Adding grpc_init/grpc_shutdown to subchannel ctor/dtor to solve bugs related to async destruction of subchannels (#26965)

* Adding grpc_init/grpc_shutdown to subchannel ctor/dtor to solve bugs related to async destruction of subchannels

* removing blank line

src/core/ext/filters/client_channel/subchannel.cc

@@ -657,6 +657,15 @@ Subchannel::Subchannel(SubchannelKey key,
       key_(std::move(key)),
       connector_(std::move(connector)),
       backoff_(ParseArgsForBackoffValues(args, &min_connect_timeout_ms_)) {
+  // A grpc_init is added here to ensure that grpc_shutdown does not happen
+  // until the subchannel is destroyed. Subchannels can persist longer than
+  // channels because they maybe reused/shared among multiple channels. As a
+  // result the subchannel destruction happens asynchronously to channel
+  // destruction. If the last channel destruction triggers a grpc_shutdown
+  // before the last subchannel destruction, then there maybe race conditions
+  // triggering segmentation faults. To prevent this issue, we call a grpc_init
+  // here and a grpc_shutdown in the subchannel destructor.
+  grpc_init();
   GRPC_STATS_INC_CLIENT_SUBCHANNELS_CREATED();
   pollset_set_ = grpc_pollset_set_create();
   grpc_resolved_address* addr =
@@ -707,6 +716,8 @@ Subchannel::~Subchannel() {
   grpc_channel_args_destroy(args_);
   connector_.reset();
   grpc_pollset_set_destroy(pollset_set_);
+  // grpc_shutdown is called here because grpc_init is called in the ctor.
+  grpc_shutdown();
 }
 
 RefCountedPtr<Subchannel> Subchannel::Create(