Bug 1657609 - statically bound the maximum number of parameters for x…

…pidl methods; r=mccr8

We used to not be able to do something like this, and therefore our xpidl
calling code had to  be fully general.  But now that we know all possible
xpidl methods at build time, we can ensure that the number of parameters
does not exceed some bound.  And we choose that bound to be the maximum
number of stack-allocated parameters our calling code supports.

Differential Revision: https://phabricator.services.mozilla.com/D86212

xpcom/reflect/xptinfo/xptcodegen.py

@@ -204,6 +204,8 @@ def interface_idx(name):
     params = []
     param_cache = {}
     methods = []
+    max_params = 0
+    method_with_max_params = None
     consts = []
     domobjects = []
     domobject_cache = {}
@@ -361,6 +363,10 @@ def lower_method(method, ifacename):
                 for idx, param in enumerate(method['params']):
                     lower_param(param, "%s[%d]" % (methodname, idx))
 
+        nonlocal max_params, method_with_max_params
+        if numparams > max_params:
+            max_params = numparams
+            method_with_max_params = methodname
         methods.append(nsXPTMethodInfo(
             "%d = %s" % (len(methods), methodname),
 
@@ -508,6 +514,12 @@ def array(ty, name, els):
     array("nsXPTType", "sTypes", types)
     array("nsXPTParamInfo", "sParams", params)
     array("nsXPTMethodInfo", "sMethods", methods)
+    # Verify that stack-allocated buffers will do for xptcall implementations.
+    msg = "Too many method arguments in %s. " \
+          "Either reduce the number of arguments " \
+          "or increase PARAM_BUFFER_COUNT." % method_with_max_params
+    fd.write("static_assert(%s <= PARAM_BUFFER_COUNT, \"%s\");\n\n"
+             % (max_params, msg))
     array("nsXPTDOMObjectInfo", "sDOMObjects", domobjects)
     array("nsXPTConstantInfo", "sConsts", consts)