The part about claiming there is decoding of the input to `shelve.open`
is sort of an odd one, since it's not the filename, but the contents of
the file that is decoded.
However, trying to only handle this problem through path injection is
not enough -- if a user is able to upload and access files through
`shelve.open` in a path injection safe manner, that still leads to code
execution.
So right now the best way we have of modeling this is to treat the
filename argument as being deserialized...
The changes to `ModificationOfParameterWithDefault.ql` and the use of
`ConditionBlock::controls` therein caused the `BasicBlock` argument to
get magicked in, resulting in the following antijoin for the `forall`:
```
[2021-10-04 12:07:46] (108s) Tuple counts for GuardedControlFlow::ConditionBlock::controls_dispred#fbf#antijoin_rhs/5@d84e94 after 1m44s:
201222345 ~7% {5} r1 = JOIN GuardedControlFlow::ConditionBlock::controls_dispred#fbf#shared#2 WITH Flow::BasicBlock::getASuccessor_dispred#ff_10#join_rhs ON FIRST 1 OUTPUT Lhs.0 'arg1', Rhs.1 'arg4', Lhs.1 'arg0', Lhs.2 'arg2', Lhs.3 'arg3'
200599933 ~4% {5} r2 = JOIN r1 WITH Flow::BasicBlock::dominates#ff ON FIRST 2 OUTPUT Lhs.2 'arg0', Lhs.0 'arg1', Lhs.3 'arg2', Lhs.4 'arg3', Lhs.1 'arg4'
0 ~0% {4} r3 = JOIN GuardedControlFlow::ConditionBlock::controls_dispred#fbf#shared#1 WITH GuardedControlFlow::ConditionBlock#class#f ON FIRST 1 OUTPUT Lhs.0 'arg3', Lhs.2 'arg1', Lhs.1 'arg0', false
0 ~0% {4} r4 = JOIN GuardedControlFlow::ConditionBlock::controls_dispred#fbf#shared WITH GuardedControlFlow::ConditionBlock#class#f ON FIRST 1 OUTPUT Lhs.0 'arg3', Lhs.2 'arg1', Lhs.1 'arg0', true
0 ~0% {4} r5 = r3 UNION r4
0 ~0% {5} r6 = JOIN r5 WITH Flow::BasicBlock::getASuccessor_dispred#ff ON FIRST 2 OUTPUT Lhs.2 'arg0', Lhs.1 'arg1', Lhs.3 'arg2', Lhs.0 'arg3', Rhs.0
200599933 ~4% {5} r7 = r2 UNION r6
return r7
```
(cancelled)
I observed that quick-eval'ing the `controls` predicate exhibit no such
bad join order (and terminated quickly) which lead me to conclude that
this was a case of bad magic.
Adding the `pragma[nomagic]` resulted in a return to the previous
performance.
