helgrind 'lock order': DBTDevice: Don't abuse mtx_connect in notify[Connected|Disconnected]() callbacks or in GATTHandler lifecycle

GATTHandler lifecycle has its own minimal mutex, mtx_gattHandler, no more [ab]using mtx_connect.

To avoid 'lock order' issues and hence potential deadlocks:

- GATTHandler construction happens now via connectGATT() into notifyConnect off-thread for LE devices.

- GATTHandler disconnectGATT() happens at disconnect() upfront above the mtx_connect lock or at notifyDisconnect() the latest.

- notify[Connected|Disconnected]() callbacks don't hold mtx_connect,
  as they only set atomic flags and notifyConnected() may issue connectGATT() off-thread (see above)

Have allowDisconnect before isConnected in notify[Connected|Disconnected](), as it is queried before.

Have disconnectGATT() be called last in notifyDisconnected(), as allowDisconnect and isConnected have precedent
and connectGATT() queries both to avoid ctor attempt if not connected or disconnecting.

getGATTService() and pingGATT() now just use getGATTHandler(), implemented merely as 'user methods',
either the GATTHandler was created at notifyConnected() and hence exists or not.

connectGATT(): Add noexcept as GATTHandler ctor is also noexcept; also don't throw an exception.
This renders connectGATT() suitable to be excecuted off-thread.

1 files changed, 1 insertions, 1 deletions

diff --git a/examples/direct_bt_scanner01/dbt_scanner01.cpp b/examples/direct_bt_scanner01/dbt_scanner01.cpp
index 655ea4a0..db7fd78c 100644
--- a/examples/direct_bt_scanner01/dbt_scanner01.cpp
+++ b/examples/direct_bt_scanner01/dbt_scanner01.cpp
@@ -255,7 +255,7 @@ int main(int argc, char *argv[])
             //
             const uint64_t t4 = getCurrentMilliseconds();
             // let's check further for full GATT
-            std::shared_ptr<GATTHandler> gatt = device->connectGATT();
+            std::shared_ptr<GATTHandler> gatt = device->getGATTHandler();
             if( nullptr != gatt ) {
                 fprintf(stderr, "GATT usedMTU %d (server) -> %d (used)\n", gatt->getServerMTU(), gatt->getUsedMTU());