os: add availableParallelism()

This commit exposes uv_available_parallelism() as an alternative
to cpus().length. uv_available_parallelism() is inspired by
Rust's available_parallelism().

Author: cjihrig

doc/api/os.md

@@ -26,6 +26,17 @@ The operating system-specific end-of-line marker.
 * `\n` on POSIX
 * `\r\n` on Windows
 
+## `os.availableParallelism()`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Returns: {integer}
+
+Returns an estimate of the default amount of parallelism a program should use.
+Always returns a non-zero value.
+
 ## `os.arch()`
 
 <!-- YAML

lib/os.js

@@ -45,6 +45,7 @@ const {
 const { validateInt32 } = require('internal/validators');
 
 const {
+  getAvailableParallelism,
   getCPUs,
   getFreeMem,
   getHomeDirectory: _getHomeDirectory,
@@ -100,6 +101,7 @@ const getOSVersion = () => version;
  */
 const getMachine = () => machine;
 
+getAvailableParallelism[SymbolToPrimitive] = () => getAvailableParallelism();
 getFreeMem[SymbolToPrimitive] = () => getFreeMem();
 getHostname[SymbolToPrimitive] = () => getHostname();
 getOSVersion[SymbolToPrimitive] = () => getOSVersion();
@@ -364,6 +366,7 @@ function userInfo(options) {
 
 module.exports = {
   arch,
+  availableParallelism: getAvailableParallelism,
   cpus,
   endianness,
   freemem: getFreeMem,

src/node_os.cc

@@ -381,6 +381,12 @@ static void GetPriority(const FunctionCallbackInfo<Value>& args) {
 }
 
 
+static void GetAvailableParallelism(const FunctionCallbackInfo<Value>& args) {
+  unsigned int parallelism = uv_available_parallelism();
+  args.GetReturnValue().Set(parallelism);
+}
+
+
 void Initialize(Local<Object> target,
                 Local<Value> unused,
                 Local<Context> context,
@@ -397,6 +403,8 @@ void Initialize(Local<Object> target,
   SetMethod(context, target, "getUserInfo", GetUserInfo);
   SetMethod(context, target, "setPriority", SetPriority);
   SetMethod(context, target, "getPriority", GetPriority);
+  SetMethod(
+      context, target, "getAvailableParallelism", GetAvailableParallelism);
   SetMethod(context, target, "getOSInformation", GetOSInformation);
   target
       ->Set(context,
@@ -417,6 +425,7 @@ void RegisterExternalReferences(ExternalReferenceRegistry* registry) {
   registry->Register(GetUserInfo);
   registry->Register(SetPriority);
   registry->Register(GetPriority);
+  registry->Register(GetAvailableParallelism);
   registry->Register(GetOSInformation);
 }
 

test/parallel/test-os.js

@@ -255,6 +255,8 @@ if (!common.isIBMi) {
   is.number(os.uptime(), 'uptime');
 }
 
+is.number(+os.availableParallelism, 'availableParallelism');
+is.number(os.availableParallelism(), 'availableParallelism');
 is.number(+os.freemem, 'freemem');
 is.number(os.freemem(), 'freemem');
 
@@ -264,3 +266,5 @@ if (common.isWindows) {
 } else {
   assert.strictEqual(devNull, '/dev/null');
 }
+
+assert.ok(os.availableParallelism() > 0);