Description
I tried this code:
#[no_mangle]
pub extern "C" fn update() {
static FOO: std::sync::Once = std::sync::Once::new();
FOO.call_once(|| {
let _ = std::fs::metadata("/mnt/c/foo.txt");
});
}I expected to see this happen:
If you compile this to a WASI library and call the update function from a WASM runtime, then it will query the metadata of the file only during the first call to update. Every other invocation the update function should do nothing.
Instead, this happened:
The update function seems to refresh the list of preopened directories on every call... and it leaks them. The WASM memory keeps growing, looking like this:
This likely is because of the way Rust lazily initializes the list of preopened directories through wasi-libc. It probably does it unconditionally, which wasi-libc probably can't handle, so it never frees any memory.
The problem also occurs if you have any file system code at all, and it never even gets called:
#[no_mangle]
pub extern "C" fn other() {
let _ = std::fs::metadata("/mnt/c/foo.txt");
}
#[no_mangle]
pub extern "C" fn update() { }If you compile this code, but never call other and instead only update, it also keeps leaking the list of preopened directories.
The WASM seems to come down to this:
(export "update" (func $update.command_export))
...
(func $update.command_export
(call $__wasm_call_ctors)
(call $update)
(call $__wasm_call_dtors)
)with
(func $__wasm_call_ctors
(call $__wasilibc_populate_preopens)
)
(func $__wasm_call_dtors
(call $dummy)
(call $dummy)
)__wasilibc_populate_preopens looks too complex. The original C code probably needs to be checked.
Meta
rustc --version --verbose:
rustc 1.70.0 (90c541806 2023-05-31)
binary: rustc
commit-hash: 90c541806f23a127002de5b4038be731ba1458ca
commit-date: 2023-05-31
host: x86_64-pc-windows-msvc
release: 1.70.0
LLVM version: 16.0.2