process: add optional timeout to process.exit()

When set, an internal timer will be set that will exit the
process at the given timeout. In the meantime, the registered
listeners for process.on('exitingSoon') will be invoked and
passed a callback to be called when the handler is ready for
the process to exit. The process will exit either when the
internal timer fires or all the callbacks are called, whichever
comes first.

This is an attempt to deal more intelligently with resource
cleanup and async op completion on exit

(see nodejs#6456).

Author: jasnell

doc/api/process.md

@@ -44,6 +44,35 @@ process.on('exit', (code) => {
   console.log('About to exit with code:', code);
 });
 ```
+## Event: 'exitingSoon'
+
+Emitted when `process.exit()` is called using the optional `timeout` argument.
+
+Calling `process.exit()` with the optional `timeout` creates an internal timer
+that will exit the Node.js process after the given period of time (in
+milliseconds). Each `exitingSoon` listener registered at the time the 
+`process.exit()` was called will be invoked and passed a callback that must be
+called when the listener has completed it's work and is ready for the process
+to exit. The process will exit either when all of the listeners have called
+the callback indicating that they are ready or when the internal exit timer
+is triggered, whichever comes first.
+
+```js
+// Some ongoing task that would normally keep the event loop active
+const timer1 = setInterval(() => {}, 1000);
+
+// Register an exitingSoon handler to clean up before exit
+process.on('exitingSoon', (ready) => {
+  setImmediate(() => {
+    // Clean up resources
+    clearInterval(timer1);
+    // Notify that we're done
+    ready();
+  });
+});
+
+process.exit(0, 10000);
+```
 
 ## Event: 'message'
 
@@ -704,7 +733,7 @@ Example:
 ```
 
 
-## process.exit([code])
+## process.exit([code][, timeout])
 
 Ends the process with the specified `code`.  If omitted, exit uses the
 'success' code `0`.
@@ -717,6 +746,32 @@ process.exit(1);
 
 The shell that executed Node.js should see the exit code as 1.
 
+When the optional `timeout` value is specified, an internal timer will be
+created that will end the process after `timeout` milliseconds. Any listeners
+registered for the `'exitingSoon'` event will be invoked and will be passed
+a callback that should be called when the listener is ready for the exit to
+proceed. The process will exit either when all listeners have signaled that
+they are ready (by invoking the callback) or when the exit timer expires,
+whichever comes first.
+
+The `timeout` argument is ignored if it is not a positive, finite number.
+
+```js
+// Some ongoing task that would normally keep the event loop active
+const timer1 = setInterval(() => {}, 1000);
+
+// Register an exitingSoon handler to clean up before exit
+process.on('exitingSoon', (ready) => {
+  setImmediate(() => {
+    // Clean up resources
+    clearInterval(timer1);
+    // Notify that we're done
+    ready();
+  });
+});
+
+process.exit(0, 10000);
+```
 
 ## process.exitCode
 

lib/internal/process.js

@@ -70,10 +70,48 @@ function setupConfig(_source) {
 
 function setupKillAndExit() {
 
-  process.exit = function(code) {
+  const kExitTimer = Symbol('kExitTimer');
+
+  function exitWithTimeout(code, timeout) {
+    // The way this works is simple. When exitWithTimeout is called, the
+    // number of listeners registered for the `exitingSoon` event on
+    // process is grabbed. A callback is created and passed to each of
+    // the listeners on emit. When each is done doing it's thing, it
+    // invokes the callback, which counts down. Once the counter hits
+    // zero, the real process.exit is called. In the meantime, an unref'd
+    // timer is created to run proecss.exit() at a given timeout just in
+    // case the cleanup code takes too long. Returns true if the method
+    // schedules the exit, returns false if an exit has already been
+    // scheduled.
+    if (process[kExitTimer] !== undefined)
+      return false;   // Already called!
+    var count = process.listenerCount('exitingSoon');
+    function ready() {
+      count--;
+      if (count <= 0) {
+        // If we're here, we beat the exit timeout, clear it and exit.
+        clearTimeout(process[kExitTimer]);
+        process.exit(code);
+      }
+    }
+    process[kExitTimer] = setTimeout(() => {
+      process.exit(code);
+    }, timeout);
+    process[kExitTimer].unref();
+    process.emit('exitingSoon', ready);
+    return true;
+  }
+
+  process.exit = function(code, timeout) {
     if (code || code === 0)
       process.exitCode = code;
 
+    if (timeout) {
+      timeout |= 0;
+      if (timeout > 0)
+        return exitWithTimeout(code, timeout);
+    }
+
     if (!process._exiting) {
       process._exiting = true;
       process.emit('exit', process.exitCode || 0);

test/parallel/test-process-exit-timeout.js

@@ -0,0 +1,29 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+// Schedule something that'll keep the loop active normally
+const timer1 = setInterval(() => {}, 1000);
+const timer2 = setInterval(() => {}, 1000);
+
+// Register an exitingSoon handler to clean up before exit
+process.on('exitingSoon', common.mustCall((ready) => {
+  // Simulate some async task
+  assert.strictEqual(process.exitCode, 0);
+  // Shouldn't be callable twice
+  assert.strictEqual(process.exit(0, 10000), false);
+  setImmediate(() => {
+    // Clean up resources
+    clearInterval(timer1);
+    // Notify that we're done
+    ready();
+  });
+}));
+
+process.on('exitingSoon', common.mustCall((ready) => {
+  clearInterval(timer2);
+  ready();
+}));
+
+assert.strictEqual(process.exit(0, 10000), true);