Auto merge of #122079 - tbu-:pr_copy_file_range_probe, r=the8472

Less syscalls for the `copy_file_range` probe

If it's obvious from the actual syscall results themselves that the syscall is supported or unsupported, don't do an extra syscall with an invalid file descriptor.

CC #122052

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

@@ -560,6 +560,12 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
     // We store the availability in a global to avoid unnecessary syscalls
     static HAS_COPY_FILE_RANGE: AtomicU8 = AtomicU8::new(NOT_PROBED);
 
+    let mut have_probed = match HAS_COPY_FILE_RANGE.load(Ordering::Relaxed) {
+        NOT_PROBED => false,
+        UNAVAILABLE => return CopyResult::Fallback(0),
+        _ => true,
+    };
+
     syscall! {
         fn copy_file_range(
             fd_in: libc::c_int,
@@ -571,25 +577,22 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
         ) -> libc::ssize_t
     }
 
-    match HAS_COPY_FILE_RANGE.load(Ordering::Relaxed) {
-        NOT_PROBED => {
-            // 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 some other error (ENOSYS or EPERM) if it's not available
-            let result = unsafe {
-                cvt(copy_file_range(INVALID_FD, ptr::null_mut(), INVALID_FD, ptr::null_mut(), 1, 0))
-            };
-
-            if matches!(result.map_err(|e| e.raw_os_error()), Err(Some(EBADF))) {
-                HAS_COPY_FILE_RANGE.store(AVAILABLE, Ordering::Relaxed);
-            } else {
-                HAS_COPY_FILE_RANGE.store(UNAVAILABLE, Ordering::Relaxed);
-                return CopyResult::Fallback(0);
-            }
+    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,
         }
-        UNAVAILABLE => return CopyResult::Fallback(0),
-        _ => {}
-    };
+    }
 
     let mut written = 0u64;
     while written < max_len {
@@ -604,6 +607,11 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
             cvt(copy_file_range(reader, ptr::null_mut(), writer, ptr::null_mut(), bytes_to_copy, 0))
         };
 
+        if !have_probed && copy_result.is_ok() {
+            have_probed = true;
+            HAS_COPY_FILE_RANGE.store(AVAILABLE, Ordering::Relaxed);
+        }
+
         match copy_result {
             Ok(0) if written == 0 => {
                 // fallback to work around several kernel bugs where copy_file_range will fail to
@@ -619,7 +627,28 @@ pub(super) fn copy_regular_files(reader: RawFd, writer: RawFd, max_len: u64) ->
                 return match err.raw_os_error() {
                     // when file offset + max_length > u64::MAX
                     Some(EOVERFLOW) => CopyResult::Fallback(written),
-                    Some(ENOSYS | EXDEV | EINVAL | EPERM | EOPNOTSUPP | EBADF) if written == 0 => {
+                    Some(raw_os_error @ (ENOSYS | EXDEV | EINVAL | EPERM | EOPNOTSUPP | EBADF))
+                        if written == 0 =>
+                    {
+                        if !have_probed {
+                            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);
+                        }
+
                         // Try fallback io::copy if either:
                         // - Kernel version is < 4.5 (ENOSYS¹)
                         // - Files are mounted on different fs (EXDEV)