process: allow monitoring uncaughtException

Installing an uncaughtException listener has a side effect that process
is not aborted. This is quite bad for monitoring/logging tools which
tend to be interested in errors but don't want to cause side effects
like swallow an exception or change the output on console.

There are some workarounds in the wild like monkey patching emit or
rethrow in the exception if monitoring tool detects that it is the only
listener but this is error prone and risky.

This PR allows to install a listener to monitor uncaughtException
without the side effect to consider the exception has handled. To avoid
conflicts with other events it exports a symbol on process which owns
this special meaning.

Author: Flarna

doc/api/process.md

@@ -262,6 +262,20 @@ nonexistentFunc();
 console.log('This will not run.');
 ```
 
+It is possible to monitor `'uncaughtException'` events without overriding the
+default behavior to exit the process by installing a listener using the symbol
+`uncaughtExceptionMonitor`.
+
+```js
+process.on(process.uncaughtExceptionMonitor, (err, origin) => {
+  MyMonitoringTool.logSync(err, origin);
+});
+
+// Intentionally cause an exception, but don't catch it.
+nonexistentFunc();
+// Still crashes Node.js
+```
+
 #### Warning: Using `'uncaughtException'` correctly
 
 `'uncaughtException'` is a crude mechanism for exception handling
@@ -2320,6 +2334,20 @@ documentation for the [`'warning'` event][process_warning] and the
 [`emitWarning()` method][process_emit_warning] for more information about this
 flag's behavior.
 
+## `process.uncaughtExceptionMonitor`
+<!-- YAML
+added: REPLACEME
+-->
+
+This symbol shall be used to install a listener for only monitoring
+`'uncaughtException'` events. Listeners installed using this symbol are called
+before the regular `'uncaughtException'` listeners and before a hook
+installed via [`process.setUncaughtExceptionCaptureCallback()`][].
+
+Installing a listener using this symbol does not change the behavior once an
+`'uncaughtException'` event is emitted, therefore the process will still crash
+if no regular `'uncaughtException'` listener is installed.
+
 ## `process.umask([mask])`
 <!-- YAML
 added: v0.1.19

lib/internal/bootstrap/node.js

@@ -206,7 +206,8 @@ ObjectDefineProperty(process, 'features', {
   const {
     onGlobalUncaughtException,
     setUncaughtExceptionCaptureCallback,
-    hasUncaughtExceptionCaptureCallback
+    hasUncaughtExceptionCaptureCallback,
+    kUncaughtExceptionMonitor
   } = require('internal/process/execution');
 
   // For legacy reasons this is still called `_fatalException`, even
@@ -220,6 +221,12 @@ ObjectDefineProperty(process, 'features', {
     setUncaughtExceptionCaptureCallback;
   process.hasUncaughtExceptionCaptureCallback =
     hasUncaughtExceptionCaptureCallback;
+  ObjectDefineProperty(process, 'uncaughtExceptionMonitor', {
+    value: kUncaughtExceptionMonitor,
+    writable: false,
+    configurable: true,
+    enumerable: true
+  });
 }
 
 const { emitWarning } = require('internal/process/warning');

lib/internal/process/execution.js

@@ -3,6 +3,7 @@
 const {
   JSONStringify,
   PromiseResolve,
+  Symbol,
 } = primordials;
 
 const path = require('path');
@@ -27,6 +28,8 @@ const {
 // communication with JS.
 const { shouldAbortOnUncaughtToggle } = internalBinding('util');
 
+const kUncaughtExceptionMonitor = Symbol('process.uncaughtExceptionMonitor');
+
 function tryGetCwd() {
   try {
     return process.cwd();
@@ -159,6 +162,7 @@ function createOnGlobalUncaughtException() {
     }
 
     const type = fromPromise ? 'unhandledRejection' : 'uncaughtException';
+    process.emit(kUncaughtExceptionMonitor, er, type);
     if (exceptionHandlerState.captureFn !== null) {
       exceptionHandlerState.captureFn(er);
     } else if (!process.emit('uncaughtException', er, type)) {
@@ -214,5 +218,6 @@ module.exports = {
   evalScript,
   onGlobalUncaughtException: createOnGlobalUncaughtException(),
   setUncaughtExceptionCaptureCallback,
-  hasUncaughtExceptionCaptureCallback
+  hasUncaughtExceptionCaptureCallback,
+  kUncaughtExceptionMonitor
 };

test/parallel/test-process-uncaught-exception-monitor.js

@@ -0,0 +1,35 @@
+'use strict';
+
+const common = require('../common');
+const assert = require('assert');
+
+const theErr = new Error('MyError');
+
+process.on(
+  process.uncaughtExceptionMonitor,
+  common.mustCall(function onUncaughtExceptionMonitor(err, origin) {
+    assert.strictEqual(err, theErr);
+    assert.strictEqual(origin, 'uncaughtException');
+  }, 2)
+);
+
+process.on('uncaughtException', common.mustCall(
+  function onUncaughtException(err, origin) {
+    assert.strictEqual(origin, 'uncaughtException');
+    assert.strictEqual(err, theErr);
+  })
+);
+
+process.nextTick(common.mustCall(
+  function withExceptionCaptureCallback() {
+    process.setUncaughtExceptionCaptureCallback(common.mustCall(
+      function uncaughtExceptionCaptureCallback(err) {
+        assert.strictEqual(err, theErr);
+      })
+    );
+
+    throw theErr;
+  })
+);
+
+throw theErr;