From 87c514d1d14960dd04e74cf74870f64426537424 Mon Sep 17 00:00:00 2001
From: oech3 <79379754+oech3@users.noreply.github.com>
Date: Sat, 4 Apr 2026 00:11:28 +0900
Subject: [PATCH] head: splice fast-path for -c

---
 Cargo.lock                           |  1 +
 src/uu/head/Cargo.toml               |  4 ++
 src/uu/head/src/head.rs              | 77 ++++++++++++++++++++++++++++
 src/uucore/src/lib/features/pipes.rs | 12 +++++
 4 files changed, 94 insertions(+)

diff --git a/Cargo.lock b/Cargo.lock
index 45107199eec..fde0ac97b92 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -3638,6 +3638,7 @@ dependencies = [
  "clap",
  "fluent",
  "memchr",
+ "rustix",
  "thiserror 2.0.18",
  "uucore",
 ]
diff --git a/src/uu/head/Cargo.toml b/src/uu/head/Cargo.toml
index 353404a775a..77bfd32c76e 100644
--- a/src/uu/head/Cargo.toml
+++ b/src/uu/head/Cargo.toml
@@ -30,6 +30,10 @@ uucore = { workspace = true, features = [
 ] }
 fluent = { workspace = true }
 
+[target.'cfg(any(target_os = "linux", target_os = "android"))'.dependencies]
+rustix = { workspace = true, features = ["fs"] }
+uucore = { workspace = true, features = ["pipes"] }
+
 [[bin]]
 name = "head"
 path = "src/main.rs"
diff --git a/src/uu/head/src/head.rs b/src/uu/head/src/head.rs
index 235ea52587b..26c2539d63e 100644
--- a/src/uu/head/src/head.rs
+++ b/src/uu/head/src/head.rs
@@ -166,6 +166,7 @@ fn wrap_in_stdout_error(err: io::Error) -> io::Error {
     )
 }
 
+#[cfg(not(any(target_os = "linux", target_os = "android")))]
 fn read_n_bytes(input: impl Read, n: u64) -> io::Result<u64> {
     // Read the first `n` bytes from the `input` reader.
     let mut reader = input.take(n);
@@ -184,6 +185,81 @@ fn read_n_bytes(input: impl Read, n: u64) -> io::Result<u64> {
     Ok(bytes_written)
 }
 
+// read_n_bytes with zero-copy fast-path
+#[cfg(any(target_os = "linux", target_os = "android"))]
+fn read_n_bytes(input: impl Read + AsFd, n: u64) -> io::Result<u64> {
+    use uucore::pipes::splice;
+    let pipe_size = uucore::pipes::MAX_ROOTLESS_PIPE_SIZE.min(n as usize);
+    let mut stdout = io::stdout();
+    let mut n = n;
+    let mut bytes_written: u64 = 0;
+    // io::copy's internal splice() breaks FUSE <https://github.com/uutils/coreutils/issues/9609#issuecomment-3636584324>
+    // we cannot always fallback to write as it needs 2 Ctrl+D on tty
+    let mut needs_fallback = uucore::pipes::might_fuse(&input);
+    if let Ok(b) = splice(&input, &stdout, n as usize) {
+        bytes_written = b as u64;
+        n -= bytes_written;
+        if n == 0 {
+            // avoid fcntl overhead for small input
+            return Ok(bytes_written);
+        }
+        // improves throughput (expected that input is already extended if it is coming from splice)
+        let _ = rustix::pipe::fcntl_setpipe_size(&stdout, pipe_size);
+        loop {
+            match splice(&input, &stdout, n as usize) {
+                Ok(0) => break,
+                Ok(s @ 1..) => {
+                    n -= s as u64;
+                    bytes_written += s as u64;
+                }
+                _ => {
+                    needs_fallback = true;
+                    break;
+                }
+            }
+        }
+    } else if let Ok((broker_r, broker_w)) = uucore::pipes::pipe_with_size(pipe_size) {
+        // both of in/output are not pipe. needs broker to use splice() with additional cost
+        loop {
+            match splice(&input, &broker_w, n as usize) {
+                Ok(0) => break,
+                Ok(s @ 1..) => {
+                    if uucore::pipes::splice_exact(&broker_r, &stdout, s).is_ok() {
+                        n -= s as u64;
+                        bytes_written += s as u64;
+                    } else {
+                        let mut drain = Vec::with_capacity(s); // bounded by pipe size
+                        broker_r.take(s as u64).read_to_end(&mut drain)?;
+                        stdout.write_all(&drain).map_err(wrap_in_stdout_error)?;
+                        needs_fallback = true;
+                        break;
+                    }
+                }
+                _ => {
+                    needs_fallback = true;
+                    break;
+                }
+            }
+        }
+    }
+
+    if !needs_fallback {
+        return Ok(bytes_written);
+    }
+    let mut reader = input.take(n);
+    let mut stdout = stdout.lock();
+    let mut buf = vec![0u8; (32 * 1024).min(n as usize)]; //use heap to avoid early allocation
+    loop {
+        match reader.read(&mut buf).map_err(wrap_in_stdout_error)? {
+            0 => return Ok(bytes_written),
+            n => {
+                stdout.write_all(&buf[..n]).map_err(wrap_in_stdout_error)?;
+                bytes_written += n as u64;
+            }
+        }
+    }
+}
+
 fn read_n_lines(input: &mut impl io::BufRead, n: u64, separator: u8) -> io::Result<u64> {
     // Read the first `n` lines from the `input` reader.
     let mut reader = take_lines(input, n, separator);
@@ -608,6 +684,7 @@ mod tests {
     }
 
     #[test]
+    #[cfg(not(any(target_os = "linux", target_os = "android")))] // missing trait for rustix::fd::AsFd
     fn read_early_exit() {
         let mut empty = io::BufReader::new(Cursor::new(Vec::new()));
         assert!(read_n_bytes(&mut empty, 0).is_ok());
diff --git a/src/uucore/src/lib/features/pipes.rs b/src/uucore/src/lib/features/pipes.rs
index 24890efca94..c952966cc6a 100644
--- a/src/uucore/src/lib/features/pipes.rs
+++ b/src/uucore/src/lib/features/pipes.rs
@@ -30,6 +30,18 @@ pub fn pipe() -> std::io::Result<(File, File)> {
     Ok((File::from(read), File::from(write)))
 }
 
+#[inline]
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub fn pipe_with_size(s: usize) -> std::io::Result<(File, File)> {
+    let (read, write) = rustix::pipe::pipe()?;
+    const DEFAULT_SIZE: usize = 64 * 1024;
+    if s > DEFAULT_SIZE {
+        let _ = fcntl_setpipe_size(&read, s);
+    }
+
+    Ok((File::from(read), File::from(write)))
+}
+
 /// Less noisy wrapper around [`rustix::pipe::splice`].
 ///
 /// Up to `len` bytes are moved from `source` to `target`. Returns the number