core/rawdb: use readonly file lock in readonly mode (#28180)

This allows using the freezer from multiple processes at once
in read-only mode.

Co-authored-by: Martin Holst Swende <martin@swende.se>

core/rawdb/freezer.go

@@ -108,7 +108,11 @@ func NewFreezer(datadir string, namespace string, readonly bool, maxTableSize ui
 	// Leveldb uses LOCK as the filelock filename. To prevent the
 	// name collision, we use FLOCK as the lock name.
 	lock := flock.New(flockFile)
-	if locked, err := lock.TryLock(); err != nil {
+	tryLock := lock.TryLock
+	if readonly {
+		tryLock = lock.TryRLock
+	}
+	if locked, err := tryLock(); err != nil {
 		return nil, err
 	} else if !locked {
 		return nil, errors.New("locking failed")

core/rawdb/freezer_test.go

@@ -283,6 +283,57 @@ func TestFreezerReadonlyValidate(t *testing.T) {
 	}
 }
 
+func TestFreezerConcurrentReadonly(t *testing.T) {
+	t.Parallel()
+
+	tables := map[string]bool{"a": true}
+	dir := t.TempDir()
+
+	f, err := NewFreezer(dir, "", false, 2049, tables)
+	if err != nil {
+		t.Fatal("can't open freezer", err)
+	}
+	var item = make([]byte, 1024)
+	batch := f.tables["a"].newBatch()
+	items := uint64(10)
+	for i := uint64(0); i < items; i++ {
+		require.NoError(t, batch.AppendRaw(i, item))
+	}
+	require.NoError(t, batch.commit())
+	if loaded := f.tables["a"].items.Load(); loaded != items {
+		t.Fatalf("unexpected number of items in table, want: %d, have: %d", items, loaded)
+	}
+	require.NoError(t, f.Close())
+
+	var (
+		wg   sync.WaitGroup
+		fs   = make([]*Freezer, 5)
+		errs = make([]error, 5)
+	)
+	for i := 0; i < 5; i++ {
+		wg.Add(1)
+		go func(i int) {
+			defer wg.Done()
+
+			f, err := NewFreezer(dir, "", true, 2049, tables)
+			if err == nil {
+				fs[i] = f
+			} else {
+				errs[i] = err
+			}
+		}(i)
+	}
+
+	wg.Wait()
+
+	for i := range fs {
+		if err := errs[i]; err != nil {
+			t.Fatal("failed to open freezer", err)
+		}
+		require.NoError(t, fs[i].Close())
+	}
+}
+
 func newFreezerForTesting(t *testing.T, tables map[string]bool) (*Freezer, string) {
 	t.Helper()