refactor(fileio): move decompression teardown order to Rust

Project the decompression-resource cleanup callbacks into Rust so the stable
teardown order remains dictionary, dstream, write pool, and read pool. C
retains ownership of dRess_t and each private resource implementation, with
its existing dstream CHECK behavior preserved. Focused tests cover order and
null or incomplete callback states.

All heavy verification was run serially with a 40 GiB virtual-memory cap and
one build job.

Test Plan:
- git diff --cached --check
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/Cargo.toml --all-targets (788 passed)
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets -- -D warnings
- ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo test --manifest-path rust/cli/Cargo.toml --all-targets (181 passed)
- ulimit -v 41943040; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check
- ulimit -v 41943040; make -j1
- ulimit -v 41943040; make -j1 -C tests test

programs/fileio.c

@@ -3648,6 +3648,51 @@ typedef struct {
     ReadPoolCtx_t *readCtx;
 } dRess_t;
 
+typedef void (*FIO_rust_freeDResourcesCallback_f)(void* context);
+typedef struct {
+    void* callbackContext;
+    FIO_rust_freeDResourcesCallback_f freeDict;
+    FIO_rust_freeDResourcesCallback_f freeDctx;
+    FIO_rust_freeDResourcesCallback_f freeWritePool;
+    FIO_rust_freeDResourcesCallback_f freeReadPool;
+} FIO_rust_freeDResourcesState;
+typedef char FIO_rust_free_d_resources_state_layout[
+        (offsetof(FIO_rust_freeDResourcesState, callbackContext) == 0
+         && offsetof(FIO_rust_freeDResourcesState, freeDict) == sizeof(void*)
+         && offsetof(FIO_rust_freeDResourcesState, freeDctx)
+                == 2 * sizeof(void*)
+         && offsetof(FIO_rust_freeDResourcesState, freeWritePool)
+                == 3 * sizeof(void*)
+         && offsetof(FIO_rust_freeDResourcesState, freeReadPool)
+                == 4 * sizeof(void*)
+         && sizeof(FIO_rust_freeDResourcesState) == 5 * sizeof(void*))
+        ? 1 : -1];
+void FIO_rust_freeDResources(const FIO_rust_freeDResourcesState* state);
+
+static void FIO_rust_freeDResources_dict(void* context)
+{
+    dRess_t* const ress = (dRess_t*)context;
+    FIO_freeDict(&(ress->dict));
+}
+
+static void FIO_rust_freeDResources_dctx(void* context)
+{
+    dRess_t* const ress = (dRess_t*)context;
+    CHECK( ZSTD_freeDStream(ress->dctx) );
+}
+
+static void FIO_rust_freeDResources_writePool(void* context)
+{
+    dRess_t* const ress = (dRess_t*)context;
+    AIO_WritePool_free(ress->writeCtx);
+}
+
+static void FIO_rust_freeDResources_readPool(void* context)
+{
+    dRess_t* const ress = (dRess_t*)context;
+    AIO_ReadPool_free(ress->readCtx);
+}
+
 static dRess_t FIO_createDResources(FIO_prefs_t* const prefs, const char* dictFileName)
 {
     int useMMap = prefs->mmapDict == ZSTD_ps_enable;
@@ -3693,10 +3738,14 @@ static dRess_t FIO_createDResources(FIO_prefs_t* const prefs, const char* dictFi
 
 static void FIO_freeDResources(dRess_t ress)
 {
-    FIO_freeDict(&(ress.dict));
+    FIO_rust_freeDResourcesState const state = {
-    CHECK( ZSTD_freeDStream(ress.dctx) );
+        &ress,
-    AIO_WritePool_free(ress.writeCtx);
+        FIO_rust_freeDResources_dict,
-    AIO_ReadPool_free(ress.readCtx);
+        FIO_rust_freeDResources_dctx,
+        FIO_rust_freeDResources_writePool,
+        FIO_rust_freeDResources_readPool
+    };
+    FIO_rust_freeDResources(&state);
 }
 
 /* FIO_passThrough() : just copy input into output, for compatibility with gzip -df mode

rust/src/fileio_prefs.rs

@@ -173,6 +173,56 @@ pub unsafe extern "C" fn FIO_rust_freeCResources(
     }
 }
 
+type FIO_freeDResourcesCallbackFn = unsafe extern "C" fn(*mut c_void);
+
+/// C-owned decompression-resource teardown projection.
+///
+/// Rust owns the stable cleanup order while C keeps `dRess_t`, dictionary,
+/// asynchronous-pool, and stream layouts private behind callbacks.
+#[repr(C)]
+pub struct FIO_rust_freeDResourcesState {
+    callback_context: *mut c_void,
+    free_dict: Option<FIO_freeDResourcesCallbackFn>,
+    free_dctx: Option<FIO_freeDResourcesCallbackFn>,
+    free_write_pool: Option<FIO_freeDResourcesCallbackFn>,
+    free_read_pool: Option<FIO_freeDResourcesCallbackFn>,
+}
+
+const _: () = {
+    assert!(offset_of!(FIO_rust_freeDResourcesState, callback_context) == 0);
+    assert!(offset_of!(FIO_rust_freeDResourcesState, free_dict) == size_of::<usize>());
+    assert!(offset_of!(FIO_rust_freeDResourcesState, free_dctx) == 2 * size_of::<usize>());
+    assert!(offset_of!(FIO_rust_freeDResourcesState, free_write_pool) == 3 * size_of::<usize>());
+    assert!(offset_of!(FIO_rust_freeDResourcesState, free_read_pool) == 4 * size_of::<usize>());
+    assert!(size_of::<Option<FIO_freeDResourcesCallbackFn>>() == size_of::<usize>());
+    assert!(size_of::<FIO_rust_freeDResourcesState>() == 5 * size_of::<usize>());
+};
+
+#[no_mangle]
+pub unsafe extern "C" fn FIO_rust_freeDResources(state: *const FIO_rust_freeDResourcesState) {
+    let Some(state) = (unsafe { state.as_ref() }) else {
+        return;
+    };
+    if state.callback_context.is_null() {
+        return;
+    }
+    let (Some(free_dict), Some(free_dctx), Some(free_write_pool), Some(free_read_pool)) = (
+        state.free_dict,
+        state.free_dctx,
+        state.free_write_pool,
+        state.free_read_pool,
+    ) else {
+        return;
+    };
+
+    unsafe {
+        free_dict(state.callback_context);
+        free_dctx(state.callback_context);
+        free_write_pool(state.callback_context);
+        free_read_pool(state.callback_context);
+    }
+}
+
 pub type FIO_createCResourcesSetParameterFn =
     unsafe extern "C" fn(*mut c_void, c_int, c_int) -> usize;
 pub type FIO_createCResourcesIsErrorFn = unsafe extern "C" fn(usize) -> c_int;
@@ -2169,6 +2219,67 @@ mod tests {
         assert_eq!(context.events, ["dict", "write", "read", "cctx"]);
     }
 
+    #[derive(Default)]
+    struct FreeDResourcesTestState {
+        events: Vec<&'static str>,
+    }
+
+    unsafe extern "C" fn free_d_resources_dict_test(context: *mut c_void) {
+        let state = unsafe { &mut *context.cast::<FreeDResourcesTestState>() };
+        state.events.push("dict");
+    }
+
+    unsafe extern "C" fn free_d_resources_dctx_test(context: *mut c_void) {
+        let state = unsafe { &mut *context.cast::<FreeDResourcesTestState>() };
+        state.events.push("dctx");
+    }
+
+    unsafe extern "C" fn free_d_resources_write_pool_test(context: *mut c_void) {
+        let state = unsafe { &mut *context.cast::<FreeDResourcesTestState>() };
+        state.events.push("write");
+    }
+
+    unsafe extern "C" fn free_d_resources_read_pool_test(context: *mut c_void) {
+        let state = unsafe { &mut *context.cast::<FreeDResourcesTestState>() };
+        state.events.push("read");
+    }
+
+    fn free_d_resources_test_state(
+        context: &mut FreeDResourcesTestState,
+    ) -> FIO_rust_freeDResourcesState {
+        FIO_rust_freeDResourcesState {
+            callback_context: context as *mut FreeDResourcesTestState as *mut c_void,
+            free_dict: Some(free_d_resources_dict_test),
+            free_dctx: Some(free_d_resources_dctx_test),
+            free_write_pool: Some(free_d_resources_write_pool_test),
+            free_read_pool: Some(free_d_resources_read_pool_test),
+        }
+    }
+
+    #[test]
+    fn free_d_resources_preserves_teardown_order() {
+        let mut context = FreeDResourcesTestState::default();
+        let state = free_d_resources_test_state(&mut context);
+
+        unsafe { FIO_rust_freeDResources(&state) };
+
+        assert_eq!(context.events, ["dict", "dctx", "write", "read"]);
+    }
+
+    #[test]
+    fn free_d_resources_rejects_null_or_incomplete_state_without_callbacks() {
+        let mut context = FreeDResourcesTestState::default();
+        let mut incomplete = free_d_resources_test_state(&mut context);
+        incomplete.free_dctx = None;
+
+        unsafe {
+            FIO_rust_freeDResources(ptr::null());
+            FIO_rust_freeDResources(&incomplete);
+        }
+
+        assert!(context.events.is_empty());
+    }
+
     #[derive(Default)]
     struct CreateCResourcesTestState {
         events: Vec<(c_int, c_int)>,