add co_cancelled

Summary: [folly] Add `co_cancelled`, effectively a shorthand for `co_error(OperationCancelled{})`.

Reviewed By: Mizuchi, lewissbaker

Differential Revision: D25627993

fbshipit-source-id: 91f80772a8b179a4de0fe3b29dfe7eecaafb8e1b

folly/experimental/coro/Collect-inl.h

@@ -206,7 +206,7 @@ auto collectAllImpl(
       // Parent task was cancelled before any child tasks failed.
       // Complete with the OperationCancelled error instead of the
       // child task's errors.
-      co_yield co_error(OperationCancelled{});
+      co_yield co_cancelled;
     }
 
     co_return std::tuple<collect_all_component_t<SemiAwaitables>...>{
@@ -320,7 +320,7 @@ auto collectAllRange(InputRange awaitables)
 
     // Cancellation was requested of the parent Task before any of the
     // child tasks failed.
-    co_yield co_error(OperationCancelled{});
+    co_yield co_cancelled;
   }
 
   std::vector<detail::collect_all_range_component_t<
@@ -617,7 +617,7 @@ auto collectAllWindowed(InputRange awaitables, std::size_t maxConcurrency)
       co_yield co_error(std::move(firstException));
     }
 
-    co_yield co_error(OperationCancelled{});
+    co_yield co_cancelled;
   }
 }
 
@@ -778,7 +778,7 @@ auto collectAllWindowed(InputRange awaitables, std::size_t maxConcurrency)
 
     // Otherwise, cancellation was requested before any of the child tasks
     // failed so complete with the OperationCancelled error.
-    co_yield co_error(OperationCancelled{});
+    co_yield co_cancelled;
   }
 
   std::vector<detail::collect_all_range_component_t<

folly/experimental/coro/Result.h

@@ -19,6 +19,7 @@
 #include <cassert>
 #include <type_traits>
 
+#include <folly/CancellationToken.h>
 #include <folly/ExceptionWrapper.h>
 #include <folly/Try.h>
 
@@ -61,5 +62,14 @@ class co_result final {
   Try<T> result_;
 };
 
+class co_cancelled_t final {
+ public:
+  /* implicit */ operator co_error() const {
+    return co_error(OperationCancelled{});
+  }
+};
+
+FOLLY_INLINE_VARIABLE constexpr co_cancelled_t co_cancelled{};
+
 } // namespace coro
 } // namespace folly

folly/experimental/coro/Retry.h

@@ -187,7 +187,7 @@ class ExponentialBackoffWithJitter {
     // Check to see if we were cancelled during the sleep.
     const auto& cancelToken = co_await co_current_cancellation_token;
     if (cancelToken.isCancellationRequested()) {
-      co_yield folly::coro::co_error(OperationCancelled{});
+      co_yield folly::coro::co_cancelled;
     }
   }
 

folly/experimental/coro/Sleep-inl.h

@@ -43,7 +43,7 @@ inline Task<void> sleep(Duration d, Timekeeper* tk) {
     co_await baton;
   }
   if (cancelled) {
-    co_yield co_error(OperationCancelled());
+    co_yield co_cancelled;
   }
   co_yield co_result(std::move(result));
 }

folly/fibers/Semaphore.cpp

@@ -171,7 +171,7 @@ coro::Task<void> Semaphore::co_wait() {
         // sure that we aren't reading it concurrently with a potential write
         // from a thread requesting cancellation.
         if (cancelled) {
-          co_yield folly::coro::co_error(folly::OperationCancelled{});
+          co_yield folly::coro::co_cancelled;
         }
 
         co_return;

folly/fibers/SemaphoreBase.cpp

@@ -147,7 +147,7 @@ coro::Task<void> SemaphoreBase::co_wait_common(int64_t tokens) {
         // sure that we aren't reading it concurrently with a potential write
         // from a thread requesting cancellation.
         if (cancelled) {
-          co_yield folly::coro::co_error(folly::OperationCancelled{});
+          co_yield folly::coro::co_cancelled;
         }
 
         co_return;