[GitHub] [arrow] sunchao commented on a change in pull request #7917: ARROW-8423: [Rust] [Parquet] Serialize Arrow schema metadata

[GitBox <gi...@apache.org>]

sunchao commented on a change in pull request #7917:
URL: https://github.com/apache/arrow/pull/7917#discussion_r472294646



##########
File path: rust/parquet/src/arrow/schema.rs
##########
@@ -83,12 +90,77 @@ where
         .map(|fields| Schema::new_with_metadata(fields, metadata))
 }
 
+/// Try to convert Arrow schema metadata into a schema
+fn get_arrow_schema_from_metadata(encoded_meta: &str) -> Option<Schema> {
+    let decoded = base64::decode(encoded_meta);
+    match decoded {
+        Ok(bytes) => {
+            let slice = if bytes[0..4] == [255u8; 4] {
+                &bytes[8..]
+            } else {
+                bytes.as_slice()
+            };
+            let message = arrow::ipc::get_root_as_message(slice);
+            message
+                .header_as_schema()
+                .map(arrow::ipc::convert::fb_to_schema)
+        }
+        Err(err) => {
+            // The C++ implementation returns an error if the schema can't be parsed.
+            // To prevent this, we explicitly log this, then compute the schema without the metadata
+            eprintln!(
+                "Unable to decode the encoded schema stored in {}, {:?}",
+                super::ARROW_SCHEMA_META_KEY,
+                err
+            );
+            None
+        }
+    }
+}
+
+/// Mutates writer metadata by encoding the Arrow schema and storing it in the metadata.
+/// If there is an existing Arrow schema metadata, it is replaced.
+pub fn add_encoded_arrow_schema_to_metadata(

Review comment:
       np - my comments are mostly cosmetic - it will be good to change the visibility though.




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