Avoid per-line MemoryStream allocation in BufferedReadStream.ReadLine by davidfowl · Pull Request #70194 · dotnet/aspnetcore

davidfowl · 2026-10-31T16:19:46Z

BufferedReadStream.ReadLine/ReadLineAsync allocated a MemoryStream(200) (object + 200-byte buffer) for every line, even when the whole line was already in the read buffer. MultipartReader calls it for every boundary remainder and every section header line (including the blank terminator line), so a multipart form pays this several times per section.

This adds a fast path: if the buffered bytes contain \r\n and the line (content + CR) is within lengthLimit, the line is decoded straight from the buffer with the same Encoding.UTF8.GetString. Otherwise (no CRLF buffered yet, or the limit would be exceeded) it falls back to the existing accumulating path, so returned strings, the limit exception, and stream position behaviour are unchanged.

Benchmark

MultipartReader over a 3-section form (28-char boundary, 5 header lines, small bodies, bodies drained), BenchmarkDotNet InProcess, 3 interleaved rounds:

Round	main	PR	main alloc	PR alloc
1	2.963 us	2.194 us	12,000 B	8,544 B
2	2.943 us	2.331 us	12,000 B	8,544 B
3	2.973 us	2.258 us	12,000 B	8,544 B

~24% faster, 29% less allocated.

Tests

Added BufferedReadStreamTests covering several buffered lines (empty line, bare \r / \n inside a line, \r\r\n, non-ASCII, trailing unterminated data, BufferedData remainder), and a buffered line exactly at / just over the length limit, for both sync and async. They pass on main and on this branch. WebUtilities.Tests (512) and Http.Tests pass.

When the CRLF-terminated line is already fully buffered and within the length limit, decode it directly from the buffer instead of copying it into a new MemoryStream(200). Otherwise fall back to the existing path, which also produces the limit exception, so results and errors are unchanged. This is the path MultipartReader uses for every boundary remainder and section header line. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Copilot

🟡 Changes recommended

The new tests leave the read buffer empty, bypassing the changed over-limit fallback and potentially missing an off-by-one regression; prime the buffer before calling ReadLine.

1 open issue

Prime the read buffer to exercise the over-limit fallback · New

What changed in this PR

This PR reduces per-line allocations in WebUtilities’ BufferedReadStream, improving multipart parsing when complete lines are already buffered.

Changes:

Adds a shared fast path for synchronous and asynchronous reads that decodes buffered CRLF-terminated lines directly.
Retains the accumulating path for incomplete or over-limit lines.
Adds tests for buffered lines, UTF-8 content, remaining data, and length limits.

File	Description
`src/Http/WebUtilities/test/BufferedReadStreamTests.cs`	Adds synchronous and asynchronous line-reading coverage.
`src/Http/WebUtilities/src/BufferedReadStream.cs`	Avoids `MemoryStream` allocation for complete buffered lines within the limit.

_{Review effort: Balanced}

Ensures the new tests exercise the buffered fast path, including the over-limit fallback. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

davidfowl · 2026-10-31T17:18:05Z

Heads-up for reviewers: this PR overlaps with #70156, which was opened earlier from the same account and applies the same TryReadLineFromBuffer fast path in BufferedReadStream.ReadLine/ReadLineAsync. #70156 also changes the MultipartReader header-value trim. The two are not meant to be merged together. Please pick one; the other can be closed. The only extra here is the test coverage that primes the buffer so the fast path is exercised (BufferedReadStreamTests), and it can be ported to #70156 if preferred. I won't add further changes here unless asked.

🤖 Posted by ASP.NET Core Perf 2 (AI Agent)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

davidfowl · 2026-10-31T18:02:42Z

Retriggered CI with an empty commit (d22cf7f). The previous run had one failure, Microsoft.AspNetCore.Mvc.FunctionalTests.RealServerUsingMinimalBackedIntegrationTests.CanResolveServices, with System.InvalidOperationException : IServerAddressesFeature.Addresses cannot be modified after the server has started. That's a startup race in the real-server test host. This PR only changes BufferedReadStream.ReadLine in WebUtilities, which that test doesn't exercise. All other 86,592 tests passed.