crush: use std::vector instead of variable length arrays

despite that variable length arrays (VLA for short) has been around
for a long time, it is an extension supported by GCC and Clang, and is
not a part of C++ standard, its implementation allocates the dynamically
sized array on stack, hence is a source of potential stack overflow.

when compiling with Clang, it complains. so in this change, we switch
to std::vector<>, which is defined by the C++ standard, and it allocates
the storage on heap, so it is immune to the possible stack overflow problem.

```
/home/kefu/dev/ceph/src/crush/CrushWrapper.h:1613:16: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
 1613 |     int rawout[maxout];
      |                ^~~~~~
/home/kefu/dev/ceph/src/crush/CrushWrapper.h:1613:16: note: function parameter 'maxout' with unknown value cannot be used in a constant expression
/home/kefu/dev/ceph/src/crush/CrushWrapper.h:1610:60: note: declared here
 1610 |   void do_rule(int rule, int x, std::vector<int>& out, int maxout,
      |                                                            ^
/home/kefu/dev/ceph/src/crush/CrushWrapper.h:1614:15: warning: variable length arrays in C++ are a Clang extension [-Wvla-cxx-extension]
 1614 |     char work[crush_work_size(crush, maxout)];
      |               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/home/kefu/dev/ceph/src/crush/CrushWrapper.h:1614:31: note: implicit use of 'this' pointer is only allowed within the evaluation of a call to a 'constexpr' member function
 1614 |     char work[crush_work_size(crush, maxout)];
      |                               ^
2 warnings generated.
```

Signed-off-by: Kefu Chai <tchaikov@gmail.com>

diff --git a/src/crush/CrushWrapper.h b/src/crush/CrushWrapper.h
index 7018ca498c67fac938f5567e4ca8d6f1a3214e59..6c592bb6ed7ce954ca436b4a0546191cd43578cf 100644 (file)
--- a/src/crush/CrushWrapper.h
+++ b/src/crush/CrushWrapper.h
@@ -1610,14 +1610,14 @@ public:
   void do_rule(int rule, int x, std::vector<int>& out, int maxout,
               const WeightVector& weight,
               uint64_t choose_args_index) const {
-    int rawout[maxout];
-    char work[crush_work_size(crush, maxout)];
-    crush_init_workspace(crush, work);
+    std::vector<int> rawout(maxout);
+    std::vector<char> work(crush_work_size(crush, maxout));
+    crush_init_workspace(crush, std::data(work));
     crush_choose_arg_map arg_map = choose_args_get_with_fallback(
       choose_args_index);
-    int numrep = crush_do_rule(crush, rule, x, rawout, maxout,
+    int numrep = crush_do_rule(crush, rule, x, std::data(rawout), maxout,
                               std::data(weight), std::size(weight),
-                              work, arg_map.args);
+                              std::data(work), arg_map.args);
     if (numrep < 0)
       numrep = 0;
     out.resize(numrep);