[webkit-dev] Set once pointer types and static analysis

[Ryosuke Niwa]

Instead of introducing a new smart pointer type like SetOnceRefPtr, we’re going to use `const RefPtr`, `const Ref`, `const unique_ptr`, and `const UniqueRef`. `const Ref` and `const UniqueRef` are used for values that are initialized in the constructor and never changed. `const RefPtr` and `const unique_ptr` are used for lazily initialized values. We introduce a `initializeOnce(ptr, v)` function which takes `const RefPtr` or `const unique_ptr` ptr and sets it to v. The static analyzer will be updated to recognize this pattern.

https://bugs.webkit.org/show_bug.cgi?id=283038
https://github.com/llvm/llvm-project/pull/115594

- R. Niwa

> On Oct 29, 2024, at 2:49 AM, youenn fablet <youennf at gmail.com> wrote:
> 
> FWIW, there are many classes with Ref<> members that are initialized in the constructor and are never intended to be changed.
> I was hoping we would cover this case at least. The RefPtr lazy initialization is a welcome addition as well.
> 
> I think this would be useful to improve readability:
> - Too many protected in the same line of code makes code harder to read for me.
> - It is easier to reason about "stable" member variables. Having an explicit type or some annotation instead of having to look at the whole code is an improvement.
> 
> Le mar. 29 oct. 2024 à 03:10, Jean-Yves Avenard via webkit-dev <webkit-dev at lists.webkit.org> a écrit :
>> Hi
>> 
>> It’s a very strong +1 for me ; I find the usage of foo->protectedBar()->method() or Ref { foo->bar() }->method()
>> particularly unsightly (and points more to the inability of the static analyser to deal with some use case than the code being inherently unsafe)
>> 
>> Having a way to reduce the unsightly pattern where we are 100% certain it’s not needed is a benefit.
>> I assume also that the static analyser will not complain with `const` members either 
>> 
>> (For the sake of disclosure, I asked Ryosuke for that feature after discussing the need with Youenn) 
>> 
>> Jean-Yves
>> 
>>> On 29 Oct 2024, at 5:48 am, Ryosuke Niwa via webkit-dev <webkit-dev at lists.webkit.org> wrote:
>>> 
>>> Hi all,
>>> 
>>> In WebKit, it’s fairly common to write a member variable as RefPtr or std::unique_ptr that later gets lazily initialized to some value but never unset or assigned of a different value after that.
>>> 
>>> e.g.
>>> 
>>> class Foo {
>>>     Bar& bar() {
>>>         if (!m_bar)
>>>             m_bar = Bar::create();
>>>         return *m_bar;
>>>     }
>>>     Ref<Bar> protectedBar() { return bar(); }
>>> 
>>>     RefPtr<Bar> m_bar;
>>> }
>>> 
>>> Assuming there is no other code modifying m_bar, foo->bar()->method() is always safe to call even if method wasn’t a trivial function. Right now, static analyzer doesn’t recognize this pattern so we’d be forced to write code like this: foo->protectedBar()->method() where ensureProtectedBar is a wrapper around ensureBar which returns Ref<Bar>.
>>> 
>>> A suggestion was made that static analyzer can recognize this patten. Specifically, if we introduced a new smart pointer types that only allow setting the value once, static analyzer can allow foo->bar()->method()and avoid ref-churn in some cases:
>>> 
>>> e.g.
>>> 
>>> class Foo {
>>>     Bar& bar() {
>>>         if (!m_bar)
>>>             m_bar = Bar::create();
>>>         return *m_bar;
>>>     }
>>> 
>>>     SetOnceRefPtr<Bar> m_bar;
>>> }
>>> 
>>> SetOnceRefPtr::operator=(T*) release asserts that m_ptr isn’t set, and doesn’t have a move constructor, operator=(nullptr_t), leakRef(), releaseNonNull(), etc… which can override the value of m_ptr after setting the value via operator= or in constructor.
>>> 
>>> We could create various variants: SetOnceRef, SetOnceUniquePtr, SetOnceUniqueRef.
>>> 
>>> Do you think this will be useful? 
>>> 
>>> - R. Niwa
>> 
>> 
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