Temp fix: Snapshot attribute in file-based encoding

Snapshot attribute must be written in Iteration::endStep() in file-based
encoding

src/Series.cpp

@@ -23,6 +23,7 @@
 #include "openPMD/IO/AbstractIOHandler.hpp"
 #include "openPMD/IO/AbstractIOHandlerHelper.hpp"
 #include "openPMD/IO/Format.hpp"
+#include "openPMD/IterationEncoding.hpp"
 #include "openPMD/ReadIterations.hpp"
 #include "openPMD/auxiliary/Date.hpp"
 #include "openPMD/auxiliary/Filesystem.hpp"
@@ -2008,7 +2009,14 @@ void Series::flushStep(bool doFlush)
 {
     auto &series = get();
     if (!series.m_currentlyActiveIterations.empty() &&
-        IOHandler()->m_frontendAccess != Access::READ_ONLY)
+        /*
+         * In file-based iteration encoding, this must be triggered by
+         * Iteration::endStep() since the "snapshot" attribute is different
+         * for each file.
+         * Also, at this point the files might have already been closed.
+         */
+        iterationEncoding() != IterationEncoding::fileBased &&
+        access::write(IOHandler()->m_frontendAccess))
     {
         /*
          * Warning: changing attribute extents over time (probably) unsupported
@@ -2022,6 +2030,7 @@ void Series::flushStep(bool doFlush)
         wAttr.resource = std::vector<unsigned long long>{
             series.m_currentlyActiveIterations.begin(),
             series.m_currentlyActiveIterations.end()};
+        series.m_currentlyActiveIterations.clear();
         wAttr.dtype = Datatype::VEC_ULONGLONG;
         IOHandler()->enqueue(IOTask(&series.iterations, wAttr));
         if (doFlush)