[Bug]: `just` uses an incorrect runtime directory causing recipes to fail.

[TomJo2000]

Problem description

Upon running any recipe defined in a justfile which runs any sort of command external to just, the recipe will fail with Read-only file system (os error 30).

What steps will reproduce the bug?

Minimal example:

bug:
    #!/usr/bin/env sh
    echo "hello world"

Output:

just bug
// error: I/O error in runtime dir `/run/user/10215/just`: Read-only file system (os error 30)

What is the expected behavior?

just bug
// hello world

System information

Termux Variables:
TERMUX_API_VERSION=0.50.1+4159c62
TERMUX_APK_RELEASE=GITHUB
TERMUX_APP_PACKAGE_MANAGER=apt
TERMUX_APP_PID=23767
TERMUX_IS_DEBUGGABLE_BUILD=1
TERMUX_MAIN_PACKAGE_FORMAT=debian
TERMUX_VERSION=0.118.1
TERMUX__USER_ID=0
Packages CPU architecture:
aarch64
Subscribed repositories:
# sources.list
deb https://packages.termux.dev/apt/termux-main stable main
# root-repo (sources.list.d/root.list)
deb https://packages.termux.dev/apt/termux-root root stable
# tur-repo (sources.list.d/tur.list)
deb https://tur.kcubeterm.com tur-packages tur tur-on-device tur-continuous
# x11-repo (sources.list.d/x11.list)
deb https://packages.termux.dev/apt/termux-x11 x11 main
Updatable packages:
All packages up to date
termux-tools version:
1.44.3
Android version:
14
Kernel build information:
Linux localhost 5.4.242-qgki-g07bc4ccc612b #1 SMP PREEMPT Fri Sep 20 17:45:34 CST 2024 aarch64 Android
Device manufacturer:
Fairphone
Device model:
FP5
LD Variables:
LD_LIBRARY_PATH=
LD_PRELOAD=/data/data/com.termux/files/usr/lib/libtermux-exec.so
Installed termux plugins:
com.termux.api versionCode:51
com.termux.x11 versionCode:14

[TomJo2000]

Turns out this is probably something we should patch in the rust package itself. just gets the value of the runtime_dir from dirs::runtime_dir() https://github.com/casey/just/blob/5db910f400b13daa87b5c37dc62fb1da1ff6533d/src/recipe.rs#L371

Which is part of the Rust standard library. https://docs.rs/dirs/latest/dirs/fn.runtime_dir.html

So any Rust program using this function would be affected.

[TomJo2000]

Okay, it looks like it uses the value of $XDG_RUNTIME_DIR, which makes sense. https://docs.rs/dirs/latest/src/dirs/lib.rs.html#173-175 https://docs.rs/dirs/latest/src/dirs/lin.rs.html#14

But I do not see where or how it is getting its fallback value if that environment variable is unset. ^{We do not currently set it by default, though setting the XDG basedir variables to sane defaults as part of the profile.d/ scripts would be something we may want to consider}

[TomJo2000]

On a related note. It looks like we will need to fix any shebangs in justfiles on the fly.

Otherwise the example given above fails with:

error: Recipe `bug` with shebang `#!/usr/bin/env sh` execution error: No such file or directory (os error 2)

I set $XDG_RUNTIME_DIR to /data/data/com.termux/files/usr/var/run/user for this example.

Edit: I think I found the place where we'd need to patch this. https://github.com/casey/just/blob/5db910f400b13daa87b5c37dc62fb1da1ff6533d/src/shebang.rs#L28 However, it took me 30 minutes to find this. And my very limited grasp of rust is at its end. So I'll leave figuring out a viable way to patch the shebangs as an exercise to the reader.