tee: improve throughput by raw syscall

src/uu/tee/Cargo.toml

@@ -24,6 +24,9 @@ clap = { workspace = true }
 uucore = { workspace = true, features = ["libc", "parser", "signals"] }
 fluent = { workspace = true }
 
+[target.'cfg(unix)'.dependencies]
+rustix = { workspace = true }
+
 [dev-dependencies]
 divan = { workspace = true }
 rustix = { workspace = true, features = ["stdio", "fs"] }

src/uu/tee/src/tee.rs

@@ -197,21 +197,16 @@ impl MultiWriter {
 
     fn write_flush(&mut self, buf: &[u8]) -> Result<()> {
         let mode = self.output_error_mode;
-        self.writers.retain_mut(|writer| {
-            let res = (|| {
-                writer.inner.write_all(buf)?;
-                writer.inner.flush()
-            })();
-            match res {
+        self.writers
+            .retain_mut(|writer| match writer.inner.write_all(buf) {
                 Ok(()) => true,
                 Err(e) => {
                     if let Err(e) = process_error(mode, e, writer, &mut self.ignored_errors) {
                         self.aborted.get_or_insert(e);
                     }
                     false
                 }
-            }
-        });
+            });
         self.aborted.take().map_or(
             if self.writers.is_empty() {
                 // This error kind will never be raised by the standard
@@ -254,18 +249,34 @@ enum Writer {
     Stdout(std::io::Stdout),
 }
 
+// raw syscall avoids buffering which is POSIX requirement
+// throughput is better than flush after write_all by unknown reason
+// since there is no rustix::io::write_all, we remove buffering by impl from write_all
+#[cfg(unix)]
 impl Write for Writer {
     fn write(&mut self, buf: &[u8]) -> Result<usize> {
         match self {
-            Self::File(f) => f.write(buf),
-            Self::Stdout(s) => s.write(buf),
+            Self::File(f) => Ok(rustix::io::write(f, buf)?),
+            Self::Stdout(s) => Ok(rustix::io::write(s, buf)?),
         }
     }
 
     fn flush(&mut self) -> Result<()> {
+        Ok(())
+    }
+}
+
+#[cfg(not(unix))]
+impl Writer {
+    // override write_all to avoid calling many flush
+    pub fn write_all(&mut self, buf: &[u8]) -> Result<()> {
         match self {
-            Self::File(f) => f.flush(),
-            Self::Stdout(s) => s.flush(),
+            // File does not have line buffering
+            Self::File(f) => f.write_all(buf),
+            Self::Stdout(s) => {
+                s.write_all(buf)?;
+                s.flush() // todo: investigate how to remove flush overhead
+            }
         }
     }
 }