Added support for DataType::Extension in metadata.

arrow-pyarrow-integration-testing/tests/test_sql.py

@@ -23,6 +23,14 @@
 import arrow_pyarrow_integration_testing
 
 
+class UuidType(pyarrow.PyExtensionType):
+    def __init__(self):
+        super().__init__(pyarrow.binary(16))
+
+    def __reduce__(self):
+        return UuidType, ()
+
+
 class TestCase(unittest.TestCase):
     def setUp(self):
         self.old_allocated_rust = (
@@ -179,3 +187,10 @@ def test_field_metadata(self):
         result = arrow_pyarrow_integration_testing.round_trip_field(field)
         assert field == result
         assert field.metadata == result.metadata
+
+    # see https://issues.apache.org/jira/browse/ARROW-13855
+    def _test_field_extension(self):
+        field = pyarrow.field("aa", UuidType())
+        result = arrow_pyarrow_integration_testing.round_trip_field(field)
+        assert field == result
+        assert field.metadata == result.metadata

src/datatypes/field.rs

@@ -277,3 +277,19 @@ impl std::fmt::Display for Field {
         write!(f, "{:?}", self)
     }
 }
+
+pub(crate) type Metadata = Option<BTreeMap<String, String>>;
+pub(crate) type Extension = Option<(String, Option<String>)>;
+
+pub(crate) fn get_extension(metadata: &Option<BTreeMap<String, String>>) -> Extension {
+    if let Some(metadata) = metadata {
+        if let Some(name) = metadata.get("ARROW:extension:name") {
+            let metadata = metadata.get("ARROW:extension:metadata").cloned();
+            Some((name.clone(), metadata))
+        } else {
+            None
+        }
+    } else {
+        None
+    }
+}

src/datatypes/mod.rs

@@ -7,6 +7,8 @@ pub use field::Field;
 pub use physical_type::*;
 pub use schema::Schema;
 
+pub(crate) use field::{get_extension, Extension, Metadata};
+
 /// The set of datatypes that are supported by this implementation of Apache Arrow.
 ///
 /// The Arrow specification on data types includes some more types.

src/ffi/schema.rs

@@ -1,7 +1,7 @@
 use std::{collections::BTreeMap, convert::TryInto, ffi::CStr, ffi::CString, ptr};
 
 use crate::{
-    datatypes::{DataType, Field, IntervalUnit, TimeUnit},
+    datatypes::{DataType, Extension, Field, IntervalUnit, Metadata, TimeUnit},
     error::{ArrowError, Result},
 };
 
@@ -91,7 +91,26 @@ impl Ffi_ArrowSchema {
             None
         };
 
-        let metadata = field.metadata().as_ref().map(metadata_to_bytes);
+        let metadata = field.metadata();
+
+        let metadata = if let DataType::Extension(name, _, extension_metadata) = field.data_type() {
+            // append extension information.
+            let mut metadata = metadata.clone().unwrap_or_default();
+
+            // metadata
+            if let Some(extension_metadata) = extension_metadata {
+                metadata.insert(
+                    "ARROW:extension:metadata".to_string(),
+                    extension_metadata.clone(),
+                );
+            }
+
+            metadata.insert("ARROW:extension:name".to_string(), name.clone());
+
+            Some(metadata_to_bytes(&metadata))
+        } else {
+            metadata.as_ref().map(metadata_to_bytes)
+        };
 
         let name = CString::new(name).unwrap();
         let format = CString::new(format).unwrap();
@@ -192,7 +211,14 @@ pub fn to_field(schema: &Ffi_ArrowSchema) -> Result<Field> {
     } else {
         to_data_type(schema)?
     };
-    let metadata = unsafe { metadata_from_bytes(schema.metadata) };
+    let (metadata, extension) = unsafe { metadata_from_bytes(schema.metadata) };
+
+    let data_type = if let Some((name, extension_metadata)) = extension {
+        DataType::Extension(name, Box::new(data_type), extension_metadata)
+    } else {
+        data_type
+    };
+
     let mut field = Field::new(schema.name(), data_type, schema.nullable());
     field.set_metadata(metadata);
     Ok(field)
@@ -412,17 +438,17 @@ unsafe fn read_bytes(ptr: *const u8, len: usize) -> &'static str {
     std::str::from_utf8(slice).unwrap()
 }
 
-unsafe fn metadata_from_bytes(
-    data: *const ::std::os::raw::c_char,
-) -> Option<BTreeMap<String, String>> {
+unsafe fn metadata_from_bytes(data: *const ::std::os::raw::c_char) -> (Metadata, Extension) {
     let mut data = data as *const u8; // u8 = i8
     if data.is_null() {
-        return None;
+        return (None, None);
     };
     let len = read_ne_i32(data);
     data = data.add(4);
 
     let mut result = BTreeMap::new();
+    let mut extension_name = None;
+    let mut extension_metadata = None;
     for _ in 0..len {
         let key_len = read_ne_i32(data) as usize;
         data = data.add(4);
@@ -432,7 +458,18 @@ unsafe fn metadata_from_bytes(
         data = data.add(4);
         let value = read_bytes(data, value_len);
         data = data.add(value_len);
-        result.insert(key.to_string(), value.to_string());
+        match key {
+            "ARROW:extension:name" => {
+                extension_name = Some(value.to_string());
+            }
+            "ARROW:extension:metadata" => {
+                extension_metadata = Some(value.to_string());
+            }
+            _ => {
+                result.insert(key.to_string(), value.to_string());
+            }
+        };
     }
-    Some(result)
+    let extension = extension_name.map(|name| (name, extension_metadata));
+    (Some(result), extension)
 }

src/io/ipc/convert.rs

@@ -17,7 +17,9 @@
 
 //! Utilities for converting between IPC types and native Arrow types
 
-use crate::datatypes::{DataType, Field, IntervalUnit, Schema, TimeUnit};
+use crate::datatypes::{
+    get_extension, DataType, Extension, Field, IntervalUnit, Metadata, Schema, TimeUnit,
+};
 use crate::endianess::is_native_little_endian;
 use crate::io::ipc::convert::ipc::UnionMode;
 
@@ -32,9 +34,6 @@ use std::collections::{BTreeMap, HashMap};
 
 use DataType::*;
 
-type Metadata = Option<BTreeMap<String, String>>;
-type Extension = Option<(String, Option<String>)>;
-
 pub fn schema_to_fb_offset<'a>(
     fbb: &mut FlatBufferBuilder<'a>,
     schema: &Schema,
@@ -84,19 +83,6 @@ fn read_metadata(field: &ipc::Field) -> Metadata {
     }
 }
 
-pub(crate) fn get_extension(metadata: &Metadata) -> Extension {
-    if let Some(metadata) = metadata {
-        if let Some(name) = metadata.get("ARROW:extension:name") {
-            let metadata = metadata.get("ARROW:extension:metadata").cloned();
-            Some((name.clone(), metadata))
-        } else {
-            None
-        }
-    } else {
-        None
-    }
-}
-
 /// Convert an IPC Field to Arrow Field
 impl<'a> From<ipc::Field<'a>> for Field {
     fn from(field: ipc::Field) -> Field {

src/io/ipc/mod.rs

@@ -13,7 +13,6 @@ mod compression;
 mod convert;
 
 pub use convert::fb_to_schema;
-pub(crate) use convert::get_extension;
 pub use gen::Message::root_as_message;
 pub mod read;
 pub mod write;

src/io/json_integration/schema.rs

@@ -25,8 +25,7 @@ use serde_json::{json, Value};
 
 use crate::error::{ArrowError, Result};
 
-use crate::datatypes::{DataType, Field, IntervalUnit, Schema, TimeUnit};
-use crate::io::ipc::get_extension;
+use crate::datatypes::{get_extension, DataType, Field, IntervalUnit, Schema, TimeUnit};
 
 pub trait ToJson {
     /// Generate a JSON representation

tests/it/ffi.rs

@@ -172,3 +172,13 @@ fn schema() -> Result<()> {
     let field = field.with_metadata(metadata);
     test_round_trip_schema(field)
 }
+
+#[test]
+fn extension() -> Result<()> {
+    let field = Field::new(
+        "a",
+        DataType::Extension("a".to_string(), Box::new(DataType::Int32), None),
+        true,
+    );
+    test_round_trip_schema(field)
+}