Fix observer initialization on fiber

Summary: Observer initialization is blocking and is using thread-local state to detect cycles/track dependencies. When running on a fiber we have to make sure the fiber doesn't get suspended during observer initialization.

Reviewed By: yfeldblum

Differential Revision: D28306797

fbshipit-source-id: 807a4553e89f9fa807e6c745a623041dbb0e47ce

folly/experimental/observer/detail/ObserverManager.h

@@ -20,6 +20,7 @@
 
 #include <folly/experimental/observer/detail/Core.h>
 #include <folly/experimental/observer/detail/GraphCycleDetector.h>
+#include <folly/fibers/FiberManager.h>
 #include <folly/futures/Future.h>
 #include <folly/synchronization/SanitizeThread.h>
 
@@ -92,12 +93,14 @@ class ObserverManager {
 
     auto& instance = getInstance();
 
-    auto inManagerThread = std::exchange(inManagerThread_, true);
-    SCOPE_EXIT { inManagerThread_ = inManagerThread; };
+    folly::fibers::runInMainContext([&] {
+      auto inManagerThread = std::exchange(inManagerThread_, true);
+      SCOPE_EXIT { inManagerThread_ = inManagerThread; };
 
-    SharedMutexReadPriority::ReadHolder rh(instance.versionMutex_);
+      SharedMutexReadPriority::ReadHolder rh(instance.versionMutex_);
 
-    core->refresh(instance.version_);
+      core->refresh(instance.version_);
+    });
   }
 
   static void waitForAllUpdates();

folly/experimental/observer/test/ObserverTest.cpp

@@ -20,6 +20,8 @@
 #include <folly/experimental/observer/Observer.h>
 #include <folly/experimental/observer/SimpleObservable.h>
 #include <folly/experimental/observer/WithJitter.h>
+#include <folly/fibers/FiberManager.h>
+#include <folly/fibers/FiberManagerMap.h>
 #include <folly/portability/GTest.h>
 #include <folly/synchronization/Baton.h>
 
@@ -860,3 +862,26 @@ TEST(Observer, MakeObserverUpdatesTracking) {
     EXPECT_EQ(i, **hazptrObserverGetLocalSnapshotCheck);
   }
 }
+
+TEST(Observer, Fibers) {
+  folly::EventBase evb;
+  auto& fm = folly::fibers::getFiberManager(evb);
+
+  auto f1 = fm.addTaskFuture([] {
+    auto o = makeObserver([] {
+      folly::futures::sleep(std::chrono::milliseconds{10}).get();
+      return 1;
+    });
+    EXPECT_EQ(1, **o);
+  });
+  auto f2 = fm.addTaskFuture([] {
+    auto o = makeObserver([] {
+      folly::futures::sleep(std::chrono::milliseconds{20}).get();
+      return 2;
+    });
+    EXPECT_EQ(2, **o);
+  });
+
+  std::move(f1).getVia(&evb);
+  std::move(f2).getVia(&evb);
+}