix is joined
// ("/etc" → "/tmp/test123/etc"). For wslFS the input is returned
// unchanged, since paths are passed through to wsl.exe as-is.
ActualPath(name string) string
Stat(name string) (isRegular bool, err error)
Truncate(name string) error
WriteFile(name string, contents []byte, perm os.FileMode) error
@@ -613,6 +628,8 @@ type directFS struct {
func (fs directFS) path(name string) string { return filepath.Join(fs.prefix, name) }
func (fs directFS) ActualPath(name string) string { return fs.path(name) }
func (fs directFS) Stat(name string) (isRegular bool, err error) {
fi, err := os.Stat(fs.path(name))
if err != nil {