gazelle/common: keep dangling BUILD symlinks across snapshot restore

gazelle/common/buildsnapshot.go

@@ -20,6 +20,8 @@ type snapshotEntry struct {
 	data   []byte
 	exists bool
 	mode   os.FileMode
+	link   bool
+	target string
 }
 
 // Reset forgets all recorded files.
@@ -42,6 +44,10 @@ func (s *BuildSnapshot) Record(path string) error {
 	}
 	info, err := os.Stat(path)
 	if errors.Is(err, os.ErrNotExist) || (err == nil && info.IsDir()) {
+		if target, lerr := os.Readlink(path); lerr == nil {
+			s.files[path] = snapshotEntry{link: true, target: target}
+			return nil
+		}
 		s.files[path] = snapshotEntry{}
 		return nil
 	}
@@ -81,6 +87,12 @@ func (s *BuildSnapshot) Restore() error {
 	var errs []error
 	for _, p := range paths {
 		e := s.files[p]
+		if e.link {
+			if err := restoreLink(p, e.target); err != nil {
+				errs = append(errs, fmt.Errorf("restore %s: %w", p, err))
+			}
+			continue
+		}
 		if !e.exists {
 			if err := os.Remove(p); err != nil && !errors.Is(err, os.ErrNotExist) {
 				errs = append(errs, fmt.Errorf("restore %s: %w", p, err))
@@ -96,3 +108,15 @@ func (s *BuildSnapshot) Restore() error {
 	}
 	return errors.Join(errs...)
 }
+
+// restoreLink keeps a link that still points at target and otherwise replaces
+// whatever is at path with a link to target.
+func restoreLink(path, target string) error {
+	if cur, err := os.Readlink(path); err == nil && cur == target {
+		return nil
+	}
+	if err := os.Remove(path); err != nil && !errors.Is(err, os.ErrNotExist) {
+		return err
+	}
+	return os.Symlink(target, path)
+}

gazelle/common/buildsnapshot_test.go

@@ -3,10 +3,56 @@ package common
 import (
 	"os"
 	"path/filepath"
+	"runtime"
 	"strings"
 	"testing"
 )
 
+func TestBuildSnapshotDanglingLink(t *testing.T) {
+	if runtime.GOOS == "windows" {
+		t.Skip("creating symlinks needs privileges on Windows")
+	}
+	dir := t.TempDir()
+	build := filepath.Join(dir, "BUILD.bazel")
+	const target = "missing/BUILD.bazel"
+	if err := os.Symlink(target, build); err != nil {
+		t.Fatal(err)
+	}
+	var s BuildSnapshot
+	if err := s.Record(build); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.Restore(); err != nil {
+		t.Fatal(err)
+	}
+	if got, err := os.Readlink(build); err != nil || got != target {
+		t.Fatalf("intact link = %q, %v; want %q", got, err, target)
+	}
+
+	if err := os.Remove(build); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.Restore(); err != nil {
+		t.Fatal(err)
+	}
+	if got, err := os.Readlink(build); err != nil || got != target {
+		t.Fatalf("removed link = %q, %v; want %q", got, err, target)
+	}
+
+	if err := os.Remove(build); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.Symlink("other/BUILD.bazel", build); err != nil {
+		t.Fatal(err)
+	}
+	if err := s.Restore(); err != nil {
+		t.Fatal(err)
+	}
+	if got, err := os.Readlink(build); err != nil || got != target {
+		t.Fatalf("repointed link = %q, %v; want %q", got, err, target)
+	}
+}
+
 func TestBuildSnapshotRestore(t *testing.T) {
 	dir := t.TempDir()
 	same := filepath.Join(dir, "same")