MB-48819: Change Cookie::engine_storage to be atomic

As seen on cluster_run built with TSan, the following race is seen
aborting a timed-out SyncWrite:

    WARNING: ThreadSanitizer: data race (pid=46769)
      Write of size 8 at 0x7b5400291698 by thread T60 (mutexes: read M542537949550136824, write M360774, read M541974810616805208, write M1133353977207195360):
        #0 Cookie::setEngineStorage(void*) kv_engine/daemon/cookie.h:432 (memcached+0x66a6e1)
        #1 EventuallyPersistentEngine::storeEngineSpecific(CookieIface const*, void*) kv_engine/engines/ep/src/ep_engine.cc:1841 (memcached+0x7c4162)
        #2 operator() kv_engine/engines/ep/src/kv_bucket.cc:2756 (memcached+0xa65268)
        #3 __invoke_impl<void, KVBucket::makeSyncWriteCompleteCB()::<...> /usr/include/c++/10/bits/invoke.h:60 (memcached+0xa65268)
        #4 __invoke_r<void, KVBucket::makeSyncWriteCompleteCB()::<...> /usr/include/c++/10/bits/invoke.h:110 (memcached+0xa65268)
        #5 _M_invoke /usr/include/c++/10/bits/std_function.h:291 (memcached+0xa65268)
        #6 std::function<void (CookieIface const*, cb::engine_errc)>::operator()(...) const /usr/include/c++/10/bits/std_function.h:622 (memcached+0x9bf920)
        #7 VBucket::notifyClientOfSyncWriteComplete(CookieIface const*, cb::engine_errc) kv_engine/engines/ep/src/vbucket.cc:1041 (memcached+0x9bf920)
        ...

      Previous write of size 8 at 0x7b5400291698 by thread T18 (mutexes: write M3809):
        #0 Cookie::setEngineStorage(void*) /home/daver/repos/couchbase/server/kv_engine/daemon/cookie.h:432 (memcached+0x66a6e1)
        #1 EventuallyPersistentEngine::storeEngineSpecific(...) kv_engine/engines/ep/src/ep_engine.cc:1841 (memcached+0x7bb0c2)
        #2 EventuallyPersistentEngine::storeIfInner(...) kv_engine/engines/ep/src/ep_engine.cc:2539 (memcached+0x7de96a)
        #3 EventuallyPersistentEngine::store_if(...) kv_engine/engines/ep/src/ep_engine.cc:478 (memcached+0x7debed)
        #4 bucket_store_if(...) kv_engine/daemon/protocol/mcbp/engine_wrapper.cc:148 (memcached+0x7517bd)
        #5 MutationCommandContext::storeItem() kv_engine/daemon/protocol/mcbp/mutation_context.cc:288 (memcached+0x733f6d)
        #6 MutationCommandContext::step() kv_engine/daemon/protocol/mcbp/mutation_context.cc:54 (memcached+0x7385f7)
        ...

When setEngineStorage is called from
EventuallyPersistentEngine::store_if (when issueing a SyncWrite) from
the frontend thread, the per-frontend thread mutex is held when
modifying Cookie:engine_storage. However when the background thread
later modifies Cookie:engine_storage the front-end thread mutex is not
held.

Address this by making Cookie::engine_storage atomic. We could have
added a mutex around it, but that would add additional space
requirements for each Cookie (of which there are potentially many), so
atomic_ptr suffices for the time being.

Change-Id: I62e25b6a74d47c2da6b500cb3dc20d7ad2b01e03
Reviewed-on: http://review.couchbase.org/c/kv_engine/+/163278
Tested-by: Build Bot <[email protected]>
Reviewed-by: Paolo Cocchi <[email protected]>

daemon/cookie.cc

@@ -68,7 +68,7 @@ nlohmann::json Cookie::toJSON() const {
  ret["ewouldblock"] = ewouldblock;
  ret["aiostat"] = to_string(cb::engine_errc(aiostat));
  ret["refcount"] = uint32_t(refcount);
- ret["engine_storage"] = cb::to_hex(uint64_t(engine_storage));
+ ret["engine_storage"] = cb::to_hex(uint64_t(engine_storage.load()));
  return ret;
 }
 

daemon/cookie.h

@@ -687,8 +687,10 @@ class Cookie : public CookieIface {
  * Pointer to engine-specific data which the engine has requested the server
  * to persist for the life of the connection.
  * See SERVER_COOKIE_API::{get,store}_engine_specific()
+ * Atomic so it can safely be read / updated from engine background threads
+ * in addition to front-end threads.
  */
- void* engine_storage{nullptr};
+ std::atomic<void*> engine_storage{nullptr};
 
  /**
  * Log a preformatted response text

programs/engine_testapp/mock_cookie.h

@@ -144,7 +144,7 @@ class MockCookie : public CookieIface {
 protected:
  static CheckPrivilegeFunction checkPrivilegeFunction;
 
- void* engine_data{nullptr};
+ std::atomic<void*> engine_data{nullptr};
  uint32_t sfd{};
  bool handle_ewouldblock{true};
  bool handle_mutation_extras{true};