Merge pull request #3679 from fonttools/cff2-specializer-maxStack

[CFF2] specializer fix stack overflow

Lib/fontTools/cffLib/specializer.py

@@ -446,9 +446,9 @@ def _convertToBlendCmds(args):
     i = 0
     while i < num_args:
         arg = args[i]
+        i += 1
         if not isinstance(arg, list):
             new_args.append(arg)
-            i += 1
             stack_use += 1
         else:
             prev_stack_use = stack_use
@@ -458,13 +458,8 @@ def _convertToBlendCmds(args):
             # up to the max stack limit.
             num_sources = len(arg) - 1
             blendlist = [arg]
-            i += 1
             stack_use += 1 + num_sources  # 1 for the num_blends arg
-            while (i < num_args) and isinstance(args[i], list):
-                blendlist.append(args[i])
-                i += 1
-                stack_use += num_sources
-                if stack_use + num_sources > maxStackLimit:
+
             # if we are here, max stack is the CFF2 max stack.
             # I use the CFF2 max stack limit here rather than
             # the 'maxstack' chosen by the client, as the default
@@ -472,7 +467,17 @@ def _convertToBlendCmds(args):
             # the other operators, this just produces a little less
             # optimization, but here it puts a hard (and low) limit
             # on the number of source fonts that can be used.
-                    break
+            #
+            # Make sure the stack depth does not exceed (maxstack - 1), so
+            # that subroutinizer can insert subroutine calls at any point.
+            while (
+                (i < num_args)
+                and isinstance(args[i], list)
+                and stack_use + num_sources < maxStackLimit
+            ):
+                blendlist.append(args[i])
+                i += 1
+                stack_use += num_sources
             # blendList now contains as many single blend tuples as can be
             # combined without exceeding the CFF2 stack limit.
             num_blends = len(blendlist)
@@ -504,6 +509,19 @@ def _addArgs(a, b):
     return a + b
 
 
+def _argsStackUse(args):
+    stackLen = 0
+    maxLen = 0
+    for arg in args:
+        if type(arg) is list:
+            # Blended arg
+            maxLen = max(maxLen, stackLen + _argsStackUse(arg))
+            stackLen += arg[-1]
+        else:
+            stackLen += 1
+    return max(stackLen, maxLen)
+
+
 def specializeCommands(
     commands,
     ignoreErrors=False,
@@ -746,7 +764,7 @@ def specializeCommands(
 
         # Make sure the stack depth does not exceed (maxstack - 1), so
         # that subroutinizer can insert subroutine calls at any point.
-        if new_op and len(args1) + len(args2) < maxstack:
+        if new_op and _argsStackUse(args1) + _argsStackUse(args2) < maxstack:
             commands[i - 1] = (new_op, args1 + args2)
             del commands[i]