investigate flaky test-worker-ref-onexit

[Trott]

Haven't seen this one before. Might be difficult to reproduce (but I haven't tried yet).

https://ci.nodejs.org/job/node-test-commit-aix/21132/nodes=aix61-ppc64/console

test-osuosl-aix61-ppc64_be-1

09:04:17 not ok 2174 parallel/test-worker-ref-onexit
09:04:17   ---
09:04:17   duration_ms: 2.331
09:04:17   severity: fail
09:04:17   exitcode: 1
09:04:17   stack: |-
09:04:17     assert.js:128
09:04:17       throw err;
09:04:17       ^
09:04:17     
09:04:17     AssertionError [ERR_ASSERTION]: function should not have been called at /home/iojs/build/workspace/node-test-commit-aix/nodes/aix61-ppc64/test/parallel/test-worker-ref-onexit.js:10
09:04:17         at Worker.mustNotCall (/home/iojs/build/workspace/node-test-commit-aix/nodes/aix61-ppc64/test/common/index.js:425:12)
09:04:17         at Worker.emit (events.js:197:13)
09:04:17         at Worker.[kOnExit] (internal/worker.js:130:10)
09:04:17         at Worker.(anonymous function).onexit (internal/worker.js:82:51)
09:04:17   ...

[addaleax]

See also #26083 (comment) … if this is an AIX-specific problem (which I guess it could be?), I’m not sure how to best debug it.

One idea could be that the Worker finishes before the current event loop turn is over. Something like this might fix it?

diff --git a/test/parallel/test-worker-ref-onexit.js b/test/parallel/test-worker-ref-onexit.js
index 0250c592ce91..968f0fe31e9e 100644
--- a/test/parallel/test-worker-ref-onexit.js
+++ b/test/parallel/test-worker-ref-onexit.js
@@ -5,6 +5,6 @@ const { Worker } = require('worker_threads');
 // Check that worker.unref() makes the 'exit' event not be emitted, if it is
 // the only thing we would otherwise be waiting for.
 
-const w = new Worker('', { eval: true });
+const w = new Worker('while(true);', { eval: true });
 w.unref();
 w.on('exit', common.mustNotCall());

[addaleax]

AIX stress test with 1000 runs: https://ci.nodejs.org/job/node-stress-single-test/2157/ (failed 34/267)
Edit: Stress test with proposed solution: https://ci.nodejs.org/job/node-stress-single-test/2158/