src: handling v8 thread pool of zero

fix: nodejs#42523

Problem:
If no platform worker exists, Node.js doesn't shut down when background
tasks exist. It keeps waiting in `NodePlatform::DrainTasks`.

Observation:
It seems that Node.js used to use V8's `DefaultPlatform` implementation,
which chooses a suitable default value in case that `--v8-pool-size=0`
is given as a command-line option. However, Node.js currently uses its
own v8::Platform implementation, `NodePlatform`. It doesn't have the
logic to handle the case.

I referred to nodejs#4344 to track the issue.

doc/api/cli.md

@@ -1368,11 +1368,9 @@ added: v5.10.0
 
 Set V8's thread pool size which will be used to allocate background jobs.
 
-If set to `0` then V8 will choose an appropriate size of the thread pool based
+If set to `0` then Node.js will choose an appropriate size of the thread pool based
 on the number of online processors.
 
-If the value provided is larger than V8's maximum, then the largest value
-will be chosen.
 
 ### `--zero-fill-buffers`
 

src/node_platform.cc

@@ -45,6 +45,19 @@ static void PlatformWorkerThread(void* data) {
   }
 }
 
+static int GetActualThreadPoolSize(int thread_pool_size) {
+  if (thread_pool_size < 1) {
+    uv_cpu_info_t* cpu_info;
+    int count;
+
+    if (uv_cpu_info(&cpu_info, &count) == 0) {
+      uv_free_cpu_info(cpu_info, count);
+      thread_pool_size = count - 1;
+    }
+  }
+  return std::max(thread_pool_size, 1);
+}
+
 }  // namespace
 
 class WorkerThreadsTaskRunner::DelayedTaskScheduler {
@@ -340,6 +353,8 @@ NodePlatform::NodePlatform(int thread_pool_size,
   // current v8::Platform instance.
   SetTracingController(tracing_controller_);
   DCHECK_EQ(GetTracingController(), tracing_controller_);
+
+  thread_pool_size = GetActualThreadPoolSize(thread_pool_size);
   worker_thread_task_runner_ =
       std::make_shared<WorkerThreadsTaskRunner>(thread_pool_size);
 }

test/parallel/test-worker-v8-pool-size-0.js

@@ -0,0 +1,15 @@
+// Flags: --v8-pool-size=0
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+const { Worker, isMainThread } = require('worker_threads');
+
+if (isMainThread) {
+  const w = new Worker(__filename);
+  w.on('online', common.mustCall());
+  w.on('exit', common.mustCall((code) => assert.strictEqual(code, 0)));
+  process.on('exit', (code) => {
+    assert.strictEqual(code, 0);
+  });
+}