refactor(cli): make decompression finish policy explicit in Rust

Extract the mixed-format decompression finish status mapping into a pure Rust policy helper and keep the existing callback side effects, diagnostics, and return values in the ABI wrapper. The explicit enum makes the success, pass-through, report, and impossible-status branches auditable and testable without widening the private fileio boundary.

Test Plan: ulimit -v 41943040; CARGO_BUILD_JOBS=1 cargo clippy --manifest-path rust/Cargo.toml --all-targets -- -D warnings; cargo +nightly fmt --manifest-path rust/Cargo.toml --all -- --check; make -j1; ./tests/rustLibSmoke; make -j1 -C tests test (all shell tests, large streaming tests, native tester, fuzzer phases, and zstream tester passed).

rust/src/fileio_asyncio.rs

@@ -5218,6 +5218,31 @@ pub unsafe extern "C" fn FIO_rust_decompressFrames(
 /// successful-file finalization.  C keeps the diagnostic and accounting
 /// callbacks because they touch `DISPLAY*` state and the private `FIO_ctx_t`
 /// layout; Rust owns the result policy and decides which callback is legal.
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+enum DecompressFinishAction {
+    Finish,
+    PassThrough,
+    ReportStatus,
+    Invalid,
+}
+
+fn classify_decompress_finish_status(status: c_int) -> DecompressFinishAction {
+    match status {
+        FIO_RUST_DECOMPRESS_OK => DecompressFinishAction::Finish,
+        FIO_RUST_DECOMPRESS_PASS_THROUGH => DecompressFinishAction::PassThrough,
+        FIO_RUST_DECOMPRESS_EMPTY_INPUT
+        | FIO_RUST_DECOMPRESS_SHORT_INPUT
+        | FIO_RUST_DECOMPRESS_GZIP_UNSUPPORTED
+        | FIO_RUST_DECOMPRESS_LZMA_UNSUPPORTED
+        | FIO_RUST_DECOMPRESS_LZ4_UNSUPPORTED
+        | FIO_RUST_DECOMPRESS_FRAME_ERROR
+        | FIO_RUST_DECOMPRESS_UNSUPPORTED_FORMAT
+        | FIO_RUST_DECOMPRESS_PASS_THROUGH_ERROR
+        | FIO_RUST_DECOMPRESS_ZSTD_UNSUPPORTED => DecompressFinishAction::ReportStatus,
+        _ => DecompressFinishAction::Invalid,
+    }
+}
+
 #[no_mangle]
 pub unsafe extern "C" fn FIO_rust_finishDecompressFrames(
     status: c_int,
@@ -5229,29 +5254,21 @@ pub unsafe extern "C" fn FIO_rust_finishDecompressFrames(
     assert!(!callbacks.is_null());
 
     let callbacks = unsafe { &*callbacks };
-    match status {
+    match classify_decompress_finish_status(status) {
-        FIO_RUST_DECOMPRESS_OK => {
+        DecompressFinishAction::Finish => {
             if let Some(finish) = callbacks.finish {
                 unsafe { finish(callbacks.opaque, src_file_name, decoded_size) };
             }
             0
         }
-        FIO_RUST_DECOMPRESS_PASS_THROUGH => 0,
+        DecompressFinishAction::PassThrough => 0,
-        FIO_RUST_DECOMPRESS_EMPTY_INPUT
+        DecompressFinishAction::ReportStatus => {
-        | FIO_RUST_DECOMPRESS_SHORT_INPUT
-        | FIO_RUST_DECOMPRESS_GZIP_UNSUPPORTED
-        | FIO_RUST_DECOMPRESS_LZMA_UNSUPPORTED
-        | FIO_RUST_DECOMPRESS_LZ4_UNSUPPORTED
-        | FIO_RUST_DECOMPRESS_FRAME_ERROR
-        | FIO_RUST_DECOMPRESS_UNSUPPORTED_FORMAT
-        | FIO_RUST_DECOMPRESS_PASS_THROUGH_ERROR
-        | FIO_RUST_DECOMPRESS_ZSTD_UNSUPPORTED => {
             if let Some(report_status) = callbacks.report_status {
                 unsafe { report_status(callbacks.opaque, status, src_file_name) };
             }
             1
         }
-        _ => {
+        DecompressFinishAction::Invalid => {
             unreachable!("unknown Rust decompression status: {status}");
         }
     }
@@ -9208,6 +9225,62 @@ mod tests {
         state.finished_sizes.push(decoded_size);
     }
 
+    #[test]
+    fn classifies_decompression_finish_statuses() {
+        for (status, expected_action) in [
+            (FIO_RUST_DECOMPRESS_OK, DecompressFinishAction::Finish),
+            (
+                FIO_RUST_DECOMPRESS_PASS_THROUGH,
+                DecompressFinishAction::PassThrough,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_EMPTY_INPUT,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_SHORT_INPUT,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_GZIP_UNSUPPORTED,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_LZMA_UNSUPPORTED,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_LZ4_UNSUPPORTED,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_FRAME_ERROR,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_UNSUPPORTED_FORMAT,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_PASS_THROUGH_ERROR,
+                DecompressFinishAction::ReportStatus,
+            ),
+            (
+                FIO_RUST_DECOMPRESS_ZSTD_UNSUPPORTED,
+                DecompressFinishAction::ReportStatus,
+            ),
+        ] {
+            assert_eq!(classify_decompress_finish_status(status), expected_action);
+        }
+
+        for status in [-1, 99] {
+            assert_eq!(
+                classify_decompress_finish_status(status),
+                DecompressFinishAction::Invalid
+            );
+        }
+    }
+
     #[test]
     fn maps_decompression_status_and_finishes_only_success() {
         let mut state = DecompressResultState {