Add os sendfile function and python test

extra_tests/snippets/stdlib_os.py

@@ -17,6 +17,18 @@ assert_raises(FileNotFoundError,
 assert_raises(FileNotFoundError,
               lambda: os.rename('DOES_NOT_EXIST', 'DOES_NOT_EXIST 2'))
 
+src_fd = os.open('README.md', os.O_RDONLY)
+dest_fd = os.open('destination.md', os.O_RDWR | os.O_CREAT)
+src_len = os.stat('README.md').st_size
+
+bytes_sent = os.sendfile(dest_fd, src_fd, 0, src_len)
+assert src_len == bytes_sent
+
+os.lseek(dest_fd, 0, 0)
+assert os.read(src_fd, src_len) == os.read(dest_fd, bytes_sent)
+os.close(src_fd)
+os.close(dest_fd)
+
 try:
     os.open('DOES_NOT_EXIST', 0)
 except OSError as err:

vm/src/stdlib/os.rs

@@ -337,6 +337,47 @@ mod _os {
         Err(vm.new_os_error("os.open not implemented on this platform".to_owned()))
     }
 
+    #[cfg(any(target_os = "linux"))]
+    #[pyfunction]
+    pub(crate) fn sendfile(
+        out_fd: i32,
+        in_fd: i32,
+        offset: i64,
+        count: u64,
+        vm: &VirtualMachine,
+    ) -> PyResult {
+        let mut file_offset = offset;
+
+        let res =
+            nix::sys::sendfile::sendfile(out_fd, in_fd, Some(&mut file_offset), count as usize)
+                .map_err(|err| err.into_pyexception(vm))?;
+        Ok(vm.ctx.new_int(res as u64))
+    }
+
+    #[cfg(any(target_os = "macos"))]
+    #[pyfunction]
+    pub(crate) fn sendfile(
+        out_fd: i64,
+        in_fd: i64,
+        offset: u64,
+        count: u64,
+        headers: OptionalArg<PyObjectRef>,
+        trailers: OptionalArg<PyObjectRef>,
+        flags: OptionalArg<i64>,
+        vm: &VirtualMachine,
+    ) -> PyResult {
+        let res = nix::sys::sendfile::sendfile(
+            in_fd,
+            out_fd,
+            offset,
+            Some(count as usize),
+            headers,
+            trailers,
+        )
+        .map_err(|err| err.into_pyexception(vm))?;
+        Ok(vm.ctx.new_int(res as u64))
+    }
+
     #[pyfunction]
     fn error(message: OptionalArg<PyStrRef>, vm: &VirtualMachine) -> PyResult {
         let msg = message.map_or("".to_owned(), |msg| msg.borrow_value().to_owned());