comm: fix comparison when reading from pipes

Use case is that two files are piped into comm, i.e. in bash
   comm <(cat file1) <(cat file2)

Before the fix, comm reads from the pipes twice. Once in "fn comm" and
once in "fn are_files_identical". As such, part of the data is skipped
in comparison which leads to wrong output.

This is fixed by skipping the file comparison in case one of the files
is not a regular file.

Author: martinkunkel2

src/uu/comm/src/comm.rs

@@ -135,6 +135,11 @@ pub fn are_files_identical(path1: &Path, path2: &Path) -> io::Result<bool> {
         return Ok(false);
     }
 
+    // only proceed if both are regular files
+    if !metadata1.is_file() || !metadata2.is_file() {
+        return Ok(false);
+    }
+
     let file1 = File::open(path1)?;
     let file2 = File::open(path2)?;
 

tests/by-util/test_comm.rs

@@ -648,3 +648,36 @@ fn test_comm_eintr_handling() {
         .stdout_contains("line2")
         .stdout_contains("line3");
 }
+
+#[test]
+#[cfg(unix)]
+fn test_anonymous_pipes() {
+    let scene = TestScenario::new(util_name!());
+    let at = &scene.fixtures;
+
+    let mut comm1_content = String::new();
+    for i in 0..10000 {
+        comm1_content.push_str(&format!("{i:05}\n"));
+    }
+    at.write("comm1", &comm1_content);
+
+    let mut comm2_content = String::new();
+    for i in 0..9000 {
+        comm2_content.push_str(&format!("{i:05}\n"));
+    }
+    // add one extra item which is only present in comm2
+    comm2_content.push_str("99999\n");
+    at.write("comm2", &comm2_content);
+
+    scene
+        .cmd("bash")
+        .arg("-c")
+        .arg(format!(
+            "{} comm -13 <(cat {}) <(cat {})",
+            scene.bin_path.display(),
+            at.plus("comm1").display(),
+            at.plus("comm2").display()
+        ))
+        .succeeds()
+        .stdout_is("99999\n");
+}