[MM-32234] Fix config.json getting reset (#16805)

* Add support for read-only config stores

* Allow read/write config store for plugin commands that require it

config/store.go

@@ -14,6 +14,12 @@ import (
 	"github.com/mattermost/mattermost-server/v5/utils/jsonutils"
 )
 
+var (
+	// ErrReadOnlyStore is returned when an attempt to modify a read-only
+	// configuration store is made.
+	ErrReadOnlyStore = errors.New("configuration store is read-only")
+)
+
 // Listener is a callback function invoked when the configuration changes.
 type Listener func(oldConfig *model.Config, newConfig *model.Config)
 
@@ -48,13 +54,13 @@ type BackingStore interface {
 }
 
 // NewStore creates a database or file store given a data source name by which to connect.
-func NewStore(dsn string, watch bool, customDefaults *model.Config) (*Store, error) {
+func NewStore(dsn string, watch, readOnly bool, customDefaults *model.Config) (*Store, error) {
 	backingStore, err := getBackingStore(dsn, watch)
 	if err != nil {
 		return nil, err
 	}
 
-	store, err := NewStoreFromBacking(backingStore, customDefaults)
+	store, err := NewStoreFromBacking(backingStore, customDefaults, readOnly)
 	if err != nil {
 		backingStore.Close()
 		return nil, errors.Wrap(err, "failed to create store")
@@ -63,10 +69,11 @@ func NewStore(dsn string, watch bool, customDefaults *model.Config) (*Store, err
 	return store, nil
 }
 
-func NewStoreFromBacking(backingStore BackingStore, customDefaults *model.Config) (*Store, error) {
+func NewStoreFromBacking(backingStore BackingStore, customDefaults *model.Config, readOnly bool) (*Store, error) {
 	store := &Store{
 		backingStore:         backingStore,
 		configCustomDefaults: customDefaults,
+		readOnly:             readOnly,
 	}
 
 	if err := store.Load(); err != nil {
@@ -96,7 +103,7 @@ func NewTestMemoryStore() *Store {
 		panic("failed to initialize memory store: " + err.Error())
 	}
 
-	configStore, err := NewStoreFromBacking(memoryStore, nil)
+	configStore, err := NewStoreFromBacking(memoryStore, nil, false)
 	if err != nil {
 		panic("failed to initialize config store: " + err.Error())
 	}
@@ -114,6 +121,7 @@ type Store struct {
 	configCustomDefaults *model.Config
 
 	persistFeatureFlags bool
+	readOnly            bool
 }
 
 // Get fetches the current, cached configuration.
@@ -156,6 +164,10 @@ func (s *Store) Set(newCfg *model.Config) (*model.Config, error) {
 	var unlockOnce sync.Once
 	defer unlockOnce.Do(s.configLock.Unlock)
 
+	if s.readOnly {
+		return nil, ErrReadOnlyStore
+	}
+
 	oldCfg := s.config.Clone()
 
 	// Really just for some tests we need to set defaults here
@@ -236,7 +248,7 @@ func (s *Store) loadLockedWithOld(oldCfg *model.Config, unlockOnce *sync.Once) e
 	if err != nil {
 		return errors.Wrap(err, "failed to marshal loaded config")
 	}
-	if len(configBytes) == 0 || !bytes.Equal(oldCfgBytes, newCfgBytes) {
+	if !s.readOnly && (len(configBytes) == 0 || !bytes.Equal(oldCfgBytes, newCfgBytes)) {
 		if err := s.backingStore.Set(s.configNoEnv); err != nil {
 			if !errors.Is(err, ErrReadOnlyConfiguration) {
 				return errors.Wrap(err, "failed to persist")
@@ -276,6 +288,9 @@ func (s *Store) GetFile(name string) ([]byte, error) {
 func (s *Store) SetFile(name string, data []byte) error {
 	s.configLock.Lock()
 	defer s.configLock.Unlock()
+	if s.readOnly {
+		return ErrReadOnlyStore
+	}
 	return s.backingStore.SetFile(name, data)
 }
 
@@ -290,6 +305,9 @@ func (s *Store) HasFile(name string) (bool, error) {
 func (s *Store) RemoveFile(name string) error {
 	s.configLock.Lock()
 	defer s.configLock.Unlock()
+	if s.readOnly {
+		return ErrReadOnlyStore
+	}
 	return s.backingStore.RemoveFile(name)
 }
 
@@ -304,3 +322,10 @@ func (s *Store) Close() error {
 	defer s.configLock.Unlock()
 	return s.backingStore.Close()
 }
+
+// IsReadOnly returns whether or not the store is read-only.
+func (s *Store) IsReadOnly() bool {
+	s.configLock.RLock()
+	defer s.configLock.RUnlock()
+	return s.readOnly
+}