szaszm commented on a change in pull request #735: MINIFICPP-1158 - Event
driven processors can starve each other
URL: https://github.com/apache/nifi-minifi-cpp/pull/735#discussion_r385330893
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File path: libminifi/include/SchedulingAgent.h
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@@ -202,7 +134,7 @@ class SchedulingAgent {
std::shared_ptr<core::ContentRepository> content_repo_;
// thread pool for components.
- utils::ThreadPool<uint64_t> thread_pool_;
+ utils::ThreadPool<utils::TaskRescheduleInfo> &thread_pool_;
Review comment:
Normally a raw pointer is only dangerous if the lifetime or ownership is
unclear or the (clear) contract is violated somewhere.
**Ownership:**
Since we use smart pointers all over the codebase to annotate ownership, all
owning pointers have a non-raw pointer type. This makes the currently
recommended practice of using raw pointers as non-owning pointers safe. We can
make the contract even more clear by introducing an annotation for observer
pointers, similarly to `gsl::owner`. See also
https://en.cppreference.com/w/cpp/experimental/observer_ptr
**Lifetime:**
`ThreadPool` is owned by `FlowController` which lives longer than
`SchedulingAgent`s. Check.
**Contract/ `delete` non-owning pointer:**
At the moment we don't have more than code reviews to check for these. In
2020, a `delete` in new code raises many eyebrows and attracts extra attention,
so I think we're safe here.
**Contract/Null:**
This is the most dangerous part is using a raw ptr for that member. Without
gsl, we don't have a safe way to annotate the pointer to be non-null. The most
we can do is add a comment next to the declaration. We're safe against null
references, since the vast majority of C++ programmers would reject `*nullptr`
instantly during code review, but `nullptr` is widely used and has valid use
cases, so we have no protection against null pointers in the proposed case.
Related talk:
https://www.infoq.com/presentations/Null-References-The-Billion-Dollar-Mistake-Tony-Hoare/
Because of the `nullptr` issue, I'm fine with keeping `thread_pool_` as a
reference. The referenced point from the C++ Core Guidelines is just a note,
not a properly explained guideline, so I'm even fine with ignoring it.
edit: formatting, because of the length
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