child_process: fix exec set stdout.setEncoding

[killagu]

cp.exec decide to use _stdout(_stdout is string) or
Buffer.concat(_stdout)(_stdout is buffer array) by options.encoding.
but std(out|err) encoding can be changed. If encoding is changed to
not null, _stdout will become string, and Buffer.concat(_stdout)
will throw TypeError. This patch will fix it, use
options.encoding and std(out|err)._readableState.encoding.

Fixes: #18969

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines

Affected core subsystem(s)

child_process

[lpinca]

What do you think about using process.{stdout,stderr}.write()?

[killagu]

process.std.write is ok. But in other test-child-process tests use the console, like encoding,env.

[hstarorg]

The stdoutLen will be affect when encoding changed.
https://github.com/killagu/node/blob/c40252d80610765e5f6efefc02ea6ba1b95cee17/lib/child_process.js#L336-L345
The stdoutLen still used old length calculate function when encoding is changed. The case is that string is Chinese.

[killagu]

@hstarorg Thanks for the advice, you are right.I have a better solution.

[killagu]

const readStream = new ReadableStream();
readStream.setEncoding(null);
// readStream._readableState.decoder.encoding equals 'utf8'
// readStream._readableState.encoding equals null;

_readableState.encoding should be same as this._readableState.decoder.encoding

[BridgeAR]

@nodejs/streams PTAL

[mcollina]

Can you please add a test for Readable  alone?

LGTM with that test included.

[BridgeAR]

Ping @killagu

[BridgeAR]

@killagu sorry that it took so long for to look at this again, it just slipped through. Would you be so kind and add a test as requested by @mcollina?

[killagu]

@BridgeAR I'm sorry it's taken me so long to get back to you. I'll add the test ASAP.

[killagu]

The test has beed added.
@mcollina @BridgeAR

[BridgeAR]

CI https://ci.nodejs.org/job/node-test-pull-request/14885/

[BridgeAR]

@nodejs/child_process PTAL

[BridgeAR]

Landed in 7d81f5d 🎉