From 8282c38fe0f69ca9a8748abeacaf3de989816494 Mon Sep 17 00:00:00 2001
From: Nathan Whitaker <17734409+nathanwhit@users.noreply.github.com>
Date: Tue, 22 Oct 2024 12:52:18 -0700
Subject: [PATCH] fix(cli): increase size of blocking task threadpool on
 windows (#26465)

Fixes #26179.

The original error reported in that issue is fixed on canary, but in
local testing on my windows machine, `next build` would just hang
forever.

After some digging, what happens is that at some point in next build,
readFile promises (from `fs/promises` ) just never resolve, and so next
hangs.

It turns out the issue is saturating tokio's blocking task thread pool.
We previously limited the number of blocking threads to 32, and at some
point those threads are all in use and there's no thread available for
the file reads.

What's taking up all of those threads? The answer turns out to be
`tokio::process`. On windows, child process stdio uses the blocking
threadpool: https://github.com/tokio-rs/tokio/pull/4824. When you poll
the child's stdio on windows, it spawns a blocking task per poll, and
calls `std::io::Read::read` in the blocking context. That call can block
until data is available.
Putting it all together, what happens is that Next.js spawns `2 * the
number of CPU cores` deno child subprocesses to do work. We implement
`child_process` with `tokio::process`. When the child processes' stdio
get polled, blocking tasks get spawned, and those blocking tasks might
block until data is available. So if you have 16 cores (as I do), there
are going to be potentially >32 blocking task threadpool threads taken
just by the child processes. That leaves no room for other tasks to make
progress

---

To fix this, for now, increase the size of the blocking threadpool on
windows. 4 * the number of CPU cores should be enough to leave room for
other tasks to make progress.

Longer term, this can be fixed more properly when we handroll our own
subprocess code (needed for detached processes and additional pipes on
windows).
---
 runtime/tokio_util.rs | 10 +++++++++-
 1 file changed, 9 insertions(+), 1 deletion(-)

diff --git a/runtime/tokio_util.rs b/runtime/tokio_util.rs
index 0d81f6e235..aa0282ece8 100644
--- a/runtime/tokio_util.rs
+++ b/runtime/tokio_util.rs
@@ -43,7 +43,15 @@ pub fn create_basic_runtime() -> tokio::runtime::Runtime {
     // parallel for deno fmt.
     // The default value is 512, which is an unhelpfully large thread pool. We
     // don't ever want to have more than a couple dozen threads.
-    .max_blocking_threads(32)
+    .max_blocking_threads(if cfg!(windows) {
+      // on windows, tokio uses blocking tasks for child process IO, make sure
+      // we have enough available threads for other tasks to run
+      4 * std::thread::available_parallelism()
+        .map(|n| n.get())
+        .unwrap_or(8)
+    } else {
+      32
+    })
     .build()
     .unwrap()
 }