Issue opening/closing character devices in Node.js

[bminer]

I am having issues opening/closing character devices in Node. How does one properly open and close a character device (i.e. a serial port or TTY device) for reading and writing?

Here's my little program that doesn't quite work as expected:

var fs = require("fs");
fs.open("/dev/ttyUSB0", "r+", function(err, fd) {
    if(err) throw err;
    var input = fs.createReadStream("/dev/ttyUSB0", {"fd": fd});
    input.on("data", function(chunk) {
        console.log("in:", chunk);
    });
    var out = fs.createWriteStream("/dev/ttyUSB0", {"fd": fd});
    console.log("out:", "ATZ\r\n");
    out.write("ATZ\r\n");
    setTimeout(function() {
        console.log("Closing file...");
        fs.close(fd, function(err) {
            console.log("File has been closed", err);
            // At this point, Node will just hang
        });
    }, 5000).unref();
});

The output of the program is something like this:

out: ATZ

in: <Buffer 41 54 5a 0a>
in: <Buffer 0a>
in: <Buffer 0a>
in: <Buffer 4f 4b 0a>
in: <Buffer 0a>
Closing file...
File has been closed null

Everything looks okay, but Node does not exit, as expected; rather, Node simply hangs at this point. Why? Is this a bug?

This problem exists even when opening a TTY terminal (i.e. /dev/tty2 for reading/writing)

[bminer]

Here is an even simpler program showing this bug:

var fs = require("fs");
var input = fs.createReadStream("/dev/tty2");
setTimeout(function() {
    console.log("Closing file...");
    input.pause();
    input.destroy();
    console.log("About to exit...");
    process.exit(0);
    console.log("This should not print.");
}, 5000);

In this case, the following output is printed:

Closing file...
About to exit...

Then Node hangs (it does not exit).

[rlidwka]

Then Node hangs (it does not exit).

confirmed, bug seems to be present on both 0.10.21 and 0.11.11

[indutny]

Should be fixed by #7111

[indutny]

Though, I must admit that close() is internal undocumented method ;)

[indutny]

Found the source of the issue: it is because of the blocking nature of ttys in libuv. Don't know about v0.10, but v0.11 for sure blocks in a read() call in a thread pool, while main thread blocks in uv_thread_join(): waiting for worker IO threads to terminate.

If you'll hit enter - it should exit. I'll see what I could do to fix it.

[bminer]

@indutny - Thanks for the update. Why are worker I/O threads blocked on a read() call after the file or I/O stream has been closed? In other words, why doesn't the read() call fail once the file descriptor is closed / cleaned up?

I don't know if I quite understand how read() works in Linux...

[indutny]

@bminer after some consideration, I came to conclusion that this is incorrect way to read from a character device. This kind of thing isn't actually a file and may block for a considerable long time if you would wish to read from it. The proper way would be to either instantiate http://nodejs.org/api/tty.html#tty_class_readstream or http://nodejs.org/api/net.html#net_new_net_socket_options with the result of fs.open as an fd option.

[bminer]

@indutny - I figured as much; however, there is no clear Node API to read from character devices at this time...

What are the next steps?

[indutny]

Have you tried creating new net.Socket({ fd: fs.open('/dev/...') })?