reader: a one-element string array is a scalar

Masters exist that store sensor_material, description and the compression name as
shape (1,) rather than as true scalars - JUNGFRAU files from an early beamline
deployment do, and the application definition permits it. ReadString() required
rank 0 and threw on anything else, so the whole file was lost: the metadata parse
never finished and no image was ever examined.

Accept any shape holding exactly one element, and keep throwing for a string
dataset that genuinely holds several, which is a different quantity and cannot be
read into one std::string. The numeric path already worked this way
(HDF5DataSet_scalar_stored_rank1); this is the string half.

Measured: three long-wavelength rotation datasets that could not be opened at all
now process end to end.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01EFEJG6WBQv8th4UJFNe53N

tests/HDF5WritingTest.cpp

@@ -137,6 +137,26 @@ TEST_CASE("HDF5DataSet_string", "[HDF5][Unit]") {
     REQUIRE (H5Fget_obj_count(H5F_OBJ_ALL, H5F_OBJ_ALL) == 0);
 }
 
+TEST_CASE("HDF5DataSet_string_stored_rank1", "[HDF5][Unit]") {
+    // Facilities write a one-element string array where the application definition allows a scalar -
+    // JUNGFRAU masters from an early beamline deployment store sensor_material and description that
+    // way - and the file is unreadable if that is refused, because the metadata parse never finishes.
+    {
+        HDF5File file("scratch2b.h5");
+        file.SaveVector("one", std::vector<std::string>{"Si"});
+        file.SaveVector("two", std::vector<std::string>{"Si", "CdTe"});
+    }
+    {
+        HDF5ReadOnlyFile file("scratch2b.h5");
+        HDF5DataSet one(file, "one");
+        CHECK(HDF5DataSpace(one).GetNumOfDimensions() == 1);
+        CHECK(one.ReadString() == "Si");
+        REQUIRE_THROWS(HDF5DataSet(file, "two").ReadString());
+    }
+    remove("scratch2b.h5");
+    REQUIRE (H5Fget_obj_count(H5F_OBJ_ALL, H5F_OBJ_ALL) == 0);
+}
+
 TEST_CASE("HDF5DataSet_vector", "[HDF5][Unit]") {
     std::vector<double> tmp_vector (16384);
     tmp_vector[0] = 599.88;

writer/HDF5Objects.cpp

@@ -908,8 +908,19 @@ HDF5DataSet& HDF5DataSet::WriteDirectChunk(const void *data, hsize_t data_size,
 
 std::string HDF5DataSet::ReadString() const {
     HDF5DataSpace file_space(*this);
-    if (file_space.GetNumOfDimensions() != 0)
-        throw JFJochException(JFJochExceptionCategory::HDF5, "Dataset tries to read string (scalar) from vector dataset");
+    // A rank-1 dataset holding exactly one string is the same thing as a scalar, and facilities
+    // write it both ways - JUNGFRAU masters from an early beamline deployment store
+    // sensor_material and description as shape (1,). Refusing those loses the whole file over a
+    // difference that carries no information.
+    if (file_space.GetNumOfDimensions() != 0) {
+        const auto dims = file_space.GetDimensions();
+        hsize_t elements = 1;
+        for (const auto d : dims)
+            elements *= d;
+        if (elements != 1)
+            throw JFJochException(JFJochExceptionCategory::HDF5,
+                                  "Dataset tries to read string (scalar) from vector dataset");
+    }
 
     HDF5DataType file_data_type(*this);
     const size_t size = file_data_type.GetElemSize();