feat(serve): Opt-in parallelism for `deno serve` (#24920)

Adds a `parallel` flag to `deno serve`. When present, we spawn multiple
workers to parallelize serving requests.


```bash
deno serve --parallel main.ts
```

Currently on linux we use `SO_REUSEPORT` and rely on the fact that the
kernel will distribute connections in a round-robin manner.

On mac and windows, we sort of emulate this by cloning the underlying
file descriptor and passing a handle to each worker. The connections
will not be guaranteed to be fairly distributed (and in practice almost
certainly won't be), but the distribution is still spread enough to
provide a significant performance increase.

---
(Run on an Macbook Pro with an M3 Max, serving `deno.com`

baseline::
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   239.78ms   13.56ms 330.54ms   79.12%
    Req/Sec   258.58     35.56   360.00     70.64%
  Latency Distribution
     50%  236.72ms
     75%  248.46ms
     90%  256.84ms
     99%  268.23ms
  15458 requests in 30.02s, 2.47GB read
Requests/sec:    514.89
Transfer/sec:     84.33MB
```

this PR (`with --parallel` flag)
```
❯ wrk -d 30s -c 125 --latency http://127.0.0.1:8000
Running 30s test @ http://127.0.0.1:8000
  2 threads and 125 connections
  Thread Stats   Avg      Stdev     Max   +/- Stdev
    Latency   117.40ms  142.84ms 590.45ms   79.07%
    Req/Sec     1.33k   175.19     1.77k    69.00%
  Latency Distribution
     50%   22.34ms
     75%  223.67ms
     90%  357.32ms
     99%  460.50ms
  79636 requests in 30.07s, 12.74GB read
Requests/sec:   2647.96
Transfer/sec:    433.71MB
```

1 files changed, 192 insertions, 0 deletions

diff --git a/cli/tools/serve.rs b/cli/tools/serve.rs
new file mode 100644
index 000000000..24666b8f6
--- /dev/null
+++ b/cli/tools/serve.rs
@@ -0,0 +1,192 @@
+// Copyright 2018-2024 the Deno authors. All rights reserved. MIT license.
+
+use std::sync::Arc;
+
+use deno_core::error::AnyError;
+use deno_core::futures::TryFutureExt;
+use deno_core::ModuleSpecifier;
+use deno_runtime::deno_permissions::Permissions;
+use deno_runtime::deno_permissions::PermissionsContainer;
+
+use super::run::check_permission_before_script;
+use super::run::maybe_npm_install;
+use crate::args::Flags;
+use crate::args::ServeFlags;
+use crate::args::WatchFlagsWithPaths;
+use crate::factory::CliFactory;
+use crate::util::file_watcher::WatcherRestartMode;
+use crate::worker::CliMainWorkerFactory;
+
+pub async fn serve(
+  flags: Arc<Flags>,
+  serve_flags: ServeFlags,
+) -> Result<i32, AnyError> {
+  check_permission_before_script(&flags);
+
+  if let Some(watch_flags) = serve_flags.watch {
+    return serve_with_watch(flags, watch_flags, serve_flags.worker_count)
+      .await;
+  }
+
+  let factory = CliFactory::from_flags(flags);
+  let cli_options = factory.cli_options()?;
+  let deno_dir = factory.deno_dir()?;
+  let http_client = factory.http_client_provider();
+
+  // Run a background task that checks for available upgrades or output
+  // if an earlier run of this background task found a new version of Deno.
+  #[cfg(feature = "upgrade")]
+  super::upgrade::check_for_upgrades(
+    http_client.clone(),
+    deno_dir.upgrade_check_file_path(),
+  );
+
+  let main_module = cli_options.resolve_main_module()?;
+
+  maybe_npm_install(&factory).await?;
+
+  let permissions = PermissionsContainer::new(Permissions::from_options(
+    &cli_options.permissions_options()?,
+  )?);
+  let worker_factory = factory.create_cli_main_worker_factory().await?;
+
+  do_serve(
+    worker_factory,
+    main_module,
+    permissions,
+    serve_flags.worker_count,
+    false,
+  )
+  .await
+}
+
+async fn do_serve(
+  worker_factory: CliMainWorkerFactory,
+  main_module: ModuleSpecifier,
+  permissions: PermissionsContainer,
+  worker_count: Option<usize>,
+  hmr: bool,
+) -> Result<i32, AnyError> {
+  let mut worker = worker_factory
+    .create_main_worker(
+      deno_runtime::WorkerExecutionMode::Serve {
+        is_main: true,
+        worker_count,
+      },
+      main_module.clone(),
+      permissions.clone(),
+    )
+    .await?;
+  let worker_count = match worker_count {
+    None | Some(1) => return worker.run().await,
+    Some(c) => c,
+  };
+
+  let main = deno_core::unsync::spawn(async move { worker.run().await });
+
+  let extra_workers = worker_count.saturating_sub(1);
+
+  let mut channels = Vec::with_capacity(extra_workers);
+  for i in 0..extra_workers {
+    let worker_factory = worker_factory.clone();
+    let main_module = main_module.clone();
+    let permissions = permissions.clone();
+    let (tx, rx) = tokio::sync::oneshot::channel();
+    channels.push(rx);
+    std::thread::Builder::new()
+      .name(format!("serve-worker-{i}"))
+      .spawn(move || {
+        deno_runtime::tokio_util::create_and_run_current_thread(async move {
+          let result =
+            run_worker(i, worker_factory, main_module, permissions, hmr).await;
+          let _ = tx.send(result);
+        });
+      })?;
+  }
+
+  let (main_result, worker_results) = tokio::try_join!(
+    main.map_err(AnyError::from),
+    deno_core::futures::future::try_join_all(
+      channels.into_iter().map(|r| r.map_err(AnyError::from))
+    )
+  )?;
+
+  let mut exit_code = main_result?;
+  for res in worker_results {
+    let ret = res?;
+    if ret != 0 && exit_code == 0 {
+      exit_code = ret;
+    }
+  }
+  Ok(exit_code)
+
+  // main.await?
+}
+
+async fn run_worker(
+  worker_count: usize,
+  worker_factory: CliMainWorkerFactory,
+  main_module: ModuleSpecifier,
+  permissions: PermissionsContainer,
+  hmr: bool,
+) -> Result<i32, AnyError> {
+  let mut worker = worker_factory
+    .create_main_worker(
+      deno_runtime::WorkerExecutionMode::Serve {
+        is_main: false,
+        worker_count: Some(worker_count),
+      },
+      main_module,
+      permissions,
+    )
+    .await?;
+  if hmr {
+    worker.run_for_watcher().await?;
+    Ok(0)
+  } else {
+    worker.run().await
+  }
+}
+
+async fn serve_with_watch(
+  flags: Arc<Flags>,
+  watch_flags: WatchFlagsWithPaths,
+  worker_count: Option<usize>,
+) -> Result<i32, AnyError> {
+  let hmr = watch_flags.hmr;
+  crate::util::file_watcher::watch_recv(
+    flags,
+    crate::util::file_watcher::PrintConfig::new_with_banner(
+      if watch_flags.hmr { "HMR" } else { "Watcher" },
+      "Process",
+      !watch_flags.no_clear_screen,
+    ),
+    WatcherRestartMode::Automatic,
+    move |flags, watcher_communicator, _changed_paths| {
+      Ok(async move {
+        let factory = CliFactory::from_flags_for_watcher(
+          flags,
+          watcher_communicator.clone(),
+        );
+        let cli_options = factory.cli_options()?;
+        let main_module = cli_options.resolve_main_module()?;
+
+        maybe_npm_install(&factory).await?;
+
+        let _ = watcher_communicator.watch_paths(cli_options.watch_paths());
+
+        let permissions = PermissionsContainer::new(Permissions::from_options(
+          &cli_options.permissions_options()?,
+        )?);
+        let worker_factory = factory.create_cli_main_worker_factory().await?;
+
+        do_serve(worker_factory, main_module, permissions, worker_count, hmr)
+          .await?;
+
+        Ok(())
+      })
+    },
+  )
+  .await?;
+  Ok(0)
+}