resource: error out if reserve_additional_memory is too large

there is chance that configured reserve_additional_memory exceeds
total_memory. as both of them can be configured at runtime either
programtically or using command line option or config file. so

- if the former is greater than the latter, we could have integer
  underflow, in that case `needed_memory` could be a very large number,
  and `needed_memory > available_memory` is likely to be evaluated
  to true, and the thrown error still sends the right message. but
  this hurts the readability.
- if reserve_additional_memory is a very large number, like
  size_t(-1), we will also have integer underflow, but `needed_memory`
  would be `c.total_memory + 1`, which still can be greater than
  `available_memory`, and no exceptions are thrown. this is not
  expected. we should have warn user that the reserve_additional_memory
  is too large.

in this change, before calculating the `needed_memory`,
`c.total_memory` is compared with `c.reserve_additional_memory`,
exception is thrown if the former is less than the latter.

Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>

Author: tchaikov

src/core/resource.cc

@@ -223,6 +223,9 @@ size_t calculate_memory(const configuration& c, size_t available_memory, float p
     if (!c.total_memory.has_value()) {
         return available_memory;
     }
+    if (*c.total_memory < c.reserve_additional_memory) {
+        throw std::runtime_error(format("insufficient total memory: reserve {} total {}", c.reserve_additional_memory, *c.total_memory));
+    }
     size_t needed_memory = *c.total_memory - c.reserve_additional_memory;
     if (needed_memory > available_memory) {
         throw std::runtime_error(format("insufficient physical memory: needed {} available {}", needed_memory, available_memory));