WasmFS: `-sNODERAWFS` creates redundant `dev` and `tmp` dirs within the CWD

[kleisauke]

Standalone test case:

$ ls -d */ | grep -E "tmp/|dev/"
$ emcc test/hello_world.c -sNODERAWFS -sWASMFS
$ node a.out.js
hello, world!
$ ls -d */ | grep -E "tmp/|dev/"
dev/
tmp/

This can be worked around by avoiding the wasmfs_create_root_dir() hook, see e.g. commit kleisauke/wasm-vips@88df6ba.

Originally mentioned in #19557 (comment)

[sbc100]

I don't think those directories should be created at all in -sNODERAWFS mode... in this mode that FS that emscripten sees should be exactly that of the host OS with no in-memory files at all (I think).

[kleisauke]

I think the reasoning behind this is to ensure that paths like /tmp, /dev/null and similar would work on non-POSIX systems when using -sNODERAWFS. For example, you can't reliably assume these paths will be valid on Windows.

The old FS also does this, see for example:

emscripten/src/lib/libfs.js

Lines 1506 to 1510 in 14da0a2

	FS.mount(MEMFS, {}, '/');
	FS.createDefaultDirectories();
	FS.createDefaultDevices();
	FS.createSpecialDirectories();

These are all created/mounted in-memory before any other FS backend is initialized.

[sbc100]

I think the reasoning behind this is to ensure that paths like /tmp, /dev/null and similar would work on non-POSIX systems when using -sNODERAWFS. For example, you can't reliably assume these paths will be valid on Windows.

The old FS also does this, see for example:

emscripten/src/lib/libfs.js

Lines 1506 to 1510 in 14da0a2

FS.mount(MEMFS, {}, '/');

FS.createDefaultDirectories();
FS.createDefaultDevices();
FS.createSpecialDirectories();
These are all created/mounted in-memory before any other FS backend is initialized.

I don't believe any of that stuff is available under NODERAWFS... i.e.NODERAWFS completely overrides the syscalls making this memfs non-accessible (I think).

[sbc100]

Also, I think the point of NODERAWFS is to expose to you the idiosyncrasies of the underlying OS filesystem.. we don't want to be faking stuff like this in that mode.

[kleisauke]

Ah, I think you're right about the syscalls. I think the issue is that it uses "." as the root directory, so it ends up creating those directories in the current working directory. It should probably skip this block in -sNODERAWFS mode:

emscripten/system/lib/wasmfs/wasmfs.cpp

Lines 109 to 124 in cbbc783

	// If the /dev/ directory does not already exist, create it. (It may already
	// exist in NODERAWFS mode, or if those files have been preloaded.)
	auto devDir = lockedRoot.insertDirectory("dev", S_IRUGO | S_IXUGO);
	if (devDir) {
	auto lockedDev = devDir->locked();
	lockedDev.mountChild("null", SpecialFiles::getNull());
	lockedDev.mountChild("stdin", SpecialFiles::getStdin());
	lockedDev.mountChild("stdout", SpecialFiles::getStdout());
	lockedDev.mountChild("stderr", SpecialFiles::getStderr());
	lockedDev.mountChild("random", SpecialFiles::getRandom());
	lockedDev.mountChild("urandom", SpecialFiles::getURandom());
	}
	// As with the /dev/ directory, it is not an error for /tmp/ to already
	// exist.
	lockedRoot.insertDirectory("tmp", S_IRWXUGO);

Alternatively, those directories could be created only as part of createMemoryBackend().

Also, I think the point of NODERAWFS is to expose to you the idiosyncrasies of the underlying OS filesystem.. we don't want to be faking stuff like this in that mode.

Agreed.