Dissector support for string_string mapping in header_field_info

[John Zwinck <jzwinck () gmail com>]

As you may know, the "strings" member of struct header_field_info can be set
to an array of value_strings which are a key/value pair of uint32 to
string.  This allows a Wireshark dissector to specify how to display values
which have an enumerated meaning.  This works when the protocol field
containing enumerated values is intrinsically a number.  But what about when
it is a string?  Some protocols (e.g. FIX) have text fields that are
enumerated values (e.g. MsgType, which in FIX can be things like "8" or "D"
or "AB").  Note that Wireshark already supports FIX, but the dissector code
calls str_to_str() and then sets the display text explicitly.  Since the
type string_string already exists in Wireshark, why not support its use in
header_field_info?

I am fairly new to Wireshark, but this seems to be an area where its support
could easily be made stronger, so I would like to know others' opinions, or
if it has been tried or rejected already.  Looking at the source makes me
think that it would involve a small patch to proto.c to call str_to_str()
for FT_STRING and FT_STRINGZ if hf_field->strings is set, and call the
existing str_to_str() function if so (this would parallel the existing calls
to val_to_str() and rval_to_str()).  There would need to be a little support
added for Lua too (which is where I actually want to use this facility).

The end goal is to improve consistency (at least in Lua it looks very odd
that mapping is supported for ints but not strings), and to reduce the need
for dissector developers to call the lower-level routines to format their
text output when dealing with fields like these.

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