Make ErrorStorage a concrete type not an interface

Since it is used in a type assertion, having it as an alias to the
error interface is the same as saying 'error', i.e. it succeeds for
all types of error.  Change to a struct which is a concrete type and
the type assertion will only succeed if the type is identical.

Signed-off-by: Bryan Boreham <bjboreham@gmail.com>

promql/engine.go

@@ -74,7 +74,7 @@ type (
 	ErrTooManySamples string
 	// ErrStorage is returned if an error was encountered in the storage layer
 	// during query handling.
-	ErrStorage error
+	ErrStorage struct{ error }
 )
 
 func (e ErrQueryTimeout) Error() string {
@@ -86,6 +86,9 @@ func (e ErrQueryCanceled) Error() string {
 func (e ErrTooManySamples) Error() string {
 	return fmt.Sprintf("query processing would load too many samples into memory in %s", string(e))
 }
+func (e ErrStorage) Error() string {
+	return e.error.Error()
+}
 
 // A Query is derived from an a raw query string and can be run against an engine
 // it is associated with.

promql/engine_test.go

@@ -193,7 +193,7 @@ func TestQueryError(t *testing.T) {
 		Timeout:       10 * time.Second,
 	}
 	engine := NewEngine(opts)
-	errStorage := ErrStorage(fmt.Errorf("storage error"))
+	errStorage := ErrStorage{fmt.Errorf("storage error")}
 	queryable := storage.QueryableFunc(func(ctx context.Context, mint, maxt int64) (storage.Querier, error) {
 		return &errQuerier{err: errStorage}, nil
 	})