Allow the CancellationToken to propagate through the co_awaitTry() algorithm

Summary:
Modify the TrySemiAwaitable adapter so that it customises the co_withCancellation() customisation-point and passes through the CancellationToken to the underlying awaitable (eg. a Task).

This fixes code that tries to do the following so that the cancellation token propagates to a Task adapted with co_awaitTry().
```
co_await co_withCancellation(cancelToken, co_awaitTry(co_foo()));
```

Reviewed By: andriigrynenko

Differential Revision: D20443798

fbshipit-source-id: 945e0f98abad4cdef0be29365642006c5687ad5f

folly/experimental/coro/ViaIfAsync.h

@@ -22,6 +22,7 @@
 #include <folly/Executor.h>
 #include <folly/Traits.h>
 #include <folly/experimental/coro/Traits.h>
+#include <folly/experimental/coro/WithCancellation.h>
 #include <folly/io/async/Request.h>
 #include <folly/lang/CustomizationPoint.h>
 
@@ -431,6 +432,16 @@ class TrySemiAwaitable {
         co_viaIfAsync(std::move(executor), std::move(self.semiAwaitable_))));
   }
 
+  friend auto co_withCancellation(
+      const CancellationToken& cancelToken,
+      TrySemiAwaitable&& awaitable) {
+    auto cancelAwaitable = folly::coro::co_withCancellation(
+        std::move(cancelToken),
+        static_cast<SemiAwaitable&&>(awaitable.semiAwaitable_));
+    return TrySemiAwaitable<decltype(cancelAwaitable)>(
+        std::move(cancelAwaitable));
+  }
+
  private:
   SemiAwaitable semiAwaitable_;
 };

folly/experimental/coro/test/TaskTest.cpp

@@ -409,6 +409,20 @@ TEST_F(TaskTest, CancellationPropagation) {
   }());
 }
 
+TEST_F(TaskTest, CancellationPropagatesThroughCoAwaitTry) {
+  folly::CancellationSource source;
+  folly::Try<int> result =
+      folly::coro::blockingWait(folly::coro::co_withCancellation(
+          source.getToken(),
+          folly::coro::co_awaitTry([&]() -> folly::coro::Task<int> {
+            auto cancelToken =
+                co_await folly::coro::co_current_cancellation_token;
+            EXPECT_TRUE(cancelToken == source.getToken());
+            co_return 42;
+          }())));
+  EXPECT_EQ(42, result.value());
+}
+
 TEST_F(TaskTest, StartInlineUnsafe) {
   folly::coro::blockingWait([]() -> folly::coro::Task<void> {
     auto executor = co_await folly::coro::co_current_executor;