Avoid throw/catch exceptions when requesting optional buffer through IBufferProtocol (#1902)

* Avoid throw/catch exceptions when requesting optional buffer through `IBufferProtocol`

* Update after review

Author: BCSharp

src/core/IronPython.Modules/_ctypes/CData.cs

@@ -125,7 +125,7 @@ public void Dispose() {
 
             private int _numExports;
 
-            IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags) {
+            IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags, bool throwOnError) {
                 if (_disposed) throw new ObjectDisposedException(GetType().Name);
                 _ = MemHolder; // check if fully initialized
                 Interlocked.Increment(ref _numExports);

src/core/IronPython.Modules/array.cs

@@ -1037,7 +1037,7 @@ bool ICollection<object>.Remove(object item) {
 
             #region IBufferProtocol Members
 
-            IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags) {
+            IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags, bool throwOnError) {
                 return _data.GetBuffer(this, _typeCode.ToString(), flags);
             }
 

src/core/IronPython.Modules/mmap.cs

@@ -1314,10 +1314,13 @@ void IWeakReferenceable.SetFinalizer(WeakRefTracker value) {
 
             #endregion
 
-            public IPythonBuffer GetBuffer(BufferFlags flags = BufferFlags.Simple) {
-                if (flags.HasFlag(BufferFlags.Writable) && IsReadOnly)
-                    throw PythonOps.BufferError("Object is not writable.");
-
+            public IPythonBuffer? GetBuffer(BufferFlags flags, bool throwOnError) {
+                if (flags.HasFlag(BufferFlags.Writable) && IsReadOnly) {
+                    if (throwOnError) {
+                        throw PythonOps.BufferError("Object is not writable.");
+                    }
+                    return null;
+                }
                 return new MmapBuffer(this, flags);
             }
 

src/core/IronPython/Runtime/BufferProtocol.cs

@@ -8,6 +8,7 @@
 using System.Buffers;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Runtime.InteropServices;
 
 using IronPython.Runtime.Exceptions;
@@ -17,7 +18,20 @@ namespace IronPython.Runtime {
     /// Equivalent functionality of CPython's <a href="https://docs.python.org/3/c-api/buffer.html">Buffer Protocol</a>.
     /// </summary>
     public interface IBufferProtocol {
-        IPythonBuffer GetBuffer(BufferFlags flags = BufferFlags.Simple);
+        /// <summary>
+        /// Exports the object's data as a buffer.
+        /// </summary>
+        /// <param name="flags">
+        /// Flags specifying the type of buffer the consumer is prepared to deal with.
+        /// </param>
+        /// <param name="throwOnError">
+        /// This parameter is advisory: if true, the method may throw <c>BufferError</c> instead of returning null if the buffer type is not supported,
+        /// with the exception's message providing more information.
+        /// </param>
+        /// <returns>
+        /// An instance of <see cref="IPythonBuffer"/> if the requested buffer type is supported, or null if not.
+        /// </returns>
+        IPythonBuffer? GetBuffer(BufferFlags flags, bool throwOnError);
     }
 
     /// <summary>
@@ -119,16 +133,31 @@ public enum BufferFlags {
         #endregion
     }
 
-    internal static class BufferProtocolExtensions {
+
+    public static class BufferProtocolExtensions {
+        public static IPythonBuffer GetBuffer(this IBufferProtocol bufferProtocol, BufferFlags flags = BufferFlags.Simple)
+            => bufferProtocol.GetBuffer(flags, throwOnError: true) ?? throw new BufferException("Buffer type not supported");
+
         internal static IPythonBuffer? GetBufferNoThrow(this IBufferProtocol bufferProtocol, BufferFlags flags = BufferFlags.Simple) {
             try {
-                return bufferProtocol.GetBuffer(flags);
+                return bufferProtocol.GetBuffer(flags, throwOnError: false);
             } catch (BufferException) {
                 return null;
             }
         }
+
+        internal static bool TryGetBuffer(this IBufferProtocol bufferProtocol, BufferFlags flags, [NotNullWhen(true)] out IPythonBuffer? buffer) {
+            try {
+                buffer = bufferProtocol.GetBuffer(flags, throwOnError: false);
+                return buffer is not null;
+            } catch (BufferException) {
+                buffer = null;
+                return false;
+            }
+        }
     }
 
+
     /// <summary>
     /// Provides low-level read-write access to byte data of the underlying object.
     /// </summary>

src/core/IronPython/Runtime/ByteArray.cs

@@ -1584,7 +1584,7 @@ private bool Equals(ReadOnlySpan<byte> other) {
 
         #region IBufferProtocol Members
 
-        IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags) {
+        IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags, bool throwOnError) {
             return _bytes.GetBuffer(this, "B", flags);
         }
 

src/core/IronPython/Runtime/Bytes.cs

@@ -1241,10 +1241,13 @@ Expression IExpressionSerializable.CreateExpression() {
 
         #region IBufferProtocol Support
 
-        IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags) {
-            if (flags.HasFlag(BufferFlags.Writable))
-                throw PythonOps.BufferError("Object is not writable.");
-
+        IPythonBuffer? IBufferProtocol.GetBuffer(BufferFlags flags, bool throwOnError) {
+            if (flags.HasFlag(BufferFlags.Writable)) {
+                if (throwOnError) {
+                    throw PythonOps.BufferError("bytes object is not writable");
+                }
+                return null;
+            }
             return new BytesView(this, flags);
         }
 

src/core/IronPython/Runtime/ConversionWrappers.cs

@@ -436,13 +436,16 @@ public MemoryBufferProtocolWrapper(Memory<byte> memory) {
             _memory = memory;
         }
 
-        public IPythonBuffer GetBuffer(BufferFlags flags) {
+        public IPythonBuffer? GetBuffer(BufferFlags flags, bool throwOnError) {
             if (_memory.HasValue) {
                 return new MemoryBufferWrapper(_memory.Value, flags);
             }
 
             if (flags.HasFlag(BufferFlags.Writable)) {
-                throw Operations.PythonOps.BufferError("ReadOnlyMemory is not writable.");
+                if (throwOnError) {
+                    throw Operations.PythonOps.BufferError("ReadOnlyMemory is not writable.");
+                }
+                return null;
             }
 
             return new MemoryBufferWrapper(_rom, flags);

src/core/IronPython/Runtime/MemoryView.cs

@@ -963,32 +963,39 @@ void IWeakReferenceable.SetFinalizer(WeakRefTracker value) {
 
         #region IBufferProtocol Members
 
-        IPythonBuffer IBufferProtocol.GetBuffer(BufferFlags flags) {
+        IPythonBuffer? IBufferProtocol.GetBuffer(BufferFlags flags, bool throwOnError) {
             CheckBuffer();
 
             if (flags.HasFlag(BufferFlags.Writable) && _isReadOnly)
-                throw PythonOps.BufferError("memoryview: underlying buffer is not writable");
+                return ReportError("memoryview: underlying buffer is not writable");
 
             if (flags.HasFlag(BufferFlags.CContiguous) && !_isCContig)
-                throw PythonOps.BufferError("memoryview: underlying buffer is not C-contiguous");
+                return ReportError("memoryview: underlying buffer is not C-contiguous");
 
             if (flags.HasFlag(BufferFlags.FContiguous) && !_isFContig)
-                throw PythonOps.BufferError("memoryview: underlying buffer is not Fortran contiguous");
+                return ReportError("memoryview: underlying buffer is not Fortran contiguous");
 
             if (flags.HasFlag(BufferFlags.AnyContiguous) && !_isCContig && !_isFContig)
-                throw PythonOps.BufferError("memoryview: underlying buffer is not contiguous");
+                return ReportError("memoryview: underlying buffer is not contiguous");
 
             // TODO: Support for suboffsets
             //if (!flags.HasFlag(!BufferFlags.Indirect) && _suboffsets != null)
-            //    throw PythonOps.BufferError("memoryview: underlying buffer requires suboffsets");
+            //    return ReportError("memoryview: underlying buffer requires suboffsets");
 
             if (!flags.HasFlag(BufferFlags.Strides) && !_isCContig)
-                throw PythonOps.BufferError("memoryview: underlying buffer is not C-contiguous");
+                return ReportError("memoryview: underlying buffer is not C-contiguous");
 
             if (!flags.HasFlag(BufferFlags.ND) && flags.HasFlag(BufferFlags.Format))
-                throw PythonOps.BufferError("memoryview: cannot cast to unsigned bytes if the format flag is present");
+                return ReportError("memoryview: cannot cast to unsigned bytes if the format flag is present");
 
             return new MemoryView(this, flags);
+
+            IPythonBuffer? ReportError(string msg) {
+                if (throwOnError) {
+                    throw PythonOps.BufferError(msg);
+                }
+                return null;
+            }
         }
 
         #endregion