Allow empty variable-based series

Author: franzpoeschel

.github/workflows/linux.yml

@@ -270,7 +270,7 @@ jobs:
         cd build
         ctest --output-on-failure
 
-        python3 -m pip install jsonschema@4.2.3 referencing
+        python3 -m pip install jsonschema==4.* referencing
         cd ../share/openPMD/json_schema
         PATH="../../../build/bin:$PATH" make -j 2
         # We need to exclude the thetaMode example since that has a different

share/openPMD/json_schema/Makefile

@@ -5,9 +5,11 @@ json_files = attribute_defs.json attributes.json dataset_defs.json iteration.jso
 .PHONY: all
 all: $(json_files)
 
+# The target file should only be created if the conversion succeeded
 $(json_files): %.json: %.toml
-	$(convert) @$^ > $@
+	$(convert) @$^ > $@.tmp
+	mv $@.tmp $@
 
 .PHONY: clean
 clean:
-	-rm $(json_files)
+	for file in $(json_files); do rm -f "$$file" "$$file.tmp"; done

share/openPMD/json_schema/series.toml

@@ -78,21 +78,34 @@ title = "Contains iterations"
 # Requirement 2.1: Either a single iteration in variable-based encoding #
 #########################################################################
 
-[[allOf.oneOf]]
+[allOf.if]
+properties.attributes.properties.iterationEncoding.oneOf = [
+    { const = { value = "variableBased", datatype = "STRING" } },
+    { const = "variableBased" },
+]
+
+[allOf.then]
 title = "Variable-based encoding"
 properties.attributes.properties.iterationEncoding.properties.value = { const = "variableBased" }
 
-[allOf.oneOf.properties.data]
+[allOf.then.properties.data]
 type = "object"
 title = "An iteration"
 description = "A single iteration."
+
+# Only require iteration data if snapshot attribute is defined
+[allOf.then.properties.data.if]
+required = ["attributes"]
+properties.attributes.required = ["snapshot"]
+
+[allOf.then.properties.data.then]
 "$ref" = "iteration.json"
 
 ############################################################################
 # Requirement 2.2: Or multiple iterations in group- or file-based encoding #
 ############################################################################
 
-[[allOf.oneOf]]
+[allOf.else]
 title = "Group-based (or file-based) encoding"
 properties.attributes.properties.iterationEncoding.properties.value = { oneOf = [
     { const = "groupBased" },
@@ -101,22 +114,22 @@ properties.attributes.properties.iterationEncoding.properties.value = { oneOf =
 
 # Base Path
 
-[allOf.oneOf.properties.data]
+[allOf.else.properties.data]
 type = "object"
 title = "Base path"
 description = "A map of all iterations/snapshots in the Series."
 
 propertyNames.pattern = "^(-?[0-9]*|attributes)$"
 
-[allOf.oneOf.properties.data.properties]
+[allOf.else.properties.data.properties]
 
-[allOf.oneOf.properties.data.properties.attributes]
+[allOf.else.properties.data.properties.attributes]
 title = "Attribute layout"
 description = "Custom attributes allowed, no required attributes defined."
 "$ref" = "attributes.json"
 
 # Base Path -> Iterations
 
-[allOf.oneOf.properties.data.patternProperties."^-?[0-9]*$"]
+[allOf.else.properties.data.patternProperties."^-?[0-9]*$"]
 title = "Iteration"
 "$ref" = "iteration.json"

src/Series.cpp

@@ -1410,24 +1410,6 @@ void Series::flushGorVBased(
     internal::FlushParams const &flushParams,
     bool flushIOHandler)
 {
-    if (iterationEncoding() == IterationEncoding::variableBased &&
-        access::writeOnly(IOHandler()->m_frontendAccess) && iterations.empty())
-    {
-        /*
-         * Note: Unlike flushFileBased, it's ok if `begin == end` since this
-         * method may be called without an explicit iteration.
-         * But since in variable-based encoding the base path is the same as the
-         * path to the (currently active) iteration, there must be at least one
-         * iteration present since the openPMD standard requires mandatory
-         * attributes.
-         * In group-based encoding, any number of iterations might be included
-         * in the base path, in variable-based encoding there must be exactly
-         * one iteration currently active.
-         */
-        throw error::WrongAPIUsage(
-            "variableBased output can not be written with no iterations.");
-    }
-
     auto &series = get();
     if (access::readOnly(IOHandler()->m_frontendAccess))
     {