yes: Cleanup

Report errors properly instead of panicking.

Replace zero_copy by a simpler specialized private module.

Do not assume splices move all data at once.

Use the modern uutils machinery.

Remove the "latency" feature. The time it takes to prepare the buffer
is drowned out by the startup time anyway.

yes: Add tests

yes: Fix long input test on Windows

tests/by-util/test_yes.rs

@@ -1 +1,72 @@
-// ToDO: add tests
+use std::io::Read;
+
+use crate::common::util::*;
+
+/// Run `yes`, capture some of the output, close the pipe, and verify it.
+fn run(args: &[&str], expected: &[u8]) {
+    let mut cmd = new_ucmd!();
+    let mut child = cmd.args(args).run_no_wait();
+    let mut stdout = child.stdout.take().unwrap();
+    let mut buf = vec![0; expected.len()];
+    stdout.read_exact(&mut buf).unwrap();
+    drop(stdout);
+    assert!(child.wait().unwrap().success());
+    assert_eq!(buf.as_slice(), expected);
+}
+
+#[test]
+fn test_simple() {
+    run(&[], b"y\ny\ny\ny\n");
+}
+
+#[test]
+fn test_args() {
+    run(&["a", "bar", "c"], b"a bar c\na bar c\na ba");
+}
+
+#[test]
+fn test_long_output() {
+    run(&[], "y\n".repeat(512 * 1024).as_bytes());
+}
+
+/// Test with an output that seems likely to get mangled in case of incomplete writes.
+#[test]
+fn test_long_odd_output() {
+    run(&["abcdef"], "abcdef\n".repeat(1024 * 1024).as_bytes());
+}
+
+/// Test with an input that doesn't fit in the standard buffer.
+#[test]
+fn test_long_input() {
+    #[cfg(not(windows))]
+    const TIMES: usize = 14000;
+    // On Windows the command line is limited to 8191 bytes.
+    // This is not actually enough to fill the buffer, but it's still nice to
+    // try something long.
+    #[cfg(windows)]
+    const TIMES: usize = 500;
+    let arg = "abcdefg".repeat(TIMES) + "\n";
+    let expected_out = arg.repeat(30);
+    run(&[&arg[..arg.len() - 1]], expected_out.as_bytes());
+}
+
+#[test]
+#[cfg(any(target_os = "linux", target_os = "freebsd", target_os = "netbsd"))]
+fn test_piped_to_dev_full() {
+    use std::fs::OpenOptions;
+
+    for &append in &[true, false] {
+        {
+            let dev_full = OpenOptions::new()
+                .write(true)
+                .append(append)
+                .open("/dev/full")
+                .unwrap();
+
+            new_ucmd!()
+                .set_stdout(dev_full)
+                .fails()
+                .stderr_contains("No space left on device");
+        }
+    }
+}