Sync child process filesystem with parent process using PROXYFS

[adamziel]

Related to:

• Prototype: Spawning PHP sub-processes in Web Workers #1031
• Blueprints: Explore switching to the PHP implementation #1051

Right now, both PRs use a full filesystem sync + sync-on-write mechanics to keep the MEMFS in sync between two separate PHP runtimes.

However, Emscripten supports PROXYFS:

This allows a module to mount another module’s file system. This is useful when separate modules need to share a file system without manually syncing file contents. For example:

// Module 2 can use the path "/fs1" to access and modify Module 1's filesystem
module2.FS.mkdir("/fs1");
module2.FS.mount(module2.PROXYFS, {
    root: "/",
    fs: module1.FS
}, "/fs1");

Let's see if this would solve the problem without any copying semantics.

[adamziel]

Done in #1069