refactor(cli): move compression metadata policy to Rust

Keep the private stat_t probe, destination opening, metadata syscalls, and format callbacks in C, but route the regular-source plus stdin/stdout exception policy through the Rust fileio preference layer. The Rust ABI bridge normalizes the scalar consumed by the existing destination lifecycle, with compile-time value assertions and focused tests covering regular, non-regular, stdin, stdout, and nonzero scalar inputs.

Test Plan: Not run by request; cargo, make, native tests, and heavy commands were intentionally avoided. Lightweight git diff --check passed.

programs/fileio.c

@@ -334,6 +334,16 @@ int FIO_rust_adjustParamsForPatchFromMode(FIO_prefs_t* prefs,
 const char* FIO_determineCompressedName(const char* srcFileName, const char* outDirName, const char* suffix);
 const char* FIO_rust_determineDstName(const char* srcFileName, const char* outDirName,
                                       const char* const* suffixList, const char* suffixListStr);
+enum {
+    FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO = 0,
+    FIO_RUST_COMPRESS_DST_TRANSFER_STAT_YES = 1
+};
+typedef char FIO_rust_compress_dst_transfer_stat_values[
+        (FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO == 0
+         && FIO_RUST_COMPRESS_DST_TRANSFER_STAT_YES == 1) ? 1 : -1];
+int FIO_rust_compressDestinationTransferStat(int sourceIsStdin,
+                                             int destinationIsStdout,
+                                             int sourceIsRegular);
 int FIO_rust_adjustMemLimitForPatchFromMode(FIO_prefs_t* prefs,
                                              unsigned long long dictSize,
                                              unsigned long long maxSrcFileSize);
@@ -3258,8 +3268,9 @@ static void FIO_rust_compressDestinationRemove(void* opaque,
 /*! FIO_compressFilename_dstFile() :
  *  open the destination, or pass through if the write pool already owns it,
  *  then start compression with FIO_compressFilename_internal().
- *  Rust owns the lifecycle ordering; C retains opaque resources, diagnostics,
+ *  Rust owns the lifecycle ordering and scalar metadata-transfer policy; C
- *  metadata operations, and format-specific compression callbacks.
+ *  retains opaque resources, the private stat probe, metadata operations, and
+ *  format-specific compression callbacks.
  *  note : ress.readCtx must already have a source file attached,
  *  so reach this function through FIO_compressFilename_srcFile().
  *  @return : 0 : compression completed correctly,
@@ -3293,10 +3304,12 @@ static int FIO_compressFilename_dstFile(FIO_ctx_t* const fCtx,
     projection.destinationAlreadyOpen = AIO_WritePool_getFile(ress.writeCtx) != NULL;
     projection.sourceIsStdin = !strcmp(srcFileName, stdinmark);
     projection.destinationIsStdout = !strcmp(dstFileName, stdoutmark);
-    projection.sourceIsRegular = (projection.sourceIsStdin
+    /* C probes the private stat_t; Rust owns the stdin/stdout exceptions and
-        || projection.destinationIsStdout)
+     * normalizes the scalar consumed by the Rust destination lifecycle. */
-        ? 0
+    projection.sourceIsRegular = FIO_rust_compressDestinationTransferStat(
-        : UTIL_isRegularFileStat(srcFileStat);
+        projection.sourceIsStdin,
+        projection.destinationIsStdout,
+        UTIL_isRegularFileStat(srcFileStat));
     projection.openDestination = FIO_rust_compressDestinationOpen;
     projection.attachDestination = FIO_rust_compressDestinationAttach;
     projection.addHandler = FIO_rust_compressAddHandler;

rust/src/fileio_prefs.rs

@@ -48,7 +48,14 @@ pub const FIO_RUST_REMOVE_FILE_ACTION_STAT_FAILED: c_int = 1;
 pub const FIO_RUST_REMOVE_FILE_ACTION_NON_REGULAR: c_int = 2;
 pub const FIO_RUST_REMOVE_FILE_ACTION_FAILED: c_int = 3;
 pub const FIO_RUST_REMOVE_FILE_ACTION_INVALID: c_int = 4;
+const FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO: c_int = 0;
+const FIO_RUST_COMPRESS_DST_TRANSFER_STAT_YES: c_int = 1;
 const UTIL_FILESIZE_UNKNOWN: u64 = u64::MAX;
+
+const _: () = {
+    assert!(FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO == 0);
+    assert!(FIO_RUST_COMPRESS_DST_TRANSFER_STAT_YES == 1);
+};
 const COMPRESSED_FILE_EXTENSIONS: &[&[u8]] = &[
     b".zst",
     b".tzst",
@@ -764,6 +771,37 @@ pub extern "C" fn FIO_rust_removeFileAction(status: c_int) -> c_int {
     remove_file_action(status)
 }
 
+#[inline]
+fn compress_destination_transfer_stat(
+    source_is_stdin: c_int,
+    destination_is_stdout: c_int,
+    source_is_regular: c_int,
+) -> c_int {
+    if source_is_stdin != 0 || destination_is_stdout != 0 {
+        FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO
+    } else {
+        c_int::from(source_is_regular != 0)
+    }
+}
+
+/// Select whether compression should transfer source metadata to its output.
+///
+/// C retains the private `stat_t` probe and the actual metadata operations.
+/// Rust owns the scalar policy: stdin and stdout never transfer metadata, and
+/// otherwise a regular source enables the transfer.
+#[no_mangle]
+pub extern "C" fn FIO_rust_compressDestinationTransferStat(
+    source_is_stdin: c_int,
+    destination_is_stdout: c_int,
+    source_is_regular: c_int,
+) -> c_int {
+    compress_destination_transfer_stat(
+        source_is_stdin,
+        destination_is_stdout,
+        source_is_regular,
+    )
+}
+
 /// Callback projection for the C-owned `--list` file operations.
 #[repr(C)]
 pub struct FIO_listMultipleFilesCallbacks_t {
@@ -2677,6 +2715,36 @@ mod tests {
         }
     }
 
+    #[test]
+    fn compression_destination_transfer_stat_preserves_metadata_exceptions() {
+        for (source_is_stdin, destination_is_stdout, source_is_regular, expected) in [
+            (0, 0, 0, FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO),
+            (0, 0, 1, FIO_RUST_COMPRESS_DST_TRANSFER_STAT_YES),
+            (-1, 0, 1, FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO),
+            (0, 1, 1, FIO_RUST_COMPRESS_DST_TRANSFER_STAT_NO),
+            (0, 0, -1, FIO_RUST_COMPRESS_DST_TRANSFER_STAT_YES),
+        ] {
+            assert_eq!(
+                compress_destination_transfer_stat(
+                    source_is_stdin,
+                    destination_is_stdout,
+                    source_is_regular,
+                ),
+                expected,
+                "unexpected metadata policy for stdin={source_is_stdin}, stdout={destination_is_stdout}, regular={source_is_regular}"
+            );
+            assert_eq!(
+                FIO_rust_compressDestinationTransferStat(
+                    source_is_stdin,
+                    destination_is_stdout,
+                    source_is_regular,
+                ),
+                expected,
+                "C ABI disagrees for stdin={source_is_stdin}, stdout={destination_is_stdout}, regular={source_is_regular}"
+            );
+        }
+    }
+
     #[test]
     fn source_exclusion_preserves_case_sensitive_suffixes_and_null_policy() {
         let cases = [