WSL ConPTY stdin truncation repro

Minimal standalone repro for a regression in WSL 2.9.x pre-release where bytes written to a ConPTY's stdin are silently dropped mid-stream.

Background

Warp bootstraps WSL sessions by writing an ~80 KB shell script to the ConPTY's stdin pipe. On WSL 2.7.10 this works correctly. On WSL 2.9.x pre-release, ~24% of the bytes are silently dropped, causing WSL sessions to never finish bootstrapping.

The drop is not at the tail of the write. It occurs in a burst window partway through the stream (e.g. bytes 11,746–12,264 of a 17,500-byte write), then delivery resumes — consistent with a momentary console input queue overflow that clears after wsl.exe drains it.

Suspected cause

Commit 5db2759f in the WSL repo — "Use overlapped IO when reading from the console" — refactored StandardInputRelay to use MultiHandleWait + ReadConsoleHandle (a RegisterWaitForSingleObject-based loop). There appears to be a race window between m_handleSignaledEvent.ResetEvent() and the next RegisterWaitForSingleObject call. Console input events that arrive during this window are never signaled, stalling the drain loop and causing the Windows console input queue to overflow. When the queue overflows, events are silently discarded.

The old implementation (a simple blocking ReadConsoleInputExW loop) kept the queue continuously drained and did not have this problem.

Results

WSL version	Bytes sent	Bytes received	Result
2.7.10	81,920	82,329 ✓	All bytes delivered
2.9.3	81,920	62,361 ✗	19,559 bytes dropped (23.9%)

Note: ${#VAR} counts the 81,920 'A' bytes plus the ~409 newlines between the 200-char lines in the heredoc, so the expected value on a working system is 82,329, not 81,920.

Gap analysis (with 2500 numbered lines on 2.9.3):

Content sent   : 17,500 bytes (2500 lines)
Bytes received : 16,987
Lines received : 2426
First missing  : line 1678  (byte offset ~11,746)
Last present   : line 2499  (byte offset ~17,493)
Missing lines  : 1678–1751 (74 lines / 518 bytes)

How the repro works

The repro ports the exact ConPTY setup that Warp uses:

1. Bidirectional named pipe — created with NtCreateNamedPipeFile (same as app/src/terminal/local_tty/windows/pipes.rs)
2. conpty.dll — the same handle is passed as both hInput and hOutput to give the ConPTY sole ownership of the pipe (same as app/src/terminal/local_tty/windows/conpty_api.rs)
3. wsl.exe -- bash spawned as a ConPTY child (same as app/src/terminal/local_tty/windows/mod.rs)
4. mio async I/O loop — reads output and writes stdin concurrently with proper WouldBlock handling (same as app/src/terminal/local_tty/event_loop.rs)
5. bash runs read -r -d '' VAR << 'EOM' ... EOM; echo ${#VAR} > /tmp/wsl_repro.txt
6. The result is read back via a plain (non-ConPTY) wsl -- cat invocation

Requirements

• Windows 10 / 11 with WSL 2 installed

• conpty.dll from Windows Terminal or Warp — the repro looks for it at these paths in order:

  • C:\Users\dev\warp\warp\target\debug\conpty.dll
  • C:\Users\dev\warp\warp\app\assets\windows\x64\conpty.dll

  You can change the paths in src/main.rs or copy conpty.dll from a Windows Terminal installation to the same directory as the binary.

• Rust toolchain (cargo)

Usage

cargo build
# Test with default WSL distro
.\target\debug\wsl-stdin-repro.exe

# Test with a specific distro
.\target\debug\wsl-stdin-repro.exe 81920 Ubuntu

# Find which specific bytes are dropped (gap analysis)
.\target\debug\wsl-stdin-repro.exe 0 Ubuntu --find-gap

Example output (WSL 2.9.3 — broken)

============================================================
WSL ConPTY stdin truncation repro (Rust)
  Content bytes : 81920 (80.0 KB)
  Distro        : Ubuntu
============================================================

Pipe: client=HANDLE(0x148)  server=HANDLE(0x144)
ConPTY: HPCON(...)
  Spawned: wsl.exe --distribution Ubuntu -- bash --norc --noprofile  PID=2512
Waiting 3 s for bash to start…
Sending 82403 raw bytes (content=81920) via ConPTY stdin…
Event loop done: 82403/82403 raw bytes delivered.
Waiting up to 30 s for bash to exit…

============================================================
RESULTS
  Content bytes sent   : 81920
  Bytes bash recorded  : Some(62361)
============================================================
  [BUG CONFIRMED] 19559 bytes dropped (23.9%)!

Example output (WSL 2.7.10 — working)

============================================================
RESULTS
  Content bytes sent   : 81920
  Bytes bash recorded  : Some(82329)
============================================================
  [OK] All bytes received. Bug not reproduced at this size.

Related

• WSL issue: microsoft/WSL#XXXXX
• Warp: https://www.warp.dev