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

[Tanu Kaskinen]

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.

> Also writing unescaped strings in situations where escaping is not necessary
> saves the overhead of looping over all the characters.
> 
> > > > core-util already contains pa_unescape() that does the same thing more
> > > > efficiently (if you drop the single quote thing).
> > > 
> > > pa_unescape() currently does not do the same thing. It removes all
> > > escape characters, while I only want to remove the characters
> > > I actually introduced (those before { or }).
> > > I can however modify pa_unescape() to take the same arguments
> > > as pa_escape().
> > 
> > I don't see the need for being selective when unescaping. Nothing
> > breaks if all (except escaped) backslashes are stripped.
> 
> You are right, if previously all backslashes in the original string
> have been escaped, nothing will break. I was still thinking of the
> old solution where I did not escape backslashes.
> 
> 
> > 
> > > > > +
> > > > > +/* 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.

-- 
Tanu

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