Remove file descriptor restriction on the stdin/stdout/stderr option

[ehmicky]

Node.js requires the streams passed to the stdio option to have an underlying file descriptor.

https://github.com/nodejs/node/blob/0afe731d357d4df9a65aeb06429bf663d8dc2975/lib/internal/child_process.js#L1060
https://github.com/nodejs/node/blob/0afe731d357d4df9a65aeb06429bf663d8dc2975/lib/internal/child_process.js#L340

Execa could remove that restriction: if a stream without a file descriptor is passed to the stdio option(s), we could pipe it to the child process after it spawned. This would re-use the code that does the same thing for web streams (#615).

This was actually already discussed in #81, although in different terms.

However, Node.js detects whether there is an underlying file descriptor using the undocumented stream._handle and stream.handle properties. Also, those must be instances of Pipe, TTY, TCP or UDP which are internal classes not accessible userland. I do not think we should build detection logic based on this, as any Node.js release might break it.

Alternatively, we could make users opt-in to that behavior. For example, wrapping the stream into an array or a plain object instead of passing stream directly, to the stdio option(s).

Or, users could turn the Node.js stream into a web stream which we now support (#603), using Readable.toWeb()/Writable.toWeb(). I.e. we would just add some documentation about this.

What are your thoughts on this @sindresorhus?

[sindresorhus]

I prefer to put minimal effort into legacy APIs like Node.js streams. So my preference would be documentation:

Or, users could turn the Node.js stream into a web stream which we now support (#603), using Readable.toWeb()/Writable.toWeb(). I.e. we would just add some documentation about this.

[ehmicky]

Yes, I was also thinking along those lines, for the same reason. 👍

[ehmicky]

When adding support for passing multiple values to the std* options (#643), we added a possible workaround for this issue, as a side effect. Namely, when passing a Node.js stream + another value, the Node.js stream will be handled by Execa instead of by child_process.spawn(). This removes the restriction above.

For example, { stdout: [nodeStream, 'pipe'] } instead of { stdout: nodeStream }.

We can document this workaround as it might help some users (done at #644).