src: print exceptions from PromiseRejectCallback

Previously, leaving the exception lying around would leave the JS engine in an invalid state, as it was not expecting exceptions to be thrown from the C++ `PromiseRejectCallback`, and lead to hard crashes under some conditions (e.g. with coverage enabled).
nodejs · addaleax · Sep 9, 2019 · Sep 12, 2019 · Sep 13, 2019 · Sep 9, 2019
commit dad139164df365aafa551ebda57e34b1800999fd
diff --git a/src/node_task_queue.cc b/src/node_task_queue.cc
@@ -10,6 +10,7 @@
 
 namespace node {
 
+using errors::TryCatchScope;
 using v8::Array;
 using v8::Context;
 using v8::Function;
@@ -111,8 +112,17 @@ void PromiseRejectCallback(PromiseRejectMessage message) {
   }
 
   Local<Value> args[] = { type, promise, value };
+
+  // V8 does not expect this callback to have a scheduled exceptions once it
+  // returns, so we print them out in a best effort to do something about it
+  // without failing silently and without crashing the process.
+  TryCatchScope try_catch(env);
   USE(callback->Call(
       env->context(), Undefined(isolate), arraysize(args), args));
+  if (try_catch.HasCaught() && !try_catch.HasTerminated()) {
+    fprintf(stderr, "Exception in PromiseRejectCallback:\n");
+    PrintCaughtException(isolate, env->context(), try_catch);
+  }
 }
 
 static void SetPromiseRejectCallback(

diff --git a/test/parallel/test-promise-reject-callback-exception.js b/test/parallel/test-promise-reject-callback-exception.js
@@ -0,0 +1,31 @@
+'use strict';
+const common = require('../common');
+const tmpdir = require('../common/tmpdir');
+const assert = require('assert');
+const path = require('path');
+const child_process = require('child_process');
+
+tmpdir.refresh();
+
+// Tests that exceptions from the PromiseRejectCallback are printed to stderr
+// when they occur as a best-effort way of handling them, and that calling
+// `console.log()` works after that. Earlier, the latter did not work because
+// of the exception left lying around by the PromiseRejectCallback when its JS
+// part exceeded the call stack limit, and when the inspector/built-in coverage
+// was enabled, it resulted in a hard crash.
+
+for (const NODE_V8_COVERAGE of ['', tmpdir.path]) {
+  const { status, signal, stdout, stderr } =
+    child_process.spawnSync(process.execPath,
+                            [path.join(__dirname, 'test-ttywrap-stack.js')],
+                            { env: { ...process.env, NODE_V8_COVERAGE } });
+
+  assert(stdout.toString('utf8')
+      .startsWith('RangeError: Maximum call stack size exceeded'),
+      `stdout: <${stdout}>`);
+  assert(stderr.toString('utf8')
+      .startsWith('Exception in PromiseRejectCallback'),
+      `stderr: <${stderr}>`);
+  assert.strictEqual(status, 0);
+  assert.strictEqual(signal, null);
+}