do direct splice syscall and probe availability to get android builds to work

Android builds use feature level 14, the libc wrapper for splice is gated
on feature level 21+ so we have to invoke the syscall directly.
Additionally the emulator doesn't seem to support it so we also have to
add ENOSYS checks.

library/std/src/sys/unix/kernel_copy.rs

@@ -58,6 +58,7 @@ use crate::os::unix::fs::FileTypeExt;
 use crate::os::unix::io::{AsRawFd, FromRawFd, RawFd};
 use crate::process::{ChildStderr, ChildStdin, ChildStdout};
 use crate::ptr;
+use crate::sync::atomic::{AtomicBool, Ordering};
 use crate::sys::cvt;
 
 #[cfg(test)]
@@ -440,7 +441,6 @@ pub(super) enum CopyResult {
 /// If the initial file offset was 0 then `Fallback` will only contain `0`.
 pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) -> CopyResult {
     use crate::cmp;
-    use crate::sync::atomic::{AtomicBool, Ordering};
 
     // Kernel prior to 4.5 don't have copy_file_range
     // We store the availability in a global to avoid unnecessary syscalls
@@ -534,6 +534,30 @@ enum SpliceMode {
 /// performs splice or sendfile between file descriptors
 /// Does _not_ fall back to a generic copy loop.
 fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) -> CopyResult {
+    static HAS_SENDFILE: AtomicBool = AtomicBool::new(true);
+    static HAS_SPLICE: AtomicBool = AtomicBool::new(true);
+
+    syscall! {
+        fn splice(
+            srcfd: libc::c_int,
+            src_offset: *const i64,
+            dstfd: libc::c_int,
+            dst_offset: *const i64,
+            len: libc::size_t,
+            flags: libc::c_int
+        ) -> libc::ssize_t
+    }
+
+    match mode {
+        SpliceMode::Sendfile if !HAS_SENDFILE.load(Ordering::Relaxed) => {
+            return CopyResult::Fallback(0);
+        }
+        SpliceMode::Splice if !HAS_SPLICE.load(Ordering::Relaxed) => {
+            return CopyResult::Fallback(0);
+        }
+        _ => (),
+    }
+
     let mut written = 0u64;
     while written < len {
         let chunk_size = crate::cmp::min(len - written, 0x7ffff000_u64) as usize;
@@ -543,7 +567,7 @@ fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) ->
                 cvt(unsafe { libc::sendfile(writer, reader, ptr::null_mut(), chunk_size) })
             }
             SpliceMode::Splice => cvt(unsafe {
-                libc::splice(reader, ptr::null_mut(), writer, ptr::null_mut(), chunk_size, 0)
+                splice(reader, ptr::null_mut(), writer, ptr::null_mut(), chunk_size, 0)
             }),
         };
 
@@ -552,8 +576,18 @@ fn sendfile_splice(mode: SpliceMode, reader: RawFd, writer: RawFd, len: u64) ->
             Ok(ret) => written += ret as u64,
             Err(err) => {
                 return match err.raw_os_error() {
-                    Some(os_err) if os_err == libc::EINVAL => {
-                        // splice/sendfile do not support this particular file descritor (EINVAL)
+                    Some(libc::ENOSYS | libc::EPERM) => {
+                        // syscall not supported (ENOSYS)
+                        // syscall is disallowed, e.g. by seccomp (EPERM)
+                        match mode {
+                            SpliceMode::Sendfile => HAS_SENDFILE.store(false, Ordering::Relaxed),
+                            SpliceMode::Splice => HAS_SPLICE.store(false, Ordering::Relaxed),
+                        }
+                        assert_eq!(written, 0);
+                        CopyResult::Fallback(0)
+                    }
+                    Some(libc::EINVAL) => {
+                        // splice/sendfile do not support this particular file descriptor (EINVAL)
                         assert_eq!(written, 0);
                         CopyResult::Fallback(0)
                     }

library/std/src/sys/unix/kernel_copy/tests.rs

@@ -121,6 +121,7 @@ fn bench_file_to_socket_copy(b: &mut test::Bencher) {
 #[cfg(any(target_os = "linux", target_os = "android"))]
 #[bench]
 fn bench_socket_pipe_socket_copy(b: &mut test::Bencher) {
+    use super::CopyResult;
     use crate::io::ErrorKind;
     use crate::process::{ChildStdin, ChildStdout};
     use crate::sys_common::FromInner;
@@ -135,6 +136,21 @@ fn bench_socket_pipe_socket_copy(b: &mut test::Bencher) {
 
     let local_end = crate::sync::Arc::new(acceptor.accept().unwrap().0);
 
+    // the data flow in this benchmark:
+    //
+    //                      socket(tx)  local_source
+    // remote_end (write)  +-------->   (splice to)
+    //                                  write_end
+    //                                     +
+    //                                     |
+    //                                     | pipe
+    //                                     v
+    //                                  read_end
+    // remote_end (read)   <---------+  (splice to) *
+    //                      socket(rx)  local_end
+    //
+    // * benchmark loop using io::copy
+
     crate::thread::spawn(move || {
         let mut sink_buf = vec![0u8; 1024 * 1024];
         remote_end.set_nonblocking(true).unwrap();
@@ -156,6 +172,24 @@ fn bench_socket_pipe_socket_copy(b: &mut test::Bencher) {
         }
     });
 
+    // check that splice works, otherwise the benchmark would hang
+    let probe = super::sendfile_splice(
+        super::SpliceMode::Splice,
+        local_end.as_raw_fd(),
+        write_end.as_raw_fd(),
+        1,
+    );
+
+    match probe {
+        CopyResult::Ended(Ok(1)) => {
+            // splice works
+        }
+        _ => {
+            eprintln!("splice failed, skipping benchmark");
+            return;
+        }
+    }
+
     let local_source = local_end.clone();
     crate::thread::spawn(move || {
         loop {