Need a way to load the _rust_crate_map_toplevel from a SO on Android

[larsbergstrom]

For Servo on Android, Rust is unable to find the _rust_crate_map_toplevel because dlsym will not search all of the loaded modules in The Normal Way. This problem has caused us to currently change the implementation of get_crate_map as follows:

        let module = if cfg!(target_os = "android") {
            dl::open_external(&Path::new("/data/data/com.example.ServoAndroid/lib/libservo.so"))
        } else {
            dl::open_internal()
        };

It would be better if there was some way to provide the runtime with the name of the binary that contains the toplevel crate map.

CC: @aydinkim

[alexcrichton]

Hm, this thread seems to indicate that this is "just how android is". As in, dlopen will not open dependent libraries, so you're forced to do the bottup-up dlopen calls yourself.

It appears that libreoffice has their own version of the function, lo_dlopen which does what I think it does on linux and co (reads the elf header and goes and dlopen's dependent libraries first).

http://motah.org.sa/dev/libreoffice.git/sal/android/lo-bootstrap.c
http://motah.org.sa/dev/libreoffice.git//android/Bootstrap/src/org/libreoffice/android/Bootstrap.java

Unsure if we'd want to bundle that inside of rust itself?

[larsbergstrom]

The dlopen stuff has been fixed:

http://code.google.com/p/android/issues/detail?id=34416&can=1&q=dlopen&colspec=ID%20Type%20Status%20Owner%20Summary%20Stars

The main problem for us (when using the windows path instead of the weak symbol path) is not that dlopen does not load dependencies, but that dlsym with RTLD_DEFAULT is not finding libservo.so and the _rust_crate_map_toplevel symbol.

[larsbergstrom]

Could you possible expose a function that I could call from C to set the toplevel crate map? We're going to be looking to upgrade Rust soon, and without a way to help the Rust runtime find the crate map when loaded as a shared library, we will have to continue having a funky android-only patch that "knows" to look in the libservo.so crate for it, ala:
https://gist.github.com/larsbergstrom/9101334

(ignore all the debugging output :-))

[alexcrichton]

Hm, I believe we could, I would imagine something along the lines of:

static mut MANUAL_CRATE_MAP: *CrateMap = 0 as *CrateMap;

#[no_mangle]
pub extern fn rust_set_crate_map(map: *CrateMap) {
    unsafe { MANUAL_CRATE_MAP = map; }
}

And then all functions using the crate map would check if MANUAL_CRATE_MAP were not null before looking for the dynamically linked or weak linked one. It seems bad that we would have to do this, but I'm not sure of a better situation for android.

Basically, yes, it could be possible.

[alexcrichton]

@larsbergstrom, would something like that work for servo?

[larsbergstrom]

Yes, definitely!

If it's easier, we can alternatively provide the module name to look in for the symbol (though I guess that only works on dlsym-using platforms.

Thanks!