fix: read output streams straight from native memory

src/typstsharp.tests/Tests.cs

@@ -1,3 +1,4 @@
+using System.Buffers;
 using System.Text;
 using UglyToad.PdfPig;
 using UglyToad.PdfPig.Content;
@@ -721,6 +722,97 @@ public async Task OutputStreamRejectsReadsAfterTheDocumentIsDisposed()
         await Assert.That(() => stream.Read(new byte[16].AsSpan())).Throws<ObjectDisposedException>();
     }
 
+    /// <summary>
+    /// A span read must not route through <see cref="Stream.Read(Span{byte})"/>, which serves the read
+    /// from an array rented from the process-wide <see cref="ArrayPool{T}"/> and returns it without
+    /// clearing it. That would leave the rendered document readable by the next unrelated component to
+    /// rent from the same pool, which is exactly what PooledBuffer goes out of its way to prevent.
+    /// </summary>
+    [Test]
+    public async Task SpanReadLeavesNoDocumentBytesInTheSharedArrayPool()
+    {
+        using var compiler = TypstCompiler.FromSource("= Not for the next renter");
+        using var document = compiler.CompileToDocument();
+
+        int length = (int)document.GetOutputLength();
+
+        // Prime the pool so that a rent of this size is served from a known array rather than a
+        // fresh allocation. Nothing may await between here and the final rent, or the thread-local
+        // pool slot the priming lands in may not be the one the read and the check see.
+        byte[] primed = ArrayPool<byte>.Shared.Rent(length);
+        primed.AsSpan().Clear();
+        ArrayPool<byte>.Shared.Return(primed);
+
+        using (var stream = document.OpenOutputStream())
+        {
+            stream.ReadExactly(new byte[length]);
+        }
+
+        byte[] afterwards = ArrayPool<byte>.Shared.Rent(length);
+        bool carriesTheDocument = afterwards.AsSpan(0, 5).SequenceEqual("%PDF-"u8);
+        ArrayPool<byte>.Shared.Return(afterwards);
+
+        await Assert.That(carriesTheDocument).IsFalse();
+    }
+
+    [Test]
+    public async Task SpanReadReturnsTheWholeBufferAndThenReportsEndOfStream()
+    {
+        using var compiler = TypstCompiler.FromSource("= Read by span");
+        using var document = compiler.CompileToDocument();
+        var expected = document.GetOutputBytes();
+
+        using var stream = document.OpenOutputStream();
+
+        // A span larger than the document is the boundary worth pinning: the read must stop at the
+        // end of the buffer rather than at the end of the span.
+        var oversized = new byte[expected.Length + 64];
+        int read = stream.Read(oversized.AsSpan());
+
+        await Assert.That(read).IsEqualTo(expected.Length);
+        await Assert.That(oversized.AsSpan(0, expected.Length).SequenceEqual(expected)).IsTrue();
+        await Assert.That(stream.Read(oversized.AsSpan())).IsEqualTo(0);
+        await Assert.That(stream.Read(Span<byte>.Empty)).IsEqualTo(0);
+    }
+
+    [Test]
+    public async Task ChunkedSpanReadsReassembleTheDocument()
+    {
+        using var compiler = TypstCompiler.FromSource(TwoPageSource);
+        using var document = compiler.CompileToDocument();
+        var expected = document.GetOutputBytes();
+
+        using var stream = document.OpenOutputStream();
+        var reassembled = new byte[expected.Length];
+
+        // Chunks that do not divide the length evenly, so the final short read is covered too.
+        const int chunk = 1000;
+        int offset = 0;
+        int read;
+        while ((read = stream.Read(reassembled.AsSpan(offset, Math.Min(chunk, reassembled.Length - offset)))) > 0)
+        {
+            offset += read;
+        }
+
+        await Assert.That(offset).IsEqualTo(expected.Length);
+        await Assert.That(reassembled.SequenceEqual(expected)).IsTrue();
+    }
+
+    [Test]
+    public async Task AsyncReadMatchesOutputBytes()
+    {
+        using var compiler = TypstCompiler.FromSource("= Read asynchronously");
+        using var document = compiler.CompileToDocument();
+        var expected = document.GetOutputBytes();
+
+        using var stream = document.OpenOutputStream();
+        var destination = new byte[expected.Length];
+        int read = await stream.ReadAsync(destination.AsMemory());
+
+        await Assert.That(read).IsEqualTo(expected.Length);
+        await Assert.That(destination.SequenceEqual(expected)).IsTrue();
+    }
+
     [Test]
     public async Task OutputLengthMatchesOutputBytes()
     {

src/typstsharp/TypstDocument.cs

@@ -326,24 +326,53 @@ private unsafe void Free()
     private sealed unsafe class OutputStream : UnmanagedMemoryStream
     {
         private readonly TypstDocument _owner;
+        private readonly byte* _pointer;
 
         internal OutputStream(TypstDocument owner, byte* pointer, long length)
             : base(pointer, length, length, FileAccess.Read)
         {
             _owner = owner;
+            _pointer = pointer;
         }
 
-        // Only the byte-array reads are overridden. Every other read path on Stream and
-        // UnmanagedMemoryStream, span and async alike, ends up calling one of these two, so this
-        // covers them all. Overriding Read(Span) as well would recurse: the UnmanagedMemoryStream
-        // override delegates to Stream.Read(Span) for any derived type, and that implementation
-        // calls back into Read(byte[]).
-        public override int Read(byte[] buffer, int offset, int count)
+        /// <summary>
+        /// Copies straight out of the native buffer, which is what keeps every span-shaped and
+        /// asynchronous read off the managed heap.
+        /// </summary>
+        /// <remarks>
+        /// <see cref="UnmanagedMemoryStream"/> only takes its own direct path when the runtime type is
+        /// exactly <see cref="UnmanagedMemoryStream"/>; for a derived type it defers to
+        /// <see cref="Stream.Read(Span{byte})"/>, which rents an array the size of the caller's span
+        /// from the shared <see cref="ArrayPool{T}"/>, reads into it, copies it out, and returns it to
+        /// the pool without clearing it. That costs a second copy of every byte and, worse, leaves the
+        /// rendered document in a process-wide pool for whatever rents from it next, which is the very
+        /// thing <c>PooledBuffer</c> clears its buffer to avoid. Reading the pointer here sidesteps
+        /// both. It does not recurse: recursion would need a call back into <c>base.Read(Span)</c>.
+        /// </remarks>
+        public override int Read(Span<byte> buffer)
         {
             ObjectDisposedException.ThrowIf(_owner.IsDisposed, _owner);
-            int read = base.Read(buffer, offset, count);
+
+            long position = Position;
+            int count = (int)Math.Min((long)buffer.Length, Length - position);
+            if (count <= 0)
+            {
+                GC.KeepAlive(_owner);
+                return 0;
+            }
+
+            new ReadOnlySpan<byte>(_pointer + position, count).CopyTo(buffer);
+            Position = position + count;
             GC.KeepAlive(_owner);
-            return read;
+            return count;
+        }
+
+        // Every other read path on Stream and UnmanagedMemoryStream, byte array and async alike,
+        // funnels into Read(Span) above.
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            ValidateBufferArguments(buffer, offset, count);
+            return Read(new Span<byte>(buffer, offset, count));
         }
 
         public override int ReadByte()