expose LockFreeRingBuffer internal address/size

Summary: This is useful if your environment takes partial dumps of the process on crash - you can tell the process which does that to include the memory range of the flight recorder in what is dumped.

Differential Revision: D27206631

fbshipit-source-id: b3679004930486094e6030b5b39ee02d01bc7617

folly/experimental/LockFreeRingBuffer.h

@@ -21,6 +21,7 @@
 #include <cstring>
 #include <memory>
 #include <type_traits>
+#include <utility>
 
 #include <folly/Portability.h>
 #include <folly/Traits.h>
@@ -158,6 +159,20 @@ class LockFreeRingBuffer {
     return Cursor(ticket - backStep);
   }
 
+  // Returns the address and length of the internal buffer.
+  // This can be useful to know if you are using LockFreeRingBuffer to store
+  // things you want to be able to retrieve after a crash, and you are not
+  // dumping all memory when a crash occurs: you can add the result of this
+  // call to the list of memory regions that are dumped.
+  // See `folly::FlightRecorder` for this use-case.
+  // It's not useful, or safe, to inspect this region at runtime (other than
+  // through the APIs above)
+  std::pair<void const*, size_t> internalBufferLocation() const {
+    return std::make_pair(
+        static_cast<void const*>(slots_.get()),
+        sizeof(detail::RingBufferSlot<T, Atom>[capacity_]));
+  }
+
   ~LockFreeRingBuffer() {}
 
  private: