Runners and wasmer_wasix::wapm don't take the [fs] table into account

[Michael-F-Bryan]

Describe the bug

The current runner implementation (used by wasmer run and wasmer run-unstable) don't respect the directory mappings in the wasmer.toml file's [fs] table.

Steps to reproduce

The easiest way to reproduce this is by creating a package that will serve static files, where the original location on the publishing machine's filesystem doesn't match the directory the WASI executable will see.

# wasmer.toml

[package]
name = "Michael-F-Bryan/wasmer-io"
version = "0.2.0"
description = "The wasmer.io website"

[dependencies]
"sharrattj/static-web-server" = "1"

[fs]
public = "out"

Expected Solution

There are two ways we could resolve this.

The first one is by making sure consumers of a *.webc file respect the [fs] mapping. At the moment our runners use the naive WebcFileSystem::init_all() method to set up the volumes (here), and that just implements a poor man's unionfs with all the volumes.

Alternatively, the wapm-targz-to-pirita could shuffle directories around as part of the general tarball-to-webc conversion.

[Michael-F-Bryan]

We should also check that people can't do something nefarious like [fs] public = "/etc/shadow".

[Michael-F-Bryan]

I haven't implemented it in #3852, but I've left some placeholders for the solution.

This is where the resolver should figure out how volumes should be mapped:

wasmer/lib/wasi/src/runtime/resolver/resolve.rs

Lines 211 to 218 in 8207c55

	fn resolve_filesystem_mapping(
	_dependency_graph: &DependencyGraph,
	) -> Result<Vec<FileSystemMapping>, ResolveError> {
	// TODO: Add filesystem mappings to summary and figure out the final mapping
	// for this dependency graph.
	// See <https://github.com/wasmerio/wasmer/issues/3744> for more.
	Ok(Vec::new())
	}

And this is where we take WEBC containers and do the actual mapping:

wasmer/lib/wasi/src/runtime/package_loader/load_package_tree.rs

Lines 232 to 241 in 8207c55

	fn filesystem(
	packages: &HashMap<PackageId, Container>,
	pkg: &ResolvedPackage,
	) -> Result<impl FileSystem + Send + Sync, Error> {
	// FIXME: Take the [fs] table into account
	// See <https://github.com/wasmerio/wasmer/issues/3744> for more
	let root = &packages[&pkg.root_package];
	let fs = WebcVolumeFileSystem::mount_all(root);
	Ok(fs)
	}