tee: don't treat a short read as end-of-file

src/uu/tee/src/tee.rs

@@ -127,23 +127,32 @@ fn copy(mut input: impl Read, mut output: impl Write) -> Result<usize> {
     };
     let mut buffer = [0u8; FIRST_BUF_SIZE];
     let mut len = 0;
-    match input.read(&mut buffer) {
-        Ok(0) => return Ok(0),
-        Ok(bytes_count) => {
-            output.write_all(&buffer[0..bytes_count])?;
-            len = bytes_count;
-            if bytes_count < FIRST_BUF_SIZE {
+    // Read with the stack buffer until we observe a full-sized read, which
+    // is a hint that the input is big enough to benefit from a larger
+    // buffer. A *short* read does not imply EOF: `read(2)` only returns 0
+    // at end-of-file, and a pipeline whose writer pauses between writes
+    // will commonly return less than the buffer size per call. We must
+    // therefore keep looping until we either see a full-sized read (and
+    // upgrade to the larger buffer) or `read` returns 0.
+    loop {
+        match input.read(&mut buffer) {
+            Ok(0) => return Ok(len), // end of file
+            Ok(bytes_count) => {
+                output.write_all(&buffer[..bytes_count])?;
                 // flush the buffer to comply with POSIX requirement that
                 // `tee` does not buffer the input.
                 output.flush()?;
-                return Ok(len);
+                len += bytes_count;
+                if bytes_count == FIRST_BUF_SIZE {
+                    break;
+                }
             }
+            Err(e) if e.kind() == ErrorKind::Interrupted => {}
+            Err(e) => return Err(e),
         }
-        Err(e) if e.kind() == ErrorKind::Interrupted => (),
-        Err(e) => return Err(e),
     }
 
-    // but optimize buffer size also for large file
+    // Optimize buffer size for large files.
     let mut buffer = vec![0u8; 4 * FIRST_BUF_SIZE]; //stack array makes code path for smaller file slower
     loop {
         match input.read(&mut buffer) {

tests/by-util/test_tee.rs

@@ -191,6 +191,64 @@ fn test_tee_output_not_buffered() {
     handle.join().unwrap();
 }
 
+#[test]
+fn test_tee_continues_after_short_read() {
+    // Regression test: `tee` must keep reading until EOF even when the
+    // first `read(2)` returns fewer bytes than its internal buffer. This
+    // happens in any pipeline where the upstream writer pauses between
+    // writes (e.g. a slow producer, a `sleep` in a shell pipeline, or a
+    // service emitting log lines in bursts). Treating a short read as
+    // end-of-file caused tee to exit prematurely and downstream producers
+    // to die with SIGPIPE on their next write.
+    //
+    // Run in a separate thread so that the test fails via timeout rather
+    // than hanging if a regression reintroduces the bug in a form where
+    // tee blocks on read instead of exiting early.
+    let handle = std::thread::spawn(move || {
+        let (at, mut ucmd) = at_and_ucmd!();
+        let file_out = "tee_short_read_out";
+
+        let mut child = ucmd
+            .arg(file_out)
+            .set_stdin(Stdio::piped())
+            .set_stdout(Stdio::piped())
+            .run_no_wait();
+
+        // First chunk — deliberately much smaller than tee's internal
+        // buffer so that `read(2)` returns a short count.
+        child.write_in(b"first\n");
+        assert_eq!(&child.stdout_exact_bytes(6), b"first\n");
+
+        // Give a buggy implementation time to exit before we try to
+        // write again.
+        child.delay(50);
+
+        // Second chunk. A correctly-implemented tee is still reading
+        // from stdin; a buggy one has already exited and this write
+        // will either fail with EPIPE or never reach the output file.
+        child.write_in(b"second\n");
+        assert_eq!(&child.stdout_exact_bytes(7), b"second\n");
+
+        // `wait` closes stdin for us before waiting on the child.
+        child.wait().unwrap().success();
+
+        assert_eq!(at.read(file_out), "first\nsecond\n");
+    });
+
+    for _ in 0..500 {
+        std::thread::sleep(Duration::from_millis(10));
+        if handle.is_finished() {
+            break;
+        }
+    }
+
+    assert!(
+        handle.is_finished(),
+        "tee did not complete within the timeout"
+    );
+    handle.join().unwrap();
+}
+
 #[cfg(target_os = "linux")]
 mod linux_only {
     use uutests::util::{AtPath, CmdResult, UCommand};