JMLX42 opened a new pull request, #7005:
URL: https://github.com/apache/opendal/pull/7005
## Summary
Adds a new `wasi-fs` backend that implements filesystem access via the
`wasi:filesystem` interface for WebAssembly components running in WASI Preview
2 runtimes (wasmtime, wasmer).
Closes #7004
## AI Assistance Disclosure
This implementation was developed with assistance from Claude (Anthropic).
The AI helped with:
- Researching OpenDAL service patterns and WASI Preview 2 APIs
- Designing the implementation architecture
- Writing the initial implementation code
- Code review and iteration
## Design Decisions
### 1. Crate Architecture
**Decision:** Split crate under `core/services/wasi-fs/` (not inline in core)
**Rationale:** Follows OpenDAL's modern service pattern (RFC 6828). Services
are being migrated to separate crates for minimal compile-time impact and
independent versioning.
### 2. WASI Bindings Approach
**Decision:** Use the `wasi` crate (v0.14.7) with pre-generated bindings
**Rationale:**
- Pre-generated bindings avoid build complexity of running `wit-bindgen`
during compilation
- Well-maintained by Bytecode Alliance, stays in sync with WASI spec
- Simpler dependency management than checking in WIT files
### 3. Async Model
**Decision:** Wrap synchronous WASI calls directly in async methods
**Rationale:**
- WASI Preview 2 filesystem operations are fundamentally synchronous from
the component's perspective
- The WASI runtime handles actual I/O scheduling, so blocking in the
component doesn't block the host thread
- No `spawn_blocking` needed since there's no separate thread pool in WASM
### 4. Preopened Directory Handling
**Decision:** Match root path to preopened directories from host runtime
**Rationale:**
- Most flexible for users - they specify the desired root path
- Builder iterates preopened directories and finds the best match
- Falls back to first preopened directory if root is `/` or not specified
- Provides clear error messages if no matching preopened directory exists
### 5. Platform Restriction
**Decision:** Entire crate guarded by `#![cfg(all(target_arch = "wasm32",
target_os = "wasi"))]`
**Rationale:**
- Crate only makes sense on `wasm32-wasip2` target
- Compiles to nothing on native targets (intentional)
- Feature flag can be safely enabled without conditional compilation issues
## Implementation
### Files Created
```
core/services/wasi-fs/
├── Cargo.toml # Crate manifest with wasi 0.14.7 dependency
└── src/
├── lib.rs # Module exports + auto-registration via
#[ctor::ctor]
├── config.rs # WasiFsConfig (root path option)
├── backend.rs # WasiFsBuilder + WasiFsBackend (Access impl)
├── core.rs # Core WASI filesystem operations
├── reader.rs # oio::Read implementation
├── writer.rs # oio::Write implementation
├── lister.rs # oio::List implementation
├── deleter.rs # oio::OneShotDelete implementation
├── error.rs # WASI ErrorCode → OpenDAL ErrorKind mapping
└── docs.md # Service documentation
```
### Capabilities
| Capability | Supported |
|------------|-----------|
| stat | ✅ |
| read | ✅ |
| write | ✅ |
| create_dir | ✅ |
| delete | ✅ |
| list | ✅ |
| copy | ✅ (via read+write) |
| rename | ✅ |
| append | ✅ |
| atomic_write | ❌ |
### Limitations
- **No atomic writes**: `abort()` returns error since WASI doesn't provide
atomic file operations
- **No file locking**: Not supported by WASI filesystem interface
- **Platform-specific**: Only compiles for `wasm32-wasip2` target
## Test Plan
- [x] `cargo check` passes on native target
- [x] `cargo check --features services-wasi-fs` passes
- [x] `cargo fmt --all` passes
- [ ] Manual testing with wasmtime (requires WASM component build)
## Usage Example
```rust
use opendal::services::WasiFs;
use opendal::Operator;
let builder = WasiFs::default().root("/data");
let op = Operator::new(builder)?.finish();
let content = op.read("hello.txt").await?;
op.write("output.txt", "Hello, WASI!").await?;
```
```bash
# Runtime invocation
wasmtime run --dir /host/path::/guest/path component.wasm
```
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