fix(wasix): Prevent recursive merge fallback when mounting packages #5799
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Pull request overview
This PR refactors the virtual filesystem handling in wasix to prevent recursive merge fallback operations when mounting packages. The core improvement is the introduction of an Overlay variant to WasiFsRoot that enables direct merging of UnionFileSystem instances without falling back to expensive recursive filesystem copies.
- Changed filesystem type handling from
BoxtoArcthroughout the codebase for better sharing semantics - Introduced
WasiFsRoot::Overlayvariant to enable efficient filesystem merging usingUnionFileSystem::merge()instead of the recursivemerge_filesystems()fallback - Extracted
RelativeOrAbsolutePathHackto its own module for better code organization
Reviewed changes
Copilot reviewed 13 out of 13 changed files in this pull request and generated 2 comments.
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| File | Description |
|---|---|
| lib/wasix/src/state/builder.rs | Updated set_fs() to accept impl Into<Arc<...>> instead of Box, added internal set_fs_root() method, removed Arc wrapper from sandbox_fs() since TmpFileSystem is Clone |
| lib/wasix/src/state/env.rs | Added handling for new WasiFsRoot::Overlay variant when injecting package commands |
| lib/wasix/src/fs/mod.rs | Refactored WasiFsRoot enum to include Overlay variant, updated conditional_union() to use UnionFileSystem::merge() for overlay case, changed default_fs_backing() return type to Arc |
| lib/wasix/src/fs/relative_path_hack.rs | New module containing extracted RelativeOrAbsolutePathHack implementation |
| lib/wasix/src/runtime/package_loader/load_package_tree.rs | Changed filesystem() to return Option<UnionFileSystem> instead of boxed trait object, removed empty OverlayFileSystem creation, added webc version validation |
| lib/wasix/src/runners/wasi_common.rs | Updated prepare_filesystem() to return WasiFsRoot and create Overlay variant, moved RelativeOrAbsolutePathHack to separate module |
| lib/wasix/src/runners/wasi.rs | Updated to use duplicate() method when cloning filesystem |
| lib/wasix/src/runners/wcgi/runner.rs | Updated to use duplicate() method when cloning filesystem |
| lib/wasix/src/bin_factory/binary_package.rs | Changed webc_fs field from Arc<dyn FileSystem> to Option<Arc<UnionFileSystem>> for more specific typing |
| lib/virtual-fs/src/union_fs.rs | Added merge() and duplicate() methods, made MountPoint cloneable |
| lib/virtual-fs/src/passthru_fs.rs | Updated to use Arc instead of Box for inner filesystem |
| lib/c-api/src/wasm_c_api/wasi/mod.rs | Added explicit type annotation for filesystem initialization |
| tests/lib/wast/src/wasi_wast.rs | Updated test utilities to use Arc instead of Box for filesystem types |
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Store the inner FS as an arc to make it more easily shareable. Preserves constructor backwards compatibility.
Add the ability to merge a union fs into another using a given merge mode.
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Pull request overview
Copilot reviewed 14 out of 14 changed files in this pull request and generated no new comments.
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Previously adding additional dependencies to a WASI environment would lead to a recursive file system merge, which would clone all files in memory, which is very expensive. This was caused by a very unstructed creation and extension logic in the WasiFs, which could take on a complicated form of <dyn Filesystem>. There previosly were attempts to downcast and extend without merging, but those would fail in common use cases. This is fixed by adding a dedicated WasiFs::Overlay variant, which is a dedicated representation for the common wasix package environment use case. This allows to deterministically extend an existing overlay with additional read-only mounts.
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Arshia001
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