Close file descriptors of pipes after use (#2591)

* Close file descriptors of pipes after use

* cat: Test file descriptor exhaustion

* fixup! cat: Test file descriptor exhaustion

tests/by-util/test_cat.rs

@@ -1,8 +1,13 @@
+// spell-checker:ignore NOFILE
+
 use crate::common::util::*;
 use std::fs::OpenOptions;
 #[cfg(unix)]
 use std::io::Read;
 
+#[cfg(target_os = "linux")]
+use rlimit::Resource;
+
 #[test]
 fn test_output_simple() {
     new_ucmd!()
@@ -87,6 +92,23 @@ fn test_fifo_symlink() {
     thread.join().unwrap();
 }
 
+#[test]
+#[cfg(target_os = "linux")]
+fn test_closes_file_descriptors() {
+    // Each file creates a pipe, which has two file descriptors.
+    // If they are not closed then five is certainly too many.
+    new_ucmd!()
+        .args(&[
+            "alpha.txt",
+            "alpha.txt",
+            "alpha.txt",
+            "alpha.txt",
+            "alpha.txt",
+        ])
+        .with_limit(Resource::NOFILE, 9, 9)
+        .succeeds();
+}
+
 #[test]
 #[cfg(unix)]
 fn test_piped_to_regular_file() {