fix #14574, cp on files >2GB (#30989)

(cherry picked from commit 05785f9)

Author: JeffBezanson

base/filesystem.jl

@@ -102,13 +102,19 @@ function close(f::File)
     return nothing
 end
 
-# sendfile is the most efficient way to copy a file (or any file descriptor)
+# sendfile is the most efficient way to copy from a file descriptor
 function sendfile(dst::File, src::File, src_offset::Int64, bytes::Int)
     check_open(dst)
     check_open(src)
-    err = ccall(:jl_fs_sendfile, Int32, (OS_HANDLE, OS_HANDLE, Int64, Csize_t),
-                src.handle, dst.handle, src_offset, bytes)
-    uv_error("sendfile", err)
+    while true
+        result = ccall(:jl_fs_sendfile, Int32, (OS_HANDLE, OS_HANDLE, Int64, Csize_t),
+                       src.handle, dst.handle, src_offset, bytes)
+        uv_error("sendfile", result)
+        nsent = result
+        bytes -= nsent
+        src_offset += nsent
+        bytes <= 0 && break
+    end
     nothing
 end