René's URL Explorer Experiment

{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Stream emitting same error twice under certain circomstances","articleBody":"* **Version**: 12.18.1 and newer\r\n* **Platform**: Darwin watson 19.6.0 Darwin Kernel Version 19.6.0: Mon Aug 31 22:12:52 PDT 2020; root:xnu-6153.141.2~1/RELEASE_X86_64 x86_64\r\n* **Subsystem**: stream\r\n\r\n### What steps will reproduce the bug?\r\n\r\nRun this code on Node.js 12.18.1 or newer and notice how the error is emitted on `b` twice. On any older Node.js version it's only emitted once.\r\n\r\n```js\r\nconst { Readable, PassThrough, Writable, pipeline } = require('stream')\r\n\r\nconst a = new Readable({\r\n  read () {\r\n    this.destroy(new Error('foo'));\r\n  }\r\n})\r\nconst b = new PassThrough()\r\nconst c = new Writable()\r\n\r\na.once('error', (err) =\u003e {\r\n  console.log('emitting error on b')\r\n  b.emit('error', err) // Note: you're not supposed to emit error manually!\r\n})\r\n\r\nb.on('error', (err) =\u003e {\r\n  console.log('b got error:', err.message)\r\n})\r\n\r\npipeline(a.pipe(b), c, (err) =\u003e {\r\n  console.log('end of pipeline, err:', err.message)\r\n})\r\n```\r\n\r\nIf I change `b.emit('error', err)` to `b.destroy(err)` the error goes away. Using `destroy` _is_ best practise and the user isn't supposed to emit `error` events manually on streams like in the above code. However, this was how we told users to do it previously, so a lot of code most likely exists in the wild expecting this to be possible and emitting the same error twice can have bad consequences.\r\n\r\nThoughts:\r\n1. It would be cool if we could warn users if they manually emitted an `error` event on a stream.\r\n1. It would be nice if we could make this odd behaviour go away even if the user emits `error` manually.\r\n\r\nThis regression was most likely introduced in #30869.\r\n\r\n\r\n","author":{"url":"https://github.com/watson","@type":"Person","name":"watson"},"datePublished":"2020-11-09T15:16:49.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":9},"url":"https://github.com/36053/node/issues/36053"}