[cffLib.specializer] Adjust stack use calculation

See comment.

Lib/fontTools/cffLib/specializer.py

@@ -764,7 +764,17 @@ 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 _argsStackUse(args1) + _argsStackUse(args2) < maxstack:
+        #
+        # The assumption is that args1, and args2, each individually
+        # can be successfully processed without a stack overflow.
+        # When combined, the stack depth to consider would be the
+        # number of items in args1 plus what it takes to build args2
+        # on top of args1, which will be already on the stack as
+        # len(args1) items.
+        #
+        # It's unfortunate that _argsStackUse() is O(n) in the number
+        # of args, but it's not a big deal hopefully.
+        if new_op and len(args1) + _argsStackUse(args2) < maxstack:
             commands[i - 1] = (new_op, args1 + args2)
             del commands[i]
 

Tests/cffLib/specializer_test.py

@@ -599,6 +599,20 @@ class CFFSpecializeProgramTest:
         stack_use = charstr_stack_use(specialized, getNumRegions=getNumRegions)
         assert maxStack - numRegions < stack_use < maxStack
 
+    def test_maxstack_blends2(self):
+        # See if two long blend sequences are merged into one
+        numRegions = 400
+        numOps = 2
+        getNumRegions = lambda iv: numRegions
+        blend_one = " ".join([str(i) for i in range(1 + numRegions)] + ["1", "blend"])
+        operands = " ".join([blend_one] * 6)
+        operator = "rrcurveto"
+        charstr = " ".join([operands, operator] * numOps)
+        specialized = charstr_specialize(
+            charstr, getNumRegions=getNumRegions, maxstack=maxStack
+        )
+        assert specialized.index("rrcurveto") == len(specialized) - len("rrcurveto")
+
 
 class CFF2VFTestSpecialize(DataFilesHandler):
     def test_blend_round_trip(self):