glcpp: error on multiple #else/#elif directives

The preprocessor currently accepts multiple else/elif-groups
per if-section. The GLSL-preprocessor is defined by the C++
specification, which defines the following parse-rule:

if-section:
	if-group elif-groups(opt) else-group(opt) endif-line

This clearly only allows a single else-group, that has to come
after any elif-groups.

So let's modify the code to follow the specification. Add test
to prevent regressions.

Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Carl Worth <cworth@cworth.org>

Cc: 10.0 <mesa-stable@lists.freedesktop.org>

src/glsl/glcpp/glcpp-parse.y

@@ -310,6 +310,11 @@ control_line:
 			_glcpp_parser_expand_and_lex_from (parser,
 							   ELIF_EXPANDED, $2);
 		}
+		else if (parser->skip_stack &&
+		    parser->skip_stack->has_else)
+		{
+			glcpp_error(& @1, parser, "#elif after #else");
+		}
 		else
 		{
 			_glcpp_parser_skip_stack_change_if (parser, & @1,
@@ -324,6 +329,11 @@ control_line:
 		{
 			glcpp_error(& @1, parser, "#elif with no expression");
 		}
+		else if (parser->skip_stack &&
+		    parser->skip_stack->has_else)
+		{
+			glcpp_error(& @1, parser, "#elif after #else");
+		}
 		else
 		{
 			_glcpp_parser_skip_stack_change_if (parser, & @1,
@@ -332,7 +342,17 @@ control_line:
 		}
 	}
 |	HASH_ELSE {
-		_glcpp_parser_skip_stack_change_if (parser, & @1, "else", 1);
+		if (parser->skip_stack &&
+		    parser->skip_stack->has_else)
+		{
+			glcpp_error(& @1, parser, "multiple #else");
+		}
+		else
+		{
+			_glcpp_parser_skip_stack_change_if (parser, & @1, "else", 1);
+			if (parser->skip_stack)
+				parser->skip_stack->has_else = true;
+		}
 	} NEWLINE
 |	HASH_ENDIF {
 		_glcpp_parser_skip_stack_pop (parser, & @1);
@@ -2025,6 +2045,7 @@ _glcpp_parser_skip_stack_push_if (glcpp_parser_t *parser, YYLTYPE *loc,
 		node->type = SKIP_TO_ENDIF;
 	}
 
+	node->has_else = false;
 	node->next = parser->skip_stack;
 	parser->skip_stack = node;
 }

src/glsl/glcpp/glcpp.h

@@ -153,6 +153,7 @@ typedef enum skip_type {
 
 typedef struct skip_node {
 	skip_type_t type;
+	bool has_else;
 	YYLTYPE loc; /* location of the initial #if/#elif/... */
 	struct skip_node *next;
 } skip_node_t;

src/glsl/glcpp/tests/118-multiple-else.c

@@ -0,0 +1,6 @@
+#if 0
+#else
+int foo;
+#else
+int bar;
+#endif

src/glsl/glcpp/tests/118-multiple-else.c.expected

@@ -0,0 +1,8 @@
+0:4(1): preprocessor error: multiple #else
+
+
+int foo;
+
+int bar;
+
+

src/glsl/glcpp/tests/119-elif-after-else.c

@@ -0,0 +1,6 @@
+#if 0
+#else
+int foo;
+#elif 0
+int bar;
+#endif

src/glsl/glcpp/tests/119-elif-after-else.c.expected

@@ -0,0 +1,8 @@
+0:4(1): preprocessor error: #elif after #else
+
+
+int foo;
+
+int bar;
+
+