[Copilot]

Responses sent through a Dev Containers forwarded port are cut short under load. Browsers report ERR_CONTENT_LENGTH_MISMATCH and curl exits with code 18. The bug still reproduces on VS Code 1.145, which already includes microsoft/vscode#338875.

This PR has no file changes. This repo has no port-forwarding code; the fix belongs in the closed-source Dev Containers extension.

Root cause

• For each forwarded connection, the extension runs docker exec -i <container> node -e <relay> (relay script quoted in DevContainer Port Forwarding terminates prematurely #11707).
• When the app's socket closes, the relay calls process.exit() immediately. Any response bytes still queued on process.stdout are lost. Under docker exec -i, stdout is a pipe (FIFO), and Node writes to pipes asynchronously on POSIX.
• This explains why it only affects Dev Containers (WSL uses a different transport), why it depends on load, and why #338875 didn't help (that change was to VS Code's side of the forwarding).
• DevContainer Port Forwarding terminates prematurely #11707 reached the same diagnosis and includes a patch its reporter tested.

Local reproduction using the extension's per-connection docker exec approach and the exact relay from #11707. Load was 1 MB responses, 40 parallel requests × 3 runs × 4 rounds (480 requests):

Relay	Complete	Truncated
Current (process.exit on close)	442	38
Waits for stdout to drain before exiting	480	0

Proposed fix (Dev Containers extension)

client.on('close', hadError => {
  const code = hadError ? 1 : 0;
  // client.pipe(process.stdout) never ends stdout, so end it explicitly or 'finish' never fires
  if (!process.stdout.writableEnded && !process.stdout.destroyed) process.stdout.end();
  if (process.stdout.writableFinished) process.exit(code);
  process.stdout.once('finish', () => process.exit(code));
  process.stdout.once('error', () => process.exit(1));
});

This is a condensed form of the #11707 patch; that original form is the one that produced the 480/480 result above.

Secondary findings

• Exit on stdin close: if the client closes its sending side after the request, the relay exits and returns 0 bytes, even with the fix above. It should call client.end() instead. This change is untested, and browsers and curl don't close their sending side this way, so it isn't behind this issue.
• Slow-reader cutoff (~12 s, reported on VS Code 1.122, before #338875): a forwarder that ignores backpressure and calls destroy() on close produces the same symptom. That matches what _mirrorGenericSocket in VS Code did before #338875. The cutoff is a fixed amount of data (~2.7 MB at both 300 KB/s and 150 KB/s), not a timer. #338875 likely fixed this, but I haven't confirmed that Dev Containers goes through that code path.

Open questions

• Hand the fix to the Dev Containers owners and link DevContainer Port Forwarding terminates prematurely #11707 to this issue.
• Close this PR, since there is nothing to merge.

• Fixes net::ERR_CONTENT_LENGTH_MISMATCH returning large content when port-forwarding from devcontainer #9969