all: Make all error implementations pointer types (#6726)

This matches the convention of the stdlib and avoids ambiguity: when
customErr{} and &customErr{} both implement error, client code needs to
check for both.

Memory use should remain the same, since storing a non-pointer type in
an interface value still copies the value to the heap.

lib/db/backend/backend.go

@@ -146,30 +146,20 @@ func OpenMemory() Backend {
 
 type errClosed struct{}
 
-func (errClosed) Error() string { return "database is closed" }
+func (*errClosed) Error() string { return "database is closed" }
 
 type errNotFound struct{}
 
-func (errNotFound) Error() string { return "key not found" }
+func (*errNotFound) Error() string { return "key not found" }
 
 func IsClosed(err error) bool {
-	if _, ok := err.(errClosed); ok {
-		return true
-	}
-	if _, ok := err.(*errClosed); ok {
-		return true
-	}
-	return false
+	_, ok := err.(*errClosed)
+	return ok
 }
 
 func IsNotFound(err error) bool {
-	if _, ok := err.(errNotFound); ok {
-		return true
-	}
-	if _, ok := err.(*errNotFound); ok {
-		return true
-	}
-	return false
+	_, ok := err.(*errNotFound)
+	return ok
 }
 
 // releaser manages counting on top of a waitgroup
@@ -209,7 +199,7 @@ func (cg *closeWaitGroup) Add(i int) error {
 	cg.closeMut.RLock()
 	defer cg.closeMut.RUnlock()
 	if cg.closed {
-		return errClosed{}
+		return &errClosed{}
 	}
 	cg.WaitGroup.Add(i)
 	return nil

lib/db/backend/badger_backend.go

@@ -402,10 +402,10 @@ func wrapBadgerErr(err error) error {
 		return nil
 	}
 	if err == badger.ErrDiscardedTxn {
-		return errClosed{}
+		return &errClosed{}
 	}
 	if err == badger.ErrKeyNotFound {
-		return errNotFound{}
+		return &errNotFound{}
 	}
 	return err
 }

lib/db/backend/leveldb_backend.go

@@ -207,14 +207,11 @@ func (it *leveldbIterator) Error() error {
 
 // wrapLeveldbErr wraps errors so that the backend package can recognize them
 func wrapLeveldbErr(err error) error {
-	if err == nil {
-		return nil
-	}
 	if err == leveldb.ErrClosed {
-		return errClosed{}
+		return &errClosed{}
 	}
 	if err == leveldb.ErrNotFound {
-		return errNotFound{}
+		return &errNotFound{}
 	}
 	return err
 }

lib/db/backend/leveldb_open.go

@@ -149,12 +149,12 @@ func open(location string, opts *opt.Options) (*leveldb.DB, error) {
 		// the database and reindexing...
 		l.Infoln("Database corruption detected, unable to recover. Reinitializing...")
 		if err := os.RemoveAll(location); err != nil {
-			return nil, errorSuggestion{err, "failed to delete corrupted database"}
+			return nil, &errorSuggestion{err, "failed to delete corrupted database"}
 		}
 		db, err = leveldb.OpenFile(location, opts)
 	}
 	if err != nil {
-		return nil, errorSuggestion{err, "is another instance of Syncthing running?"}
+		return nil, &errorSuggestion{err, "is another instance of Syncthing running?"}
 	}
 
 	if debugEnvValue("CompactEverything", 0) != 0 {
@@ -227,6 +227,6 @@ type errorSuggestion struct {
 	suggestion string
 }
 
-func (e errorSuggestion) Error() string {
+func (e *errorSuggestion) Error() string {
 	return fmt.Sprintf("%s (%s)", e.inner.Error(), e.suggestion)
 }

lib/db/db_test.go

@@ -477,7 +477,7 @@ func TestDowngrade(t *testing.T) {
 	// Pretend we just opened the DB and attempt to update it again
 	err := UpdateSchema(db)
 
-	if err, ok := err.(databaseDowngradeError); !ok {
+	if err, ok := err.(*databaseDowngradeError); !ok {
 		t.Fatal("Expected error due to database downgrade, got", err)
 	} else if err.minSyncthingVersion != dbMinSyncthingVersion {
 		t.Fatalf("Error has %v as min Syncthing version, expected %v", err.minSyncthingVersion, dbMinSyncthingVersion)

lib/db/schemaupdater.go

@@ -38,7 +38,7 @@ type databaseDowngradeError struct {
 	minSyncthingVersion string
 }
 
-func (e databaseDowngradeError) Error() string {
+func (e *databaseDowngradeError) Error() string {
 	if e.minSyncthingVersion == "" {
 		return "newer Syncthing required"
 	}
@@ -67,7 +67,7 @@ func (db *schemaUpdater) updateSchema() error {
 	}
 
 	if prevVersion > dbVersion {
-		err := databaseDowngradeError{}
+		err := &databaseDowngradeError{}
 		if minSyncthingVersion, ok, dbErr := miscDB.String("dbMinSyncthingVersion"); dbErr != nil {
 			return dbErr
 		} else if ok {