uucore: add a function returning /dev/null to use splice() for wc,dd,tail

Author: oech3

src/uu/wc/src/count_fast.rs

@@ -9,8 +9,6 @@ use uucore::hardware::SimdPolicy;
 
 use super::WordCountable;
 
-#[cfg(any(target_os = "linux", target_os = "android"))]
-use std::fs::OpenOptions;
 use std::io::{self, ErrorKind, Read};
 
 #[cfg(unix)]
@@ -41,18 +39,7 @@ const BUF_SIZE: usize = 256 * 1024;
 #[inline]
 #[cfg(any(target_os = "linux", target_os = "android"))]
 fn count_bytes_using_splice(fd: &impl AsFd) -> Result<usize, usize> {
-    let null_file = OpenOptions::new()
-        .write(true)
-        .open("/dev/null")
-        .map_err(|_| 0_usize)?;
-    let null_rdev = rustix::fs::fstat(null_file.as_fd())
-        .map_err(|_| 0_usize)?
-        .st_rdev as libc::dev_t;
-    if (libc::major(null_rdev), libc::minor(null_rdev)) != (1, 3) {
-        // This is not a proper /dev/null, writing to it is probably bad
-        // Bit of an edge case, but it has been known to happen
-        return Err(0);
-    }
+    let null_file = uucore::pipes::dev_null().ok_or(0_usize)?;
     // todo: avoid generating broker if input is pipe (fcntl_setpipe_size succeed) and directly splice() to /dev/null to save RAM usage
     let (pipe_rd, pipe_wr) = pipe().map_err(|_| 0_usize)?;
 

src/uucore/src/lib/features/pipes.rs

@@ -18,6 +18,7 @@ pub const MAX_ROOTLESS_PIPE_SIZE: usize = 1024 * 1024;
 /// Returns two `File` objects: everything written to the second can be read
 /// from the first.
 /// This is used only for resolving the limitation for splice: one of a input or output should be pipe
+#[inline]
 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub fn pipe() -> std::io::Result<(File, File)> {
     let (read, write) = rustix::pipe::pipe()?;
@@ -36,6 +37,7 @@ pub fn pipe() -> std::io::Result<(File, File)> {
 /// To get around this requirement, consider splicing from your source into
 /// a [`pipe`] and then from the pipe into your target (with `splice_exact`):
 /// this is still very efficient.
+#[inline]
 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub fn splice(source: &impl AsFd, target: &impl AsFd, len: usize) -> std::io::Result<usize> {
     Ok(rustix::pipe::splice(
@@ -53,6 +55,7 @@ pub fn splice(source: &impl AsFd, target: &impl AsFd, len: usize) -> std::io::Re
 /// Exactly `len` bytes are moved from `source` into `target`.
 ///
 /// Panics if `source` runs out of data before `len` bytes have been moved.
+#[inline]
 #[cfg(any(target_os = "linux", target_os = "android"))]
 pub fn splice_exact(source: &impl AsFd, target: &impl AsFd, len: usize) -> std::io::Result<()> {
     let mut left = len;
@@ -63,3 +66,22 @@ pub fn splice_exact(source: &impl AsFd, target: &impl AsFd, len: usize) -> std::
     }
     Ok(())
 }
+
+/// Return verified /dev/null
+///
+/// `splice` to /dev/null is faster than `read` when we skip or count the input which is not able to seek
+#[inline]
+#[cfg(any(target_os = "linux", target_os = "android"))]
+pub fn dev_null() -> Option<File> {
+    let null = std::fs::OpenOptions::new()
+        .write(true)
+        .open("/dev/null")
+        .ok()?;
+    let stat = rustix::fs::fstat(&null).ok()?;
+    let dev = stat.st_rdev;
+    if (rustix::fs::major(dev), rustix::fs::minor(dev)) == (1, 3) {
+        Some(null)
+    } else {
+        None
+    }
+}