[pulseaudio-discuss] [PATCH 6/8] message-params: Allow parameter strings to contain escaped curly braces

Fri Jul 27 08:08:25 UTC 2018

On Thu, 2018-07-26 at 18:02 +0200, Georg Chini wrote:
> On 26.07.2018 12:37, Tanu Kaskinen wrote:
> > On Sun, 2018-07-22 at 21:11 +0200, Georg Chini wrote:
> > > On 22.07.2018 17:48, Tanu Kaskinen wrote:
> > > > On Sun, 2018-07-22 at 16:02 +0200, Georg Chini wrote:
> > > > > On 21.07.2018 20:17, Tanu Kaskinen wrote:
> > > > > > On Mon, 2018-04-09 at 19:35 +0200, Georg Chini wrote:
> > > > > > > The patch adds the possibility to escape curly braces within parameter strings
> > > > > > > and introduces several new functions that can be used for writing parameters.
> > > > > > > 
> > > > > > > For writing, the structure pa_message_param, which is a wrapper for pa_strbuf
> > > > > > > has been created. Following new write functions are available:
> > > > > > > 
> > > > > > > pa_message_param_new() - creates a new pa_message_param structure
> > > > > > > pa_message_param_to_string() - converts a pa_message_param to string and frees
> > > > > > > the structure
> > > > > > > The function pa_message_param_write_string()
> > > > > > > has a parameter do_escape.
> > > > > > 
> > > > > > Why not do escaping always?
> > > > > 
> > > > > Because what you are writing as a string may be a list that you have
> > > > > prepared
> > > > > previously. Then you will not want escaping. You may for example create
> > > > > a list
> > > > > from an array and then insert this list as one string into the final
> > > > > parameter list
> > > > > or have a function that converts a certain structure to a parameter
> > > > > string and
> > > > > then write the result of this function as one element of the final list.
> > > > 
> > > > My mental model is that parameters have types, list type is different
> > > > than string type, and write_string() is only meant for writing values
> > > > of the string type.
> > > > 
> > > > Can you add a write_raw() function?
> > > 
> > > Yes, this is done in patch 7. But the raw write function differs from what
> > > write_string() is doing. write_string() writes one element of a list,
> > > that is
> > > it encloses the string in braces. The raw write function is intended for
> > > situations where you can't write a complete element with one write, so
> > > it does not add any braces. I am still of the opinion, that a structure
> > > or array converted to a parameter string is a string, so writing something
> > > like this should be done with write_string().
> > 
> > They are different kinds of strings, different abstraction levels. When
> > you're writing an array "as a string", in that context it's just a C
> > string. write_string() with escaping deals with strings in the "message
> > params type system". I don't know if this makes any sense to you.
> > Probably not... In any case, the do_escape flag seems unnecessary
> > complexity to me.
> 
> The alternative would be a function to write an unescaped string in
> addition to the write_raw() function. If you don't like the flag, would
> you be OK with a write_unescaped_string() function? I think it is just
> more comfortable than using write_raw().

Thanks for the concession, I was afraid we'll get stuck on this point.

By comfortable, do you refer to that write_raw() doesn't add braces
around the value? How about adding an add_braces flag to write_raw()?

> > > > > > > +
> > > > > > > +/* Read functions */
> > > > > > > +
> > > > > > >     /* Split the specified string into elements. An element is defined as
> > > > > > >      * a sub-string between curly braces. The function is needed to parse
> > > > > > >      * the parameters of messages. Each time it is called returns the position
> > > > > > >      * of the current element in result and the state pointer is advanced to
> > > > > > > - * the next list element.
> > > > > > > + * the next list element. On return, the parameter *is_unpacked indicates
> > > > > > > + * if the string is plain text or contains a sub-list. is_unpacked may
> > > > > > > + * be NULL.
> > > > > > 
> > > > > > is_unpacked looks like unnecessary complexity.
> > > > > > pa_message_params_read_string() should always unescape the value.
> > > > > 
> > > > > It may be possible, that the string you read is a list. Consider the
> > > > > following
> > > > > parameter list: {string1}{some nested structure}{string2}. You can now
> > > > > read this list as three strings and then continue to read the elements of
> > > > > the nested structure from the second string. You might even create a
> > > > > function
> > > > > that takes a string and outputs a structure. So you are not forced to go
> > > > > to the full depth of nesting on the first pass. This makes it much easier
> > > > > to handle deeply nested parameter lists. For me this behavior is an
> > > > > important
> > > > > feature and I do not want to drop it. See also my comment on why I do
> > > > > not always want escaping.
> > > > 
> > > > Doesn't split_list() already allow this, why do you want to use
> > > > read_string() to do the same thing as split_list()?
> > > 
> > > read_string() and split_list() are very similar and we could live
> > > without read_string(). It is provided as a counterpart to write_string()
> > > and for convenience additionally does the unescaping if necessary
> > > like write_string does the escaping.
> > > I don't see why this is a problem. It depends on the context which
> > > is the better function to use.
> > 
> > Again, in my mind a structure is not a string, they are different
> > types, and I think read_string() should only deal with the string type.
> > is_unpacked makes the API more complicated, so I'd like to get rid of
> > it.
> > 
> 
> I don't see your point. is_unpacked is not part of the read_string() 
> arguments
> or return value. In split_list() it is definitively needed to indicate 
> if the returned
> string (in the C sense) contains another list. I can imagine many 
> situations where
> you might either pass an array or just a single value or even NULL. 
> is_unpacked
> allows to differentiate between the situations.

Can you give an example? You say is_unpacked is definitely needed, but
so far the only use case has been read_string(), which you wanted to be
used for reading not only string values but everything else too. If
read_string() is changed to only read string values, then it doesn't
need is_unpacked from split_list().

The parameter types are known beforehand, so i don't see the need for
looking at the data to figure out the type. If introspection support is
desired, then that's a separate project (the is_unpacked flag isn't
sufficient for proper introspection).

-- 
Tanu

https://www.patreon.com/tanuk
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