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Construct an empty struct for empty ROS2 message definitions #11488

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Construct an empty struct for empty ROS2 message definitions #11488

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18 changes: 14 additions & 4 deletions crates/utils/re_mcap/src/layers/ros2_reflection.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -5,8 +5,8 @@ use arrow::{
array::{
ArrayBuilder, ArrowPrimitiveType, BooleanBuilder, FixedSizeListBuilder, Float32Builder,
Float64Builder, Int8Builder, Int16Builder, Int32Builder, Int64Builder, ListBuilder,
PrimitiveBuilder, StringBuilder, StructBuilder, UInt8Builder, UInt16Builder, UInt32Builder,
UInt64Builder,
PrimitiveBuilder, StringBuilder, StructArray, StructBuilder, UInt8Builder, UInt16Builder,
UInt32Builder, UInt64Builder,
},
datatypes::{
DataType, Field, Fields, Float32Type, Float64Type, Int8Type, Int16Type, Int32Type,
Expand Down Expand Up @@ -96,11 +96,21 @@ impl ArrayBuilder for MessageStructBuilder {
}

fn finish(&mut self) -> arrow::array::ArrayRef {
Arc::new(self.builder.finish())
// Special case for empty structs, e.g. `std_msgs/Empty`
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@emilk emilk Oct 13, 2025 •
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I can see from the code that we're special-casing it, but I cannot see why.

Comment the why, not the what

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This is an attempt to fix an issue when querying out these empty columns. However, it looks like it might be a datafusion issue: RR-2653.

Marking this as draft until then.

if self.spec.fields.is_empty() {
Arc::new(StructArray::new_empty_fields(self.builder.len(), None))
} else {
Arc::new(self.builder.finish())
}
}

fn finish_cloned(&self) -> arrow::array::ArrayRef {
Arc::new(self.builder.finish_cloned())
// Special case for empty structs, e.g. `std_msgs/Empty`
if self.spec.fields.is_empty() {
Arc::new(StructArray::new_empty_fields(self.builder.len(), None))
} else {
Arc::new(self.builder.finish_cloned())
}
}
}

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