commands: change property expansion format string

This affects property format strings like they are used in the
"show_text" input command, for --playing-msg, and other places.

To quote the documentation comment on m_properties_expand_string():

    ${NAME} is expanded to the value of property NAME.
    If NAME starts with '=', use the raw value of the property.
    ${NAME:STR} expands to the property, or STR if the property is not
    available.
    ${?NAME:STR} expands to STR if the property is available.
    ${!NAME:STR} expands to STR if the property is not available.
    STR is recursively expanded using the same rules.
    "$$" can be used to escape "$", and "$}" to escape "}".
    "$>" disables parsing of "$" for the rest of the string.

Most importantly, "?(property:str)" becomes "${?property:str}".

Make the simple fallback case easier, e.g. "${property:fallback}"
instead of "${property}?(!property:fallback)".

Add the ability to escape the format meta characters. "$" is used for
escaping, because escaping with "\" is taken by the commands parser in
the layer below. "$>" can be used to disable interpretation of format
strings (of course escapes by the commands parser can't be canceled).

By default, properties which are unavailable or don't exist are turned
into a string signaling the status (e.g. "(unavailable)"), instead of
an empty string. If an empty string is desired, this has to be done
explicitly: "${property:}" (the fallback part is an empty string). Raw
properties still return an empty string on error.

m_properties_expand_string() now returns a talloc'ed pointer, instead of
a malloc'ed one.

1 files changed, 12 insertions, 6 deletions

diff --git a/m_property.h b/m_property.h
index b5b8af26ee..94dd36f050 100644
--- a/m_property.h
+++ b/m_property.h
@@ -110,12 +110,18 @@ int m_property_do(const struct m_option* prop_list, const char* property_name,
 void m_properties_print_help_list(const struct m_option* list);
 
 // Expand a property string.
-/* This function allows to print strings containing property values.
- *  ${NAME} is expanded to the value of property NAME or an empty
- *  string in case of error. $(NAME:STR) expand STR only if the property
- *  NAME is available.
- */
-char* m_properties_expand_string(const struct m_option* prop_list, char* str,
+// This function allows to print strings containing property values.
+//  ${NAME} is expanded to the value of property NAME.
+//  If NAME starts with '=', use the raw value of the property.
+//  ${NAME:STR} expands to the property, or STR if the property is not
+//  available.
+//  ${?NAME:STR} expands to STR if the property is available.
+//  ${!NAME:STR} expands to STR if the property is not available.
+// General syntax: "${" ["?" | "!"] ["="] NAME ":" STR "}"
+// STR is recursively expanded using the same rules.
+// "$$" can be used to escape "$", and "$}" to escape "}".
+// "$>" disables parsing of "$" for the rest of the string.
+char* m_properties_expand_string(const struct m_option* prop_list, char *str,
                                  void *ctx);
 
 // Trivial helpers for implementing properties.