[Fusion] Parse newline-delimited JSONL records as a single arena span (#10077)

Author: michaelstaib

src/HotChocolate/AspNetCore/src/Transport.Http/JsonLines/JsonLinesReader.cs

@@ -24,6 +24,10 @@ internal class JsonLinesReader(HttpResponseMessage message, IMemoryArenaSource a
 internal class JsonLinesReader(HttpResponseMessage message) : IAsyncEnumerable<OperationResult>
 #endif
 {
+#if FUSION
+    private const int MaxSingleSpanRecordLength = 16 * 1024;
+
+#endif
     private static readonly StreamPipeReaderOptions s_options = new(
         pool: MemoryPool<byte>.Shared,
         bufferSize: 4096,
@@ -98,9 +102,37 @@ public async IAsyncEnumerator<OperationResult> GetAsyncEnumerator(
 #if FUSION
     private static SourceResultDocument ParseDocument(IMemoryArena arena, ReadOnlySequence<byte> lineBuffer)
     {
-        // Each record is one newline-delimited line. Its content bytes are filled once, directly into
-        // the arena's geometric segments using the same per-record mechanic as the SSE reader, then
-        // parsed in place via ParseFilled so no bytes are copied a second time.
+        var length = lineBuffer.Length;
+
+        // A record whose length fits within MaxSingleSpanRecordLength is filled once into a single
+        // exact-length arena chunk and parsed in place as one span. This skips the geometric ramp fill
+        // and the multi-segment reader it produces. The record length is known from newline framing, so
+        // this does not depend on a Content-Length header.
+        //
+        // The threshold is capped at 16 KB because the request-scoped arena is shared by all concurrent
+        // subgraph fetches and lives until the response is written. A large exact-length rent that does
+        // not fit the current page strands the whole remaining page tail for the rest of the request,
+        // while the geometric ramp's small leading chunks can fill those tails. Small records, which
+        // cover all realistic traffic, get the single-span win. Large records keep the tail-filling ramp.
+        if (length > 0 && length <= MaxSingleSpanRecordLength)
+        {
+            var lineLength = (int)length;
+            var buffer = arena.Rent(lineLength);
+            lineBuffer.CopyTo(buffer.Span);
+
+            var segments = arena.RentSegmentTable(1);
+            segments[0] = buffer;
+
+            return SourceResultDocument.ParseFilled(
+                arena,
+                segments,
+                usedChunks: 1,
+                lastLength: lineLength);
+        }
+
+        // Fallback: larger records (and the degenerate empty line) are filled across the geometric data
+        // chunk schedule and parsed as a multi-segment sequence, using the same per-record mechanic as
+        // the SSE reader.
         var chunks = arena.RentSegmentTable(64);
         var chunkIndex = 0;
         var chunkSize = SourceResultDocument.GetDataChunkSize(chunkIndex);