fix(sort): GNU sort-continue.sh test (#9107)

* feat: dynamically adjust merge batch size based on file descriptor limits

- Add `effective_merge_batch_size()` function to calculate batch size considering fd soft limit, with minimums and safety margins.
- Generalize fd limit handling from Linux-only to Unix systems using `fd_soft_limit()`.
- Update merge logic to use dynamic batch size instead of fixed `settings.merge_batch_size` to prevent fd exhaustion.

* fix(sort): update rlimit fetching to use fd_soft_limit with error handling

Replace direct call to get_rlimit()? with fd_soft_limit(), adding a check for None value to return a usage error if rlimit cannot be fetched. This improves robustness on Linux by ensuring proper error handling when retrieving the file descriptor soft limit.

* refactor(sort): restrict nix::libc and fd_soft_limit to Linux

Update conditional compilation attributes from #[cfg(unix)] to #[cfg(target_os = "linux")]
for the nix::libc import and fd_soft_limit function implementations, ensuring these
features are only enabled on Linux systems to improve portability and avoid issues
on other Unix-like platforms.

* refactor: improve thread management and replace unsafe libc calls

Replace unsafe libc::getrlimit calls in fd_soft_limit with safe nix crate usage.
Update Rayon thread configuration to use ThreadPoolBuilder instead of environment variables for better control.
Add documentation comment to effective_merge_batch_size function for clarity.

* refactor(linux): improve error handling in fd_soft_limit function

Extract the rlimit fetching logic into a separate `get_rlimit` function that returns `UResult<usize>` and properly handles errors with `UUsageError`, instead of silently returning `None` on failure or infinity. This provides better error reporting for resource limit issues on Linux platforms.

* refactor(sort): reorder imports in get_rlimit for consistency

Reordered the nix::sys::resource imports to group constants first (RLIM_INFINITY), then types (Resource), and finally functions (getrlimit), improving code readability and adhering to import style guidelines.

src/uu/sort/src/merge.rs

@@ -30,7 +30,7 @@ use uucore::error::{FromIo, UResult};
 use crate::{
     GlobalSettings, Output, SortError,
     chunks::{self, Chunk, RecycledChunk},
-    compare_by, open,
+    compare_by, fd_soft_limit, open,
     tmp_dir::TmpDirWrapper,
 };
 
@@ -62,6 +62,28 @@ fn replace_output_file_in_input_files(
     Ok(())
 }
 
+/// Determine the effective merge batch size, enforcing a minimum and respecting the
+/// file-descriptor soft limit after reserving stdio/output and a safety margin.
+fn effective_merge_batch_size(settings: &GlobalSettings) -> usize {
+    const MIN_BATCH_SIZE: usize = 2;
+    const RESERVED_STDIO: usize = 3;
+    const RESERVED_OUTPUT: usize = 1;
+    const SAFETY_MARGIN: usize = 1;
+    let mut batch_size = settings.merge_batch_size.max(MIN_BATCH_SIZE);
+
+    if let Some(limit) = fd_soft_limit() {
+        let reserved = RESERVED_STDIO + RESERVED_OUTPUT + SAFETY_MARGIN;
+        let available_inputs = limit.saturating_sub(reserved);
+        if available_inputs >= MIN_BATCH_SIZE {
+            batch_size = batch_size.min(available_inputs);
+        } else {
+            batch_size = MIN_BATCH_SIZE;
+        }
+    }
+
+    batch_size
+}
+
 /// Merge pre-sorted `Box<dyn Read>`s.
 ///
 /// If `settings.merge_batch_size` is greater than the length of `files`, intermediate files will be used.
@@ -94,18 +116,21 @@ pub fn merge_with_file_limit<
     output: Output,
     tmp_dir: &mut TmpDirWrapper,
 ) -> UResult<()> {
-    if files.len() <= settings.merge_batch_size {
+    let batch_size = effective_merge_batch_size(settings);
+    debug_assert!(batch_size >= 2);
+
+    if files.len() <= batch_size {
         let merger = merge_without_limit(files, settings);
         merger?.write_all(settings, output)
     } else {
         let mut temporary_files = vec![];
-        let mut batch = vec![];
+        let mut batch = Vec::with_capacity(batch_size);
         for file in files {
             batch.push(file);
-            if batch.len() >= settings.merge_batch_size {
-                assert_eq!(batch.len(), settings.merge_batch_size);
+            if batch.len() >= batch_size {
+                assert_eq!(batch.len(), batch_size);
                 let merger = merge_without_limit(batch.into_iter(), settings)?;
-                batch = vec![];
+                batch = Vec::with_capacity(batch_size);
 
                 let mut tmp_file =
                     Tmp::create(tmp_dir.next_file()?, settings.compress_prog.as_deref())?;
@@ -115,7 +140,7 @@ pub fn merge_with_file_limit<
         }
         // Merge any remaining files that didn't get merged in a full batch above.
         if !batch.is_empty() {
-            assert!(batch.len() < settings.merge_batch_size);
+            assert!(batch.len() < batch_size);
             let merger = merge_without_limit(batch.into_iter(), settings)?;
 
             let mut tmp_file =

src/uu/sort/src/sort.rs

@@ -1073,13 +1073,27 @@ fn make_sort_mode_arg(mode: &'static str, short: char, help: String) -> Arg {
 
 #[cfg(target_os = "linux")]
 fn get_rlimit() -> UResult<usize> {
-    use nix::sys::resource::{Resource, getrlimit};
+    use nix::sys::resource::{RLIM_INFINITY, Resource, getrlimit};
 
-    getrlimit(Resource::RLIMIT_NOFILE)
-        .map(|(rlim_cur, _)| rlim_cur as usize)
+    let (rlim_cur, _rlim_max) = getrlimit(Resource::RLIMIT_NOFILE)
+        .map_err(|_| UUsageError::new(2, translate!("sort-failed-fetch-rlimit")))?;
+    if rlim_cur == RLIM_INFINITY {
+        return Err(UUsageError::new(2, translate!("sort-failed-fetch-rlimit")));
+    }
+    usize::try_from(rlim_cur)
         .map_err(|_| UUsageError::new(2, translate!("sort-failed-fetch-rlimit")))
 }
 
+#[cfg(target_os = "linux")]
+pub(crate) fn fd_soft_limit() -> Option<usize> {
+    get_rlimit().ok()
+}
+
+#[cfg(not(target_os = "linux"))]
+pub(crate) fn fd_soft_limit() -> Option<usize> {
+    None
+}
+
 const STDIN_FILE: &str = "-";
 
 /// Legacy `+POS1 [-POS2]` syntax is permitted unless `_POSIX2_VERSION` is in
@@ -1232,12 +1246,12 @@ fn default_merge_batch_size() -> usize {
     #[cfg(target_os = "linux")]
     {
         // Adjust merge batch size dynamically based on available file descriptors.
-        match get_rlimit() {
-            Ok(limit) => {
+        match fd_soft_limit() {
+            Some(limit) => {
                 let usable_limit = limit.saturating_div(LINUX_BATCH_DIVISOR);
                 usable_limit.clamp(LINUX_BATCH_MIN, LINUX_BATCH_MAX)
             }
-            Err(_) => 64,
+            None => 64,
         }
     }
 
@@ -1366,9 +1380,15 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
         settings.threads = matches
             .get_one::<String>(options::PARALLEL)
             .map_or_else(|| "0".to_string(), String::from);
-        unsafe {
-            env::set_var("RAYON_NUM_THREADS", &settings.threads);
-        }
+        let num_threads = match settings.threads.parse::<usize>() {
+            Ok(0) | Err(_) => std::thread::available_parallelism()
+                .map(|n| n.get())
+                .unwrap_or(1),
+            Ok(n) => n,
+        };
+        let _ = rayon::ThreadPoolBuilder::new()
+            .num_threads(num_threads)
+            .build_global();
     }
 
     if let Some(size_str) = matches.get_one::<String>(options::BUF_SIZE) {
@@ -1419,7 +1439,15 @@ pub fn uumain(args: impl uucore::Args) -> UResult<()> {
 
                         translate!(
                             "sort-maximum-batch-size-rlimit",
-                            "rlimit" =>  get_rlimit()?
+                            "rlimit" => {
+                                let Some(rlimit) = fd_soft_limit() else {
+                                    return Err(UUsageError::new(
+                                        2,
+                                        translate!("sort-failed-fetch-rlimit"),
+                                    ));
+                                };
+                                rlimit
+                            }
                         )
                     }
                     #[cfg(not(target_os = "linux"))]