WASI Libraries

So I'm using wasm with wasmer + cranelift for a kind of scripting system for my application. So users can write little WebAssembly modules to write plugins. Over the weekend I've looked into integrating WASI to give them access to the operating system in a limited fashion (I'm planning to only let them read files and print to stdout / stderr initially). However it seems like so far (at least from Rust, not sure if this is an overall limitation) you can really only create WASI binaries, not libraries. The way my plugin system works is that my application expects certain exports to be there and I call into them, while providing WASI imports + other plugin specific imports to the wasm files. However at least the way it currently is in Rust and the wasi libc sysroot, WASI really only works for the duration of a main function that needs to be there. Calling the wasi functions from anywhere else (through the Rust std) segfaults because libpreopen isn't initialized (anymore). I'm not sure how much this is a design limitation or a just a bug, but overall I'd like my main application to have control over which functions to call. With a main function, the WebAssembly module takes control over the control flow, which isn't great for such an "extension" kind of use case.

The issue names no repository files or tests. Start by reproducing the Rust and wasi libc behavior with a module that exports callable plugin functions and uses WASI imports, then compare it with the main-function path in Wasmer and Cranelift. Done means establishing whether WASI libraries are supported or defining the required interface and initialization behavior.