bpo-41625: Expose the splice() system call in the os module (GH-21947)

Doc/library/os.rst

@@ -1419,6 +1419,38 @@ or `the MSDN <https://msdn.microsoft.com/en-us/library/z0kc8e3z.aspx>`_ on Windo
    .. versionadded:: 3.3
 
 
+.. function:: splice(src, dst, count, offset_src=None, offset_dst=None)
+
+   Transfer *count* bytes from file descriptor *src*, starting from offset
+   *offset_src*, to file descriptor *dst*, starting from offset *offset_dst*.
+   At least one of the file descriptors must refer to a pipe. If *offset_src*
+   is None, then *src* is read from the current position; respectively for
+   *offset_dst*. The offset associated to the file descriptor that refers to a
+   pipe must be ``None``. The files pointed by *src* and *dst* must reside in
+   the same filesystem, otherwise an :exc:`OSError` is raised with
+   :attr:`~OSError.errno` set to :data:`errno.EXDEV`.
+
+   This copy is done without the additional cost of transferring data
+   from the kernel to user space and then back into the kernel. Additionally,
+   some filesystems could implement extra optimizations. The copy is done as if
+   both files are opened as binary.
+
+   Upon successful completion, returns the number of bytes spliced to or from
+   the pipe. A return value of 0 means end of input. If *src* refers to a
+   pipe, then this means that there was no data to transfer, and it would not
+   make sense to block because there are no writers connected to the write end
+   of the pipe.
+
+   .. availability:: Linux kernel >= 2.6.17 or glibc >= 2.5
+
+   .. versionadded:: 3.10
+
+
+.. data:: SPLICE_F_MOVE
+          SPLICE_F_NONBLOCK
+          SPLICE_F_MORE
+
+
 .. function:: readv(fd, buffers)
 
    Read from a file descriptor *fd* into a number of mutable :term:`bytes-like