perform copy_file_range until EOF is reached instead of basing things on file size

This solves several problems

- race conditions where a file is truncated while copying from it. if we blindly trusted
  the file size this would lead to an infinite loop
- proc files appearing empty to copy_file_range but not to read/write
  https://github.com/coreutils/coreutils/commit/4b04a0c
- copy_file_range returning 0 for some filesystems (overlay? bind mounts?)
  inside docker, again leading to an infinite loop

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

@@ -1140,14 +1140,14 @@ unsafe fn copy_file_range(
     }
 
     let (mut reader, reader_metadata) = open_from(from)?;
-    let len = reader_metadata.len();
+    let max_len = u64::MAX;
     let (mut writer, _) = open_to_and_set_permissions(to, reader_metadata)?;
 
     let has_copy_file_range = HAS_COPY_FILE_RANGE.load(Ordering::Relaxed);
     let mut written = 0u64;
-    while written < len {
+    while written < max_len {
         let copy_result = if has_copy_file_range {
-            let bytes_to_copy = cmp::min(len - written, usize::MAX as u64) as usize;
+            let bytes_to_copy = cmp::min(max_len - written, usize::MAX as u64) as usize;
             let copy_result = unsafe {
                 // We actually don't have to adjust the offsets,
                 // because copy_file_range adjusts the file offset automatically
@@ -1173,6 +1173,15 @@ unsafe fn copy_file_range(
             Err(io::Error::from_raw_os_error(libc::ENOSYS))
         };
         match copy_result {
+            Ok(0) if written == 0 => {
+                // fallback to work around several kernel bugs where copy_file_range will fail to
+                // copy any bytes and return 0 instead of an error if
+                // - reading virtual files from the proc filesystem which appear to have 0 size
+                //   but are not empty. noted in coreutils to affect kernels at least up to 5.6.19.
+                // - copying from an overlay filesystem in docker. reported to occur on fedora 32.
+                return io::copy(&mut reader, &mut writer);
+            }
+            Ok(0) => return Ok(written), // reached EOF
             Ok(ret) => written += ret as u64,
             Err(err) => {
                 match err.raw_os_error() {