Add element to specify expression that defines length of a struct

Currently the layout of a struct is used to compute its size. This works
fine in case of structs of fixed size. However this introduces
forwards-compatibility problems in cases when the struct has multiple
variants and the exact variant is specified by the value of some field
(e.g. in the case of <switch> elements). Future revisions of protocols
may introduce new layout variants, in which case the old code does not
know the size of the struct variant and can't parse the incoming byte
stream.

Instead of relying on knowledge about the layout of data structures we
should instead use the length field for length information. This way
when old client libxcb communicates with newer server it can at least
ignore unknown struct variants.

1 files changed, 6 insertions, 0 deletions

diff --git a/xcbgen/xtypes.py b/xcbgen/xtypes.py
index e47189d..3359a09 100644
--- a/xcbgen/xtypes.py
+++ b/xcbgen/xtypes.py
@@ -503,6 +503,8 @@ class ComplexType(Type):
 
     Public fields added:
     fields is an array of Field objects describing the structure fields.
+    length_expr is an expression that defines the length of the structure.
+
     '''
     def __init__(self, name, elt):
         Type.__init__(self, name)
@@ -512,6 +514,7 @@ class ComplexType(Type):
         self.nmemb = 1
         self.size = 0
         self.lenfield_parent = [self]
+        self.length_expr = None
 
         # get required_start_alignment
         required_start_align_element = elt.find("required_start_align")
@@ -573,6 +576,9 @@ class ComplexType(Type):
                 type = module.get_type('INT32')
                 type.make_fd_of(module, self, fd_name)
                 continue
+            elif child.tag == 'length':
+                self.length_expr = Expression(list(child)[0], self)
+                continue
             else:
                 # Hit this on Reply
                 continue