Pull header reading code into libdwP.h

[Mark Wielaard]

Hi Timm,

On Tue, 2021-02-09 at 12:45 +0100, Timm Bäder via Elfutils-devel wrote:
> There is quite a bit of code duplication between src/readelf.c and
> libdw/dwarf_getsrclines.c when parsing header_length, unit_length,
> etc.
> 
> Pull the code out into libdwP.h and use it in both of those files.
> 
> It doesn't save a much code as I'd hoped and I'm not sure about the
> naming, but here's the patch.

BTW. In these cases I really like having a ChangeLog entry to guide
review.

I think the struct should be named line_header_state.
And the functions __libdw_line_header_state_parse and
__libdw_line_header_get_increment to make clear they are internal
__libdw functions.

I think it would be nicer if __libdw_line_header_state_parse returned a
bool (true on success, false on failure) and set any error itself using
__libdw_seterrno when returning false. This might give read_srclines a
more specific error and the readelf.c code can then add dwarf_errmsg (-
1) to its error message.

This changes the readelf case a bit. It used to print the header even
if some of the fields (version, address_size, segment_selector_size)
weren't supported. It might be good if __libdw_line_header_state_parse
would just set the values in the line_state_header and let the caller
decide what to do with them.

Cheers,

Mark