feat(fileio): move filename collision checks to Rust

Implement basename extraction, bytewise sorting, collision reporting, and allocation failure handling in the Rust fileio preferences module. Keep programs/fileio.c as the stable ABI shim and preserve non-UTF-8 filenames without lossy conversion.

Test Plan:

- cargo test --manifest-path rust/cli/Cargo.toml --no-default-features --features cli,compression,decompression

- cargo clippy --manifest-path rust/cli/Cargo.toml --all-targets --no-default-features --features cli,compression,decompression

- make -B -C tests -j2 test-cli-tests

- git diff --cached --check

programs/fileio.c

@@ -306,6 +306,7 @@ static int FIO_shouldDisplayMultipleFileSummary(FIO_ctx_t const* fCtx)
 
 /* These symbols are implemented by rust/src/fileio_prefs.rs.  The declarations
  * in fileio.h remain the C ABI shims while all actual file I/O stays here. */
+int FIO_rust_checkFilenameCollisions(const char** filenameTable, unsigned nbFiles);
 
 
 /*-*************************************
@@ -678,35 +679,7 @@ static void FIO_initDict(FIO_Dict_t* dict, const char* fileName, FIO_prefs_t* co
  * Checks for and warns if there are any files that would have the same output path
  */
 int FIO_checkFilenameCollisions(const char** filenameTable, unsigned nbFiles) {
-    const char **filenameTableSorted, *prevElem, *filename;
-    unsigned u;
-
-    filenameTableSorted = (const char**) malloc(sizeof(char*) * nbFiles);
-    if (!filenameTableSorted) {
-        DISPLAYLEVEL(1, "Allocation error during filename collision checking \n");
-        return 1;
-    }
-
-    for (u = 0; u < nbFiles; ++u) {
-        filename = strrchr(filenameTable[u], PATH_SEP);
-        if (filename == NULL) {
-            filenameTableSorted[u] = filenameTable[u];
-        } else {
-            filenameTableSorted[u] = filename+1;
-        }
-    }
-
-    qsort((void*)filenameTableSorted, nbFiles, sizeof(char*), UTIL_compareStr);
-    prevElem = filenameTableSorted[0];
-    for (u = 1; u < nbFiles; ++u) {
-        if (strcmp(prevElem, filenameTableSorted[u]) == 0) {
-            DISPLAYLEVEL(2, "WARNING: Two files have same filename: %s\n", prevElem);
-        }
-        prevElem = filenameTableSorted[u];
-    }
-
-    free((void*)filenameTableSorted);
-    return 0;
+    return FIO_rust_checkFilenameCollisions(filenameTable, nbFiles);
 }
 
 char* UTIL_createFilenameFromOutDir(const char* path, const char* outDirName,

rust/src/fileio_prefs.rs

@@ -10,7 +10,8 @@
 //! implementation.  In particular, the four legacy fields which C leaves
 //! unspecified are not zero-filled here either.
 
-use std::ffi::c_void;
+use std::ffi::{c_void, CStr};
+use std::io::{self, Write};
 use std::mem::size_of;
 use std::os::raw::{c_char, c_int, c_uint};
 use std::ptr;
@@ -475,9 +476,91 @@ pub unsafe extern "C" fn FIO_determineHasStdinInput(
     }
 }
 
+#[inline]
+fn filename_path_separator() -> u8 {
+    if cfg!(windows) {
+        b'\\'
+    } else {
+        b'/'
+    }
+}
+
+/// Return the byte-level basename used for collision checks.
+///
+/// A null entry has no filename and is ignored by the exported checker.  It
+/// still maps to an empty key here so the key operation is safe to exercise
+/// independently and never passes a null pointer to `CStr::from_ptr`.
+unsafe fn filename_collision_key<'a>(filename: *const c_char) -> &'a [u8] {
+    if filename.is_null() {
+        return &[];
+    }
+    let bytes = unsafe { CStr::from_ptr(filename).to_bytes() };
+    match bytes
+        .iter()
+        .rposition(|&byte| byte == filename_path_separator())
+    {
+        Some(index) => &bytes[index + 1..],
+        None => bytes,
+    }
+}
+
+unsafe fn same_filename_collision_key(left: *const c_char, right: *const c_char) -> bool {
+    unsafe { filename_collision_key(left) == filename_collision_key(right) }
+}
+
+fn display_filename_collision_warning(filename: *const c_char) {
+    let enabled = unsafe { (*display_prefs()).displayLevel >= 2 };
+    if !enabled {
+        return;
+    }
+
+    let basename = unsafe { filename_collision_key(filename) };
+    let mut stderr = io::stderr().lock();
+    let _ = stderr.write_all(b"WARNING: Two files have same filename: ");
+    let _ = stderr.write_all(basename);
+    let _ = stderr.write_all(b"\n");
+}
+
+/// Rust implementation of the filename collision checker used by
+/// `FIO_checkFilenameCollisions` in `programs/fileio.c`.
+#[no_mangle]
+pub unsafe extern "C" fn FIO_rust_checkFilenameCollisions(
+    filename_table: *const *const c_char,
+    nb_files: c_uint,
+) -> c_int {
+    if filename_table.is_null() || nb_files == 0 {
+        return 0;
+    }
+
+    let mut filenames = Vec::<*const c_char>::new();
+    if filenames.try_reserve_exact(nb_files as usize).is_err() {
+        display(1, "Allocation error during filename collision checking \n");
+        return 1;
+    }
+
+    for index in 0..nb_files as usize {
+        let filename = unsafe { *filename_table.add(index) };
+        if !filename.is_null() {
+            filenames.push(filename);
+        }
+    }
+
+    filenames.sort_unstable_by(|left, right| unsafe {
+        filename_collision_key(*left).cmp(filename_collision_key(*right))
+    });
+    for pair in filenames.windows(2) {
+        if unsafe { same_filename_collision_key(pair[0], pair[1]) } {
+            display_filename_collision_warning(pair[0]);
+        }
+    }
+
+    0
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
+    use std::ffi::CString;
     use std::mem::{align_of, offset_of, size_of};
 
     #[test]
@@ -516,6 +599,39 @@ mod tests {
         assert_eq!(size_of::<FIO_ctx_t>(), total_bytes + 2 * word);
     }
 
+    #[test]
+    fn filename_collision_checker_handles_null_and_empty_inputs() {
+        let empty = CString::new("").unwrap();
+        let names = [ptr::null(), empty.as_ptr()];
+
+        unsafe {
+            assert_eq!(FIO_rust_checkFilenameCollisions(ptr::null(), 0), 0);
+            assert_eq!(FIO_rust_checkFilenameCollisions(names.as_ptr(), 2), 0);
+            assert!(same_filename_collision_key(ptr::null(), empty.as_ptr()));
+        }
+    }
+
+    #[test]
+    fn filename_collision_keys_compare_basename_bytes_without_utf8_conversion() {
+        let first = CString::new(b"left/\xffname".to_vec()).unwrap();
+        let duplicate = CString::new(b"right/\xffname".to_vec()).unwrap();
+        let different = CString::new(b"right/\xfename".to_vec()).unwrap();
+        let names = [first.as_ptr(), duplicate.as_ptr()];
+
+        unsafe {
+            assert_eq!(filename_collision_key(first.as_ptr()), b"\xffname");
+            assert!(same_filename_collision_key(
+                first.as_ptr(),
+                duplicate.as_ptr()
+            ));
+            assert!(!same_filename_collision_key(
+                first.as_ptr(),
+                different.as_ptr()
+            ));
+            assert_eq!(FIO_rust_checkFilenameCollisions(names.as_ptr(), 2), 0);
+        }
+    }
+
     #[test]
     fn preference_defaults_match_fileio_c() {
         let prefs = unsafe { FIO_createPreferences() };