fix: non-seekable json streams would fail to load as a document

src/Microsoft.OpenApi/Reader/OpenApiModelFactory.cs

@@ -403,20 +403,12 @@
                 // Inspect the format from the buffered portion
                 format ??= InspectStreamFormat(bufferStream);
 
-                // If format is JSON, no need to buffer further — use the original stream.
-                if (format.Equals(OpenApiConstants.Json, StringComparison.OrdinalIgnoreCase))
-                {
-                    preparedStream = input;
-                }
-                else
-                {
-                    // YAML or other non-JSON format; copy remaining input to a new stream.
-                    preparedStream = new MemoryStream();
-                    bufferStream.Position = 0;
-                    await bufferStream.CopyToAsync(preparedStream, 81920, token).ConfigureAwait(false); // Copy buffered portion
-                    await input.CopyToAsync(preparedStream, 81920, token).ConfigureAwait(false); // Copy remaining data
-                    preparedStream.Position = 0;
-                }
+                // we need to copy the stream to memory string we've already started reading it and can't reposition it
+                preparedStream = new MemoryStream();
+                bufferStream.Position = 0;
+                await bufferStream.CopyToAsync(preparedStream, 81920, token).ConfigureAwait(false); // Copy buffered portion
+                await input.CopyToAsync(preparedStream, 81920, token).ConfigureAwait(false); // Copy remaining data
+                preparedStream.Position = 0;
             }
             else
             {

test/Microsoft.OpenApi.Tests/Reader/OpenApiModelFactoryTests.cs

@@ -3,6 +3,7 @@
 using System.Threading.Tasks;
 using System.IO;
 using System;
+using System.Threading;
 
 namespace Microsoft.OpenApi.Tests.Reader;
 
@@ -119,4 +120,169 @@ await File.WriteAllTextAsync(tempFilePathReferrer,
         Assert.NotNull(readResult.Document.Components);
         Assert.Equal(baseUri, readResult.Document.BaseUri);
     }
+    [Fact]
+    public async Task CanLoadANonSeekableStream()
+    {
+        // Given
+        var documentJson = 
+"""
+{
+    "openapi": "3.1.0",
+    "info": {
+        "title": "Sample API",
+        "version": "1.0.0"
+    },
+    "paths": {}
+}
+""";
+        using var memoryStream = new MemoryStream(System.Text.Encoding.UTF8.GetBytes(documentJson));
+        using var nonSeekableStream = new NonSeekableStream(memoryStream);
+
+        // When
+        var (document, _) = await OpenApiDocument.LoadAsync(nonSeekableStream);
+
+        // Then
+        Assert.NotNull(document);
+        Assert.Equal("Sample API", document.Info.Title);
+    }
+
+    public sealed class NonSeekableStream : Stream
+    {
+        private readonly Stream _innerStream;
+        public NonSeekableStream(Stream stream) : base()
+        {
+            _innerStream = stream;
+        }
+        public override bool CanSeek => false;
+
+        public override long Position { get => _innerStream.Position; set => throw new InvalidOperationException("Seeking is not supported."); }
+
+        public override bool CanRead => _innerStream.CanRead;
+
+        public override bool CanWrite => _innerStream.CanWrite;
+
+        public override long Length => _innerStream.Length;
+        public override IAsyncResult BeginRead(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+        {
+            return _innerStream.BeginRead(buffer, offset, count, callback, state);
+        }
+
+        public override void Flush()
+        {
+            _innerStream.Flush();
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            return _innerStream.Read(buffer, offset, count);
+        }
+
+        public override long Seek(long offset, SeekOrigin origin)
+        {
+            throw new NotSupportedException("Seeking is not supported.");
+        }
+
+        public override void SetLength(long value)
+        {
+            _innerStream.SetLength(value);
+        }
+
+        public override void Write(byte[] buffer, int offset, int count)
+        {
+            _innerStream.Write(buffer, offset, count);
+        }
+        protected override void Dispose(bool disposing)
+        {
+            _innerStream.Dispose();
+            base.Dispose(disposing);
+        }
+
+        public override async ValueTask DisposeAsync()
+        {
+            await _innerStream.DisposeAsync();
+            await base.DisposeAsync();
+        }
+
+        public override Task CopyToAsync(Stream destination, int bufferSize, CancellationToken cancellationToken)
+        {
+            return _innerStream.CopyToAsync(destination, bufferSize, cancellationToken);
+        }
+
+        public override bool CanTimeout => _innerStream.CanTimeout;
+
+        public override IAsyncResult BeginWrite(byte[] buffer, int offset, int count, AsyncCallback callback, object state)
+        {
+            return _innerStream.BeginWrite(buffer, offset, count, callback, state);
+        }
+
+        public override void CopyTo(Stream destination, int bufferSize)
+        {
+            _innerStream.CopyTo(destination, bufferSize);
+        }
+
+        public override void Close()
+        {
+            _innerStream.Close();
+        }
+
+        public override int EndRead(IAsyncResult asyncResult)
+        {
+            return _innerStream.EndRead(asyncResult);
+        }
+
+        public override void EndWrite(IAsyncResult asyncResult)
+        {
+            _innerStream.EndWrite(asyncResult);
+        }
+
+        public override int ReadByte()
+        {
+            return _innerStream.ReadByte();
+        }
+
+        public override void WriteByte(byte value)
+        {
+            _innerStream.WriteByte(value);
+        }
+
+        public override Task FlushAsync(CancellationToken cancellationToken)
+        {
+            return _innerStream.FlushAsync(cancellationToken);
+        }
+
+        public override int Read(Span<byte> buffer)
+        {
+            return _innerStream.Read(buffer);
+        }
+
+        public override Task<int> ReadAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            return _innerStream.ReadAsync(buffer, offset, count, cancellationToken);
+        }
+
+        public override ValueTask<int> ReadAsync(Memory<byte> buffer, CancellationToken cancellationToken = default)
+        {
+            return _innerStream.ReadAsync(buffer, cancellationToken);
+        }
+
+        public override int ReadTimeout { get => _innerStream.ReadTimeout; set => _innerStream.ReadTimeout = value; }
+
+        public override void Write(ReadOnlySpan<byte> buffer)
+        {
+            _innerStream.Write(buffer);
+        }
+
+        public override Task WriteAsync(byte[] buffer, int offset, int count, CancellationToken cancellationToken)
+        {
+            return _innerStream.WriteAsync(buffer, offset, count, cancellationToken);
+        }
+
+        public override ValueTask WriteAsync(ReadOnlyMemory<byte> buffer, CancellationToken cancellationToken = default)
+        {
+            return _innerStream.WriteAsync(buffer, cancellationToken);
+        }
+
+        public override int WriteTimeout { get => _innerStream.WriteTimeout; set => _innerStream.WriteTimeout = value; }
+
+    }
 }