sort: Rework merge batching logic

Fix bug #6944
Rework the way batching is done with sort such that it doesn't open
more input files than necessary. Previously, the code would always
open one extra input file which causes problems in ulimit scenarios.
Add additional test case.

src/uu/sort/src/ext_sort.rs

@@ -98,12 +98,12 @@ fn reader_writer<
     )?;
     match read_result {
         ReadResult::WroteChunksToFile { tmp_files } => {
-            let merger = merge::merge_with_file_limit::<_, _, Tmp>(
+            merge::merge_with_file_limit::<_, _, Tmp>(
                 tmp_files.into_iter().map(|c| c.reopen()),
                 settings,
+                output,
                 tmp_dir,
             )?;
-            merger.write_all(settings, output)?;
         }
         ReadResult::SortedSingleChunk(chunk) => {
             if settings.unique {

src/uu/sort/src/merge.rs

@@ -25,7 +25,6 @@ use std::{
 };
 
 use compare::Compare;
-use itertools::Itertools;
 use uucore::error::UResult;
 
 use crate::{
@@ -67,58 +66,63 @@ fn replace_output_file_in_input_files(
 ///
 /// If `settings.merge_batch_size` is greater than the length of `files`, intermediate files will be used.
 /// If `settings.compress_prog` is `Some`, intermediate files will be compressed with it.
-pub fn merge<'a>(
+pub fn merge(
     files: &mut [OsString],
-    settings: &'a GlobalSettings,
-    output: Option<&str>,
+    settings: &GlobalSettings,
+    output: Output,
     tmp_dir: &mut TmpDirWrapper,
-) -> UResult<FileMerger<'a>> {
-    replace_output_file_in_input_files(files, output, tmp_dir)?;
+) -> UResult<()> {
+    replace_output_file_in_input_files(files, output.as_output_name(), tmp_dir)?;
+    let files = files
+        .iter()
+        .map(|file| open(file).map(|file| PlainMergeInput { inner: file }));
     if settings.compress_prog.is_none() {
-        merge_with_file_limit::<_, _, WriteablePlainTmpFile>(
-            files
-                .iter()
-                .map(|file| open(file).map(|file| PlainMergeInput { inner: file })),
-            settings,
-            tmp_dir,
-        )
+        merge_with_file_limit::<_, _, WriteablePlainTmpFile>(files, settings, output, tmp_dir)
     } else {
-        merge_with_file_limit::<_, _, WriteableCompressedTmpFile>(
-            files
-                .iter()
-                .map(|file| open(file).map(|file| PlainMergeInput { inner: file })),
-            settings,
-            tmp_dir,
-        )
+        merge_with_file_limit::<_, _, WriteableCompressedTmpFile>(files, settings, output, tmp_dir)
     }
 }
 
 // Merge already sorted `MergeInput`s.
 pub fn merge_with_file_limit<
-    'a,
     M: MergeInput + 'static,
     F: ExactSizeIterator<Item = UResult<M>>,
     Tmp: WriteableTmpFile + 'static,
 >(
     files: F,
-    settings: &'a GlobalSettings,
+    settings: &GlobalSettings,
+    output: Output,
     tmp_dir: &mut TmpDirWrapper,
-) -> UResult<FileMerger<'a>> {
-    if files.len() > settings.merge_batch_size {
-        let mut remaining_files = files.len();
-        let batches = files.chunks(settings.merge_batch_size);
-        let mut batches = batches.into_iter();
+) -> UResult<()> {
+    if files.len() <= settings.merge_batch_size {
+        let merger = merge_without_limit(files, settings);
+        merger?.write_all(settings, output)
+    } else {
         let mut temporary_files = vec![];
-        while remaining_files != 0 {
-            // Work around the fact that `Chunks` is not an `ExactSizeIterator`.
-            remaining_files = remaining_files.saturating_sub(settings.merge_batch_size);
-            let merger = merge_without_limit(batches.next().unwrap(), settings)?;
+        let mut batch = vec![];
+        for file in files {
+            batch.push(file);
+            if batch.len() >= settings.merge_batch_size {
+                assert_eq!(batch.len(), settings.merge_batch_size);
+                let merger = merge_without_limit(batch.into_iter(), settings)?;
+                batch = vec![];
+
+                let mut tmp_file =
+                    Tmp::create(tmp_dir.next_file()?, settings.compress_prog.as_deref())?;
+                merger.write_all_to(settings, tmp_file.as_write())?;
+                temporary_files.push(tmp_file.finished_writing()?);
+            }
+        }
+        // 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);
+            let merger = merge_without_limit(batch.into_iter(), settings)?;
+
             let mut tmp_file =
                 Tmp::create(tmp_dir.next_file()?, settings.compress_prog.as_deref())?;
             merger.write_all_to(settings, tmp_file.as_write())?;
             temporary_files.push(tmp_file.finished_writing()?);
         }
-        assert!(batches.next().is_none());
         merge_with_file_limit::<_, _, Tmp>(
             temporary_files
                 .into_iter()
@@ -127,10 +131,9 @@ pub fn merge_with_file_limit<
                         dyn FnMut(Tmp::Closed) -> UResult<<Tmp::Closed as ClosedTmpFile>::Reopened>,
                     >),
             settings,
+            output,
             tmp_dir,
         )
-    } else {
-        merge_without_limit(files, settings)
     }
 }
 
@@ -260,7 +263,7 @@ struct PreviousLine {
 }
 
 /// Merges files together. This is **not** an iterator because of lifetime problems.
-pub struct FileMerger<'a> {
+struct FileMerger<'a> {
     heap: binary_heap_plus::BinaryHeap<MergeableFile, FileComparator<'a>>,
     request_sender: Sender<(usize, RecycledChunk)>,
     prev: Option<PreviousLine>,
@@ -269,12 +272,12 @@ pub struct FileMerger<'a> {
 
 impl FileMerger<'_> {
     /// Write the merged contents to the output file.
-    pub fn write_all(self, settings: &GlobalSettings, output: Output) -> UResult<()> {
+    fn write_all(self, settings: &GlobalSettings, output: Output) -> UResult<()> {
         let mut out = output.into_write();
         self.write_all_to(settings, &mut out)
     }
 
-    pub fn write_all_to(mut self, settings: &GlobalSettings, out: &mut impl Write) -> UResult<()> {
+    fn write_all_to(mut self, settings: &GlobalSettings, out: &mut impl Write) -> UResult<()> {
         while self.write_next(settings, out) {}
         drop(self.request_sender);
         self.reader_join_handle.join().unwrap()

src/uu/sort/src/sort.rs

@@ -1567,8 +1567,7 @@ fn exec(
     tmp_dir: &mut TmpDirWrapper,
 ) -> UResult<()> {
     if settings.merge {
-        let file_merger = merge::merge(files, settings, output.as_output_name(), tmp_dir)?;
-        file_merger.write_all(settings, output)
+        merge::merge(files, settings, output, tmp_dir)
     } else if settings.check {
         if files.len() > 1 {
             Err(UUsageError::new(2, "only one file allowed with -c"))

tests/by-util/test_sort.rs

@@ -3,7 +3,7 @@
 // For the full copyright and license information, please view the LICENSE
 // file that was distributed with this source code.
 
-// spell-checker:ignore (words) ints
+// spell-checker:ignore (words) ints (linux) NOFILE
 #![allow(clippy::cast_possible_wrap)]
 
 use std::time::Duration;
@@ -1084,6 +1084,31 @@ fn test_merge_batch_size() {
         .stdout_only_fixture("merge_ints_interleaved.expected");
 }
 
+#[test]
+#[cfg(any(target_os = "linux", target_os = "android"))]
+fn test_merge_batch_size_with_limit() {
+    use rlimit::Resource;
+    // Currently need...
+    // 3 descriptors for stdin, stdout, stderr
+    // 2 descriptors for CTRL+C handling logic (to be reworked at some point)
+    // 2 descriptors for the input files (i.e. batch-size of 2).
+    let limit_fd = 3 + 2 + 2;
+    TestScenario::new(util_name!())
+        .ucmd()
+        .limit(Resource::NOFILE, limit_fd, limit_fd)
+        .arg("--batch-size=2")
+        .arg("-m")
+        .arg("--unique")
+        .arg("merge_ints_interleaved_1.txt")
+        .arg("merge_ints_interleaved_2.txt")
+        .arg("merge_ints_interleaved_3.txt")
+        .arg("merge_ints_interleaved_3.txt")
+        .arg("merge_ints_interleaved_2.txt")
+        .arg("merge_ints_interleaved_1.txt")
+        .succeeds()
+        .stdout_only_fixture("merge_ints_interleaved.expected");
+}
+
 #[test]
 fn test_sigpipe_panic() {
     let mut cmd = new_ucmd!();