FFM implementation returns -1 when using reader().read(timeout) (MacOS M3 related?) #1879

I've attached a minimal repro project to showcase what I'm running into.

jline-ffm-read-eof.zip

The README from said project:

This project showcases (I think?) a bug with the FFM terminal representation when trying to call
terminal.reader().read(timeout)

Repro steps

This project uses JDK25 (via Gradle toolchain) and has a dependency on jline 4.1.0 (see gradle/libs.versions.toml).
Probably earlier JDKs might also work, but my understanding is JDK25 is the first version that FFM is really mature.

In case it matters, my machine is an M3 MacBook Pro. (uname -m ➔ arm64)

This project has only a single code file in it: src/main/kotlin/main.kt

• Unzip the attached project into a location of your preference. It will pull out a folder called jline-ffm-read-eof so you don't have to worry about files exploding all over your current directory like a mess.
• In a terminal....
• Make sure you have JDK25 or newer installed (java -version)
• cd into this project's root
• ./gradlew installDist
• ./build/install/jline-ffm-read-eof/bin/jline-ffm-read-eof

The program should immediately print out the line: Terminal class: org.jline.terminal.impl.ffm.FfmUnixSysTerminal

Expected behavior

Then, as a user, you should just be able to press keys, e.g. press "q" and the terminal will say: "Read: 113"

Actual behavior

The program exits immediately before I get a chance to type, printing: "Got EOF (-1), exiting"

Additionally, if you change main.kt to look like this:

// Before
if (c >= 0) {
    println("Read: $c")
}
if (c == -1) {
    println("Got EOF (-1), exiting")
    quit = true
}

// After
if (c >= -1) {
   println("Read: $c")
}

you will see that the FFM implementation does read keypresses, but it also spits out -1's at a regular cadence.
Here is the output I got when typing "Hello":

Terminal class: org.jline.terminal.impl.ffm.FfmUnixSysTerminal
Read: -1
Read: -1
Read: -1
Read: -1
Read: -1
Read: -1
Read: 104
Read: -1
Read: -1
Read: 101
Read: -1
Read: 108
Read: -1
Read: 108
Read: -1
Read: -1
Read: 111
Read: -1
Read: -1
Read: -1
Read: -1
Read: -1

My understanding

I'll write out my assumptions, just in case this bug is really just user error.

When calling reader().read(...), essentially, this value should either return:

• a real character code value (a user just pressed a key)
• -2 if the read call timed out (which is fine, just ignore it)
• -1 to indicate the input stream ended (e.g. user pressed ctrl+D), which means you should quit the loop

In other words:

var quit = false
while (!quit) {
    val c = terminal.reader().read(16)
    if (c >= 0) {
        // Valid character received. Process it!
    } else if (c == -1) {
        quit = true
    }
}

I've been using Jline for years and this has worked fine for me using the Exec fallback.

But with FFM, it seems to sporadically give me -1's, which kills my while loop almost immediately.

I assume this is a bug. It may also be related to my OS, I'm not sure.

Hopefully the team will be able to reproduce this issue on their end.