Fix blocking / non-blocking stdio woes

[piscisaureus]

Currently process.stdin / stdout / stderr is blocking, except when it is a pipe on windows. Weird and surprising. Very unpractical in cases where stdio is used as an IPC mechanism between node processes.

Discussion:http://logs.nodejs.org/libuv/2012-06-29#00:40:38.256

Preliminary results: have process.stdout.setBlocking(true|false) or process.stdout.writeSync.

Feel free to post thoughts here.

[cowboy]

I've experienced a similar issue with stdio in Windows, and have somewhat worked around it in the meantime with this horrible, horrible hack.

While my mini exit lib works well in place of process.exit in some cases, it completely fails to help in this case:

var exitHack = require('./exit').exit;

// If there are pending stdout / stderr writes when "condition" is met...
if (condition) {
  exitHack();
}

// ...this code will still do something. Whoops!
doSomething();

I haven't seen this problem in OS X or Linux. It would be great if Node.js behaved the same cross-OS.

[bpasero]

Any update? Will this land in 0.10?

[isaacs]

This issue has been stagnant for months. In order for this to happen, it will need a champion.

Are you that champion? Would you like to see stdio have a more consistently blocking/nonblocking interface, which works the same on Windows and Unix?

Heed the call. Build the bits that need to go into libuv. Sketch out an API in node for it.

It seems that it won't make the 0.10 cut-off, I'm afraid. Maybe next pass.

[HenryRawas]

I have started to look into this issue. Here is what I have found so far.
If stdout is not piped, then the output goes through libuv tty code which is blocking as expected.
If stdout is piped, the output goes through libuv pipe code.
The pipe code checks to see if the pipe supports overlapped IO.
In the case of piped stdout it does not - NtQueryInformationFile returns a mode of FILE_SYNCHRONOUS_IO_NONALERT, which means:
All operations on the file are performed synchronously.
Wait requests in the system that must synchronize I/O queuing and completion are not subject to alerts
In this case the pipe code uses the thread pool to perform all IO.
So when the process exits, the write requests are queued up for the thread pool to execute, but they are never executed.

I made a quick hack to simply let the Write operations execute synchronously when the pipe is in this mode.
In the tests that I have, this fixes the problem of data being lost when the process exits.

This approach means that stdout would have blocking IO behavior even when piped, without requiring any new API.

Other pipes that have mode FILE_SYNCHRONOUS_IO_NONALERT or FILE_SYNCHRONOUS_IO_ALERT would also end up being blocking.
These modes may be set on a pipe or file when creating it, but libuv never sets it.

Does this seem like an acceptable way of fixing this issue?

[bpasero]

Excellent! Any chance of getting this into node 0.10.x? This fix will allow me to get rid of ugly workarounds I need to use currently to not loose process output.

[bnoordhuis]

@piscisaureus and/or @sblom: request for comment, please.