Schema-level metadata is not encoded in Flight responses

[stuartcarnie]

Describe the bug

When preparing schema for encoding into a Flight response, the schema-level metadata from the source schema is dropped:

arrow-rs/arrow-flight/src/encode.rs

Line 326 in e7eb304

Schema::new(fields)

To Reproduce

Index: arrow-flight/src/encode.rs
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/arrow-flight/src/encode.rs b/arrow-flight/src/encode.rs
--- a/arrow-flight/src/encode.rs	(revision e7eb304dac442a943c434f8ea248de909f82aa88)
+++ b/arrow-flight/src/encode.rs	(date 1677611826249)
@@ -453,6 +453,7 @@
 
 #[cfg(test)]
 mod tests {
+    use std::collections::HashMap;
     use arrow::{
         array::{UInt32Array, UInt8Array},
         compute::concat_batches,
@@ -502,6 +503,16 @@
         );
     }
 
+    #[test]
+    fn test_schema_metadata_encoded() {
+        let schema = Schema::new(vec![
+            Field::new("data", DataType::Int32, false),
+        ]).with_metadata(HashMap::from([("some_key".to_owned(), "some_value".to_owned())]));
+
+        let got = prepare_schema_for_flight(&schema);
+        assert!(got.metadata().contains_key("some_key"));
+    }
+
     #[test]
     fn test_encode_no_column_batch() {
         let batch = RecordBatch::try_new_with_options(

Expected behavior

Schema-level metadata should be included permitting test to pass.

Additional context

[stuartcarnie]

@alamb I can confirm that with this patch:

Index: arrow-flight/src/encode.rs
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/arrow-flight/src/encode.rs b/arrow-flight/src/encode.rs
--- a/arrow-flight/src/encode.rs	(revision bbc1469077e13ba2e5a61f130917ad7eccfcb569)
+++ b/arrow-flight/src/encode.rs	(date 1677712010122)
@@ -323,7 +323,7 @@
         })
         .collect();
 
-    Schema::new(fields)
+    Schema::new(fields).with_metadata(schema.metadata().clone())
 }
 
 /// Split [`RecordBatch`] so it hopefully fits into a gRPC response.
@@ -453,6 +453,7 @@
 
 #[cfg(test)]
 mod tests {
+    use std::collections::HashMap;
     use arrow::{
         array::{UInt32Array, UInt8Array},
         compute::concat_batches,
@@ -502,6 +503,16 @@
         );
     }
 
+    #[test]
+    fn test_schema_metadata_encoded() {
+        let schema = Schema::new(vec![
+            Field::new("data", DataType::Int32, false),
+        ]).with_metadata(HashMap::from([("some_key".to_owned(), "some_value".to_owned())]));
+
+        let got = prepare_schema_for_flight(&schema);
+        assert!(got.metadata().contains_key("some_key"));
+    }
+
     #[test]
     fn test_encode_no_column_batch() {
         let batch = RecordBatch::try_new_with_options(

and minor fixes in IOx, the schema metadata propagates across RPC requests

[tustvold]

label_issue.py automatically added labels {'arrow'} from #3811

[tustvold]

label_issue.py automatically added labels {'arrow-flight'} from #3811