In contrast to normal functions, coroutines can suspend and later resume their execution. Depending on the program, the coroutine may resume on a
different thread of execution than the one it was started or run previously on.
Therefore, the access to the "same" variable with thread_local storage may produce different values, as illustrated below:
thread_local std::vector<Decorator> decorators;
lazy<Thingy> doSomething() {
// evaluation started on thread t1
/* .... */
const std::size_t decoratorCount = decorators.size(); // value specific to thread t1
auto result = co_await produceThingy();
// after co_await, execution resumes on thread t2
for (std::size_t i = 0; i < decoratorCount; ++i) {
decorators[i].modify(result); // access value specific to t2
// miss some tasks if t1:decorators.size() < t2:decorators.size()
// undefined behavior if t1:decorators.size() > t2:decorators.size()
}
co_return result;
}
This behavior is surprising and unintuitive compared to normal functions that are always evaluated on a single thread. The same issue can happen
for the use of different thread_local variables if their values are interconnected (e.g., one is the address of the buffer, and the other
is the number of elements in the buffer).
Moreover, access to thread_local variables defined inside the coroutine may read uninitialized memory. Each such variable is
initialized when a specific thread enters the function for the first time, and if the function was never called from a thread on which the coroutine
is resumed, it is uninitialized.
This rule raises an issue on the declaration of thread_local variables and access to thread_local variables in
coroutines.
Noncompliant code example
thread_local std::vector<Decorator> decorators;
lazy<Thingy> doSomething() {
thread_local Decorator localDecorator; // Noncompliant
const std::size_t decoratorCount = decorators.size(); // Noncompliant
/* ... */
auto result = co_await produceThingy();
for (std::size_t i = 0; i < taskCount; ++i) {
decorators[i].modify(result);
}
localDecorator.modify(result); // Noncompliant
co_return result;
}
