[GitHub] [arrow] pitrou commented on a diff in pull request #34133: GH-33856: [C#] C Data interface for schemas and types

["pitrou (via GitHub)" <gi...@apache.org>]

pitrou commented on code in PR #34133:
URL: https://github.com/apache/arrow/pull/34133#discussion_r1148414553


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csharp/src/Apache.Arrow/C/CArrowSchemaImporter.cs:
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@@ -0,0 +1,285 @@
+// Licensed to the Apache Software Foundation (ASF) under one or more
+// contributor license agreements. See the NOTICE file distributed with
+// this work for additional information regarding copyright ownership.
+// The ASF licenses this file to You under the Apache License, Version 2.0
+// (the "License"); you may not use this file except in compliance with
+// the License.  You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using Apache.Arrow.Types;
+
+namespace Apache.Arrow.C
+{
+    public static class CArrowSchemaImporter
+    {
+        /// <summary>
+        /// Import C pointer as an <see cref="ArrowType"/>.
+        /// </summary>
+        /// <examples>
+        /// Typically, you will allocate a uninitialized CArrowSchema pointer,
+        /// pass that to external function, and then use this method to import
+        /// the result.
+        /// 
+        /// <code>
+        /// CArrowSchema* importedPtr = CArrowSchema.New();
+        /// foreign_export_function(importedPtr);
+        /// ArrowType importedType = CArrowSchemaImporter.ImportType(importedPtr);
+        /// CArrowSchema.Free(importedPtr);
+        /// </code>
+        /// </examples>
+        public static unsafe ArrowType ImportType(CArrowSchema* ptr)
+        {
+            using var importedType = new ImportedArrowSchema(ptr);
+            return importedType.GetAsType();
+        }
+
+        /// <summary>
+        /// Import C pointer as an <see cref="Field"/>.
+        /// </summary>
+        /// <examples>
+        /// Typically, you will allocate a uninitialized CArrowSchema pointer,
+        /// pass that to external function, and then use this method to import
+        /// the result.
+        /// 
+        /// <code>
+        /// CArrowSchema* importedPtr = CArrowSchema.New();
+        /// foreign_export_function(importedPtr);
+        /// Field importedField = CArrowSchemaImporter.ImportField(importedPtr);
+        /// CArrowSchema.Free(importedPtr);
+        /// </code>
+        /// </examples>
+        public static unsafe Field ImportField(CArrowSchema* ptr)
+        {
+            using var importedField = new ImportedArrowSchema(ptr);
+            return importedField.GetAsField();
+        }
+
+        /// <summary>
+        /// Import C pointer as an <see cref="Schema"/>.
+        /// </summary>
+        /// <examples>
+        /// Typically, you will allocate a uninitialized CArrowSchema pointer,
+        /// pass that to external function, and then use this method to import
+        /// the result.
+        /// 
+        /// <code>
+        /// CArrowSchema* importedPtr = CArrowSchema.New();
+        /// foreign_export_function(importedPtr);
+        /// Field importedSchema = CArrowSchemaImporter.ImportSchema(importedPtr);
+        /// CArrowSchema.Free(importedPtr);
+        /// </code>
+        /// </examples>
+        public static unsafe Schema ImportSchema(CArrowSchema* ptr)
+        {
+            using var importedSchema = new ImportedArrowSchema(ptr);
+            return importedSchema.GetAsSchema();
+        }
+
+        private sealed unsafe class ImportedArrowSchema : IDisposable
+        {
+            private readonly CArrowSchema* _data;
+            private readonly bool _isRoot;
+
+            public ImportedArrowSchema(CArrowSchema* handle)
+            {
+                _data = (CArrowSchema*)handle;
+                if (_data->release == null)
+                {
+                    throw new ArgumentException("Tried to import a schema that has already been released.");
+                }
+                _isRoot = true;
+            }
+
+            public ImportedArrowSchema(CArrowSchema* handle, bool isRoot) : this(handle)
+            {
+                _isRoot = isRoot;
+            }
+
+            public void Dispose()
+            {
+                // We only call release on a root-level schema, not child ones.
+                if (_isRoot && _data->release != null)
+                {
+                    _data->release(_data);

Review Comment:
   > I feel this is a bad pattern - releasing resources that you don't own/didn't create
   
   This is the entire point. Someone (the producer) decided to share those resources with you, and they need to know when you are finished using those resources.
   
   As @wjones127 said, the release callback will generally not be a straight deallocation; instead, it may decrement a dedicated reference count (or unpin the backing object if the producer is implemented in a GC-based language).
   



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