ptmx.Read() blocks on Darwin

[noahlt]

I'm writing a daemon that controls an instance of bash, using creack/pty to create the pseudoterminal, and writing commands to the pty to send them to bash and reading the output from the pty. On occasion, calling ptmx.Read() blocks the goroutine indefinitely.

I expect ptmx.Read() to block until the program on the tty ("slave") side of the pty writes something to read. Instead it seems to block forever. To test this, I wrote a program that tries to run echo $USER in a loop, which I have uploaded as a gist — occasionally the program runs the print on line 47 then hangs. The nondeterminism makes me suspect there's a race condition between my program and bash, but I still expected that ptmx.Read() would block until bash writes to the pty and then ptmx.Read() would unblock.

I also found that resizing my terminal (and sending sigwinch to the pty) causes the program to become unblocked. Not sure what to make of that.

Honestly I'm not sure whether this is a problem with pty_darwin.go or with my own code — so sorry if that's the case.

Environment details, in case they're helpful:

• macOS 10.14.6
• golang 1.16
• creack/pty v1.1.11

@creack thanks for maintaining this project; it's excellent. Incidentally, let me know if you're open to doing some consulting work with pty.

[creack]

The issue is most likely in the readUntil function, could you provide it as well? I suspect it simply reads more than what you expect so the following ptmx.Read doesn't have anything more to read. Sending a SIGWINCH results in new activity on the slave side, so the read unblocks.

I did manage to reproduce the random blocking behavior using this naive implementation of readUntil:

const PromptChar = '\x19'

func readUntil(r io.Reader, sep, buf []byte) int {
        n, err := r.Read(buf)
        if err != nil {
                panic(err)
        }
        idx := bytes.Index(buf[:n], sep)
        if idx < 0 {
                log.Printf("sep %q not found in %q\r\n", sep, buf[:n])
        }
        return idx
}

A simple fix would be to use bufio.Reader. Here is a slightly modified version of the provided snippet which doesn't block: https://gist.github.com/creack/68448647d726fbfd4fb7bd04ae5358dd

Using a pty in a daemon is usually a bad idea, it is better to use the stdlib's exec.Command directly, which avoids having to deal with all prompt management / echo'd commands and terminal overhead.
There are some use-cases where it can be useful and maybe indeed you have such use-case, but I'd recommend checking and make sure that is the case.

In a daemon, stdin would be closed, so the whole logic around SIGWINCH and Raw mode would be superfluous and should be removed.

[creack]

Closing as it is not related to the lib. Feel free to reach out if you need more help. Thank you for the sponsorship.