settings: Add reference feature

Similar to the `also` keyword in ipsec.conf, the new syntax allows adding
one or more references to other sections, which means all the settings and
subsections defined there are inherited (values may be overridden, even
with an empty value to clear it).

It's important to note that all subsections are inherited, so if this is
used to reference a connection in swanctl.conf all auth rounds and
children are inherited.  There is currently no syntax to limit the
inclusion level or clear inherited sections (but as mentioned, settings
in those inherited sections may be overridden).

Another property is that inherited settings or sections always follow
explicitly defined entries in the current section when they are enumerated.
This is relevant if the order is important (e.g. for auth rounds if `round`
is not specified).

References are evaluated dynamically at runtime, so referring to
sections later in the config file or included via other files is no
problem.

The colon used as separator to reference other sections may be used in
section names by writing :: (e.g. for Windows log file paths).

This is based on a patch originally written in 2016.

src/libstrongswan/settings/settings_parser.y

@@ -1,6 +1,6 @@
 %{
 /*
- * Copyright (C) 2014 Tobias Brunner
+ * Copyright (C) 2014-2018 Tobias Brunner
  * HSR Hochschule fuer Technik Rapperswil
  *
  * This program is free software; you can redistribute it and/or modify it
@@ -49,6 +49,7 @@ static section_t *push_section(parser_helper_t *ctx, char *name);
 static section_t *pop_section(parser_helper_t *ctx);
 static void add_section(parser_helper_t *ctx, section_t *section);
 static void add_setting(parser_helper_t *ctx, kv_t *kv);
+static void add_references(parser_helper_t *ctx, array_t *references);
 
 /**
  * Make sure to call lexer with the proper context
@@ -78,20 +79,24 @@ static int yylex(YYSTYPE *lvalp, parser_helper_t *ctx)
 	char *s;
 	struct section_t *sec;
 	struct kv_t *kv;
+	array_t *refs;
 }
 %token <s> NAME STRING
+%token REFS ":"
 %token NEWLINE STRING_ERROR
 
 /* ...and other symbols */
 %type <s> value valuepart
 %type <sec> section_start section
 %type <kv> setting
+%type <refs> references
 
 /* properly destroy string tokens that are strdup()ed on error */
 %destructor { free($$); } NAME STRING value valuepart
 /* properly destroy parse results on error */
 %destructor { pop_section(ctx); settings_section_destroy($$, NULL); } section_start section
 %destructor { settings_kv_destroy($$, NULL); } setting
+%destructor { array_destroy_function($$, (void*)free, NULL); } references
 
 /* there are two shift/reduce conflicts because of the "NAME = NAME" and
  * "NAME {" ambiguity, and the "NAME =" rule) */
@@ -133,9 +138,24 @@ section_start:
 		$$ = push_section(ctx, $NAME);
 	}
 	|
-	NAME NEWLINE '{'
+	NAME ":" references '{'
 	{
 		$$ = push_section(ctx, $NAME);
+		add_references(ctx, $references);
+		array_destroy($references);
+	}
+	;
+
+references:
+	NAME
+	{
+		$$ = array_create(0, 0);
+		array_insert($$, ARRAY_TAIL, $1);
+	}
+	| references ',' NAME
+	{
+		array_insert($1, ARRAY_TAIL, $3);
+		$$ = $1;
 	}
 	;
 
@@ -238,6 +258,27 @@ static void add_setting(parser_helper_t *ctx, kv_t *kv)
 	settings_kv_add(section, kv, NULL);
 }
 
+/**
+ * Adds the given references to the section on top of the stack
+ */
+static void add_references(parser_helper_t *ctx, array_t *references)
+{
+	array_t *sections = (array_t*)ctx->context;
+	section_t *section;
+	enumerator_t *refs;
+	char *ref;
+
+	array_get(sections, ARRAY_TAIL, &section);
+
+	refs = array_create_enumerator(references);
+	while (refs->enumerate(refs, &ref))
+	{
+		settings_reference_add(section, ref, FALSE);
+		array_remove_at(references, refs);
+	}
+	refs->destroy(refs);
+}
+
 /**
  * Parse the given file and add all sections and key/value pairs to the
  * given section.