internal/poll: drop redundant ENOSYS in CopyFileRange

Update CL 425881 and CL 428396

I browsed the source code related to copy_file_range in the kernel and found that the latest kernel may still return EXDEV errors in copy_file_range(2) due to certain cases, for details see: https://elixir.bootlin.com/linux/v5.19.7/source/fs/read_write.c#L1559, https://elixir.bootlin.com/linux/v5.19.7/source/fs/read_write.c#L1479, and
https://elixir.bootlin.com/linux/v5.19.7/source/fs/read_write.c#L1439.

Therefore, the EXDEV still needs to be kept, but the ENOSYS error can be safely removed.

Change-Id: I47026b8dd33f7ffc4de1306af6b67c7b4d2062d2
Reviewed-on: https://go-review.googlesource.com/c/go/+/428555
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Andy Pan <panjf2000@gmail.com>

diff --git a/src/internal/poll/copy_file_range_linux.go b/src/internal/poll/copy_file_range_linux.go
index ba33f5145d4896cb34bfe1a14d34fba35e727669..66408e459040b506a66cbab6cd6dfa5a6ac5f1a0 100644 (file)
--- a/src/internal/poll/copy_file_range_linux.go
+++ b/src/internal/poll/copy_file_range_linux.go
@@ -41,22 +41,18 @@ func CopyFileRange(dst, src *FD, remain int64) (written int64, handled bool, err
                }
                n, err := copyFileRange(dst, src, int(max))
                switch err {
-               case syscall.ENOSYS:
-                       // copy_file_range(2) was introduced in Linux 4.5.
-                       // Go supports Linux >= 2.6.33, so the system call
-                       // may not be present.
-                       //
-                       // If we see ENOSYS, we have certainly not transferred
-                       // any data, so we can tell the caller that we
-                       // couldn't handle the transfer and let them fall
-                       // back to more generic code.
-                       return 0, false, nil
                case syscall.EXDEV, syscall.EINVAL, syscall.EIO, syscall.EOPNOTSUPP, syscall.EPERM:
                        // Prior to Linux 5.3, it was not possible to
-                       // copy_file_range across file systems. Similarly to
-                       // the ENOSYS case above, if we see EXDEV, we have
-                       // not transferred any data, and we can let the caller
-                       // fall back to generic code.
+                       // copy_file_range across file systems. An attempt
+                       // to do this will result in a EXDEV error.
+                       //
+                       // Even though we have checked the kernel version and blocked
+                       // the attempts to copy_file_range(2) when the kernel version
+                       // is older than 5.3, but until now the latest kernel (5.19.x)
+                       // may still return EXDEV error in certain cases.
+                       //
+                       // If we see EXDEV, we have not transferred any data,
+                       // and we can let the caller fall back to generic code.
                        //
                        // As for EINVAL, that is what we see if, for example,
                        // dst or src refer to a pipe rather than a regular