Lift the probe code of copy_file_range into a function

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

@@ -577,6 +577,23 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
         ) -> libc::ssize_t
     }
 
+    fn probe_copy_file_range_support() -> u8 {
+        // In some cases, we cannot determine availability from the first
+        // `copy_file_range` call. In this case, we probe with an invalid file
+        // descriptor so that the results are easily interpretable.
+        match unsafe {
+            cvt(copy_file_range(INVALID_FD, ptr::null_mut(), INVALID_FD, ptr::null_mut(), 1, 0))
+                .map_err(|e| e.raw_os_error())
+        } {
+            Err(Some(EPERM | ENOSYS)) => UNAVAILABLE,
+            Err(Some(EBADF)) => AVAILABLE,
+            Ok(_) => panic!("unexpected copy_file_range probe success"),
+            // Treat other errors as the syscall
+            // being unavailable.
+            Err(_) => UNAVAILABLE,
+        }
+    }
+
     let mut written = 0u64;
     while written < max_len {
         let bytes_to_copy = cmp::min(max_len - written, usize::MAX as u64);
@@ -614,35 +631,20 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
                         if written == 0 =>
                     {
                         if !have_probed {
-                            let available = match raw_os_error {
-                                EPERM => {
-                                    // EPERM can indicate seccomp filters or an
-                                    // immutable file. To distinguish these
-                                    // cases we probe with invalid file
-                                    // descriptors which should result in EBADF
-                                    // if the syscall is supported and EPERM or
-                                    // ENOSYS if it's not available.
-                                    match unsafe {
-                                        cvt(copy_file_range(
-                                            INVALID_FD,
-                                            ptr::null_mut(),
-                                            INVALID_FD,
-                                            ptr::null_mut(),
-                                            1,
-                                            0,
-                                        ))
-                                        .map_err(|e| e.raw_os_error())
-                                    } {
-                                        Err(Some(EPERM | ENOSYS)) => UNAVAILABLE,
-                                        Err(Some(EBADF)) => AVAILABLE,
-                                        Ok(_) => panic!("unexpected copy_file_range probe success"),
-                                        // Treat other errors as the syscall
-                                        // being unavailable.
-                                        Err(_) => UNAVAILABLE,
-                                    }
-                                }
-                                ENOSYS => UNAVAILABLE,
-                                _ => AVAILABLE,
+                            let available = if matches!(raw_os_error, ENOSYS | EOPNOTSUPP | EPERM) {
+                                // EPERM can indicate seccomp filters or an
+                                // immutable file. To distinguish these
+                                // cases we probe with invalid file
+                                // descriptors which should result in EBADF
+                                // if the syscall is supported and EPERM or
+                                // ENOSYS if it's not available.
+                                //
+                                // For EOPNOTSUPP, see below. In the case of
+                                // ENOSYS, we try to cover for faulty FUSE
+                                // drivers.
+                                probe_copy_file_range_support()
+                            } else {
+                                AVAILABLE
                             };
                             HAS_COPY_FILE_RANGE.store(available, Ordering::Relaxed);
                         }