[BUG] Fixed async event reporting and mutex lock ordering for groups

docs/API-functions.md

@@ -631,15 +631,15 @@ different families (that is, both `local_name` and `remote_name` must be `AF_INE
 
 ### srt_connect_callback
 ```
-int srt_connect_callback(SRTSOCKET clr, srt_connect_callback_fn* hook_fn, void* hook_opaque);
+int srt_connect_callback(SRTSOCKET u, srt_connect_callback_fn* hook_fn, void* hook_opaque);
 ```
 
-This call installs a callback hook, which will be executed on a given `clr`
-socket just after the pending connection situation in the background has been
-resolved (that is, when the connection succeeded or failed). Note that this
-function is not guaranteed to be called if the `clr` socket is set to blocking
-mode (`SRTO_RCVSYN` option set to true). It is guaranteed to be called when
-a socket is in non-blocking mode, or when you use a group.
+This call installs a callback hook, which will be executed on a given `u`
+socket or all member sockets of `u` group, just after the pending connection
+situation in the background has been resolved and the connection has failed.
+Note that this function is not guaranteed to be called if the `u` socket is set
+to blocking mode (`SRTO_RCVSYN` option set to true). It is guaranteed to be
+called when a socket is in non-blocking mode, or when you use a group.
 
 This function is mainly intended to be used with group connections. Note that
 even if you use a group connection in blocking mode, after the group is considered
@@ -650,13 +650,15 @@ or all fail - in such a case those failures also happen only in the background,
 while the connecting function blocks until all connections are resolved. 
 When all links fail, you will only get a general error code for the group.
 This mechanism allows you to get individual errors for particular member
-connections.
+connection failures.
 
 You can also use this mechanism as an alternative method for a single-socket
 connection in non-blocking mode to trigger an action when the connection
-process is finished.
+process is finished. It is recommended, however, that you use this callback
+only to collect failure information, as the call will happen in one of the
+internal SRT threads.
 
-* `clr`: Socket that will be used for connecting and for which the hook is installed
+* `u`: Socket or group that will be used for connecting and for which the hook is installed
 * `hook_fn`: The callback hook function pointer
 * `hook_opaque`: The pointer value that will be passed to the callback function
 

docs/LowLevelInfo.md

@@ -0,0 +1,239 @@
+# Low Level Information for the SRT project
+
+## Introduction
+
+This documnent will contain loose information for various topis referring to
+the SRT source code describing some cross-source analysis that would be
+unobvious for a source code reviewer. It's not a complete documentation of
+anything, rather a collection of various kind of information retrieved during
+development and even reverse engineering.
+
+## Mutex locking
+
+This analysis is a result of detected lots of cascade mutex locking in the
+SRT code. A more detailed analysis would be required as to which mutex is
+going to protect what kind of data later.
+
+Here is the info collected so far:
+
+### Data structures
+
+The overall structure of the object database, involving sockets and groups
+is as follows (pseudo-language):
+
+```
+CUDTUnited (singleton) {
+     CONTAINER<CUDTSocket> m_Sockets;
+     CONTAINER<CUDTSocket> m_ClosedSockets;
+     CONTAINER<CUDTGroup> m_Groups;
+     CONTAINER<CUDTGroup> m_ClosedGroups;
+}
+
+CUDTGroup {
+     type SocketData { CUDTSocket* ps; SRTSOCKET id; int state; ... }
+     CONTAINER<SocketData> m_Group;
+}
+```
+
+Dead sockets (either closed manually or broken after connection) are
+moved first from `m_Sockets` to `m_ClosedSockets`. The GC thread will take
+care to delete them physically after making sure all inside facilities
+do not contain any remaining data of interest.
+
+Groups may only be manually closed, however a closed group is moved
+to `m_ClosedGroups`. The GC thread will take care to delete them, as long
+as their usage counter is 0. Every call to an API function (as well as
+TSBPD thread) increases the usage counter in the beginning and decreases
+upon exit. A group may be closed in one thread and still being used in
+another. The group will persist up to the time when the current API function
+using it exits and decreases the usage counter back to 0.
+
+Containers and contents guarded by mutex:
+
+`CUDTUnited::m_GlobControlLock` - guards all containers in CUDTUnited.
+
+`CUDTSocket::m_ControlLock` - guards internal operation performed on particular
+socket, with its existence assumed (this is because a socket will always exist
+until it's deleted while being in `m_ClosedSockets`, and when the socket is in
+`m_ClosedSockets` it will not be deleted until it's free from any operation,
+while the socket is assumed nonexistent for any newly called API function even
+if it exists physically, but is moved to `m_ClosedSockets`).
+
+`CUDTGroup::m_GroupLock` - guards the `m_Group` container inside a group that
+collects member sockets.
+
+There are unfortunately many situations when multiple locks have to be applied
+at a time. This is then the hierarchy of the mutexes that must be preserved
+everywhere in the code.
+
+As mutexes cannot be really ordered unanimously, below are two trees, with also
+some possible branches inside. The mutex marked with (T) is terminal, that is,
+no other locks shall be allowed in the section where this mutex is locked.
+
+### Mutex ordering information
+
+Note that the list isn't exactly complete, but it should contain all
+mutexes for which the locking order must be preserved.
+
+```
+
+ - CUDTSocket::m_ControlLock
+
+ - CUDT::m_ConnectionLock
+
+        - CRendezvousQueue::m_RIDVectorLock
+
+ - CUDTUnited::m_GlobControlLock
+
+ - CUDTGroup::m_GroupLock
+
+    - CUDT::m_RecvAckLock  || CEPoll::m_EPollLock(T)
+
+----------------
+ - CUDTUnited::m_GlobControlLock
+
+        - CUDTGroup::m_GroupLock  || CSndUList::m_ListLock(T)
+
+     - CUDT::m_ConnectionLock
+
+              - CRendezvousQueue::m_RIDVectorLock
+
+ - CUDT::m_SendLock
+
+    - CUDT::m_RecvLock
+
+       - CUDT::m_RecvBufferLock
+
+ - CUDT::m_RecvAckLock || CUDT::m_SendBlockLock
+------------------
+
+ANALYSIS ON: m_ConnectionLock
+
+
+-- CUDT::startConnect flow
+
+CUDTUnited::connectIn -- > [LOCKED s->m_ControlLock]
+    CUDT::open -- > [MAYBE_LOCKED m_ConnectionLock, if bind() not called]
+        CUDT::clearData --> [LOCKED m_StatsLock]
+    CUDTUnited::updateMux  -- > [LOCKED m_GlobControlLock]
+    {
+       [SCOPE UNLOCK s->m_ControlLock, if blocking mode]
+       CUDT::startConnect  -- > [LOCKED m_ConnectionLock]
+           CRcvQueue::registerConnector
+                CRendezvousQueue::insert --> [LOCKED CRendezvousQueue::m_RIDVectorLock]
+    }
+END.
+
+-- CUDT::groupConnect flow
+
+CUDT::groupConnect (no locks)
+   CUDT::setOpt [LOCKS m_ConnectionLock, m_SendLock, m_RecvLock]
+   { [LOCKS m_GlobControlLock]
+        CUDTGroup::add [LOCKS m_GroupLock]
+   }
+CUDT::connectIn --> continue with startConnect flow
+
+-- CUDTUnited::listen (API function)
+
+CUDTUnited::listen
+    CUDTUnited::locateSocket [LOCKS m_GlobControlLock]
+    {
+        [SCOPE LOCK s->m_ControlLock]
+        CUDT::setListenState -- > [LOCKED m_ConnectionLock]
+             CRcvQueue::setListener -- > [LOCKED m_LSLock]
+    }
+
+-- CUDT::processAsyncConnectRequest
+
+CRcvQueue::worker ->
+...
+CRcvQueue::worker_TryAsyncRend_OrStore
+     CUDT::processAsyncConnectResponse -- > [LOCKED m_ConnectionLock]
+         CUDT::processConnectResponse
+             CUDT::postConnect
+                 CUDT::interpretSrtHandshake ->
+                 [IF group extension found]
+                 CUDT::interpretGroup
+                 {
+                     [SCOPE LOCK m_GlobControlLock]
+                     [IF Responder]
+                     {
+                         CUDT::makeMePeerOf
+                             [LOCKS m_GroupLock]
+                                 CUDTGroup::syncWithSocket
+                             CUDTGroup::find --> [LOCKED m_GroupLock]
+                     }
+                     debugGroup -- > [LOCKED m_GroupLock]
+                 }
+
+
+-- CUDT::acceptAndRespond
+
+CRcvQueue::worker_ProcessConnectionRequest
+{
+     [SCOPE LOCK m_LSLock]
+     CUDT::processConnectRequest
+         CUDTUnited::newConnection
+             locateSocket -- > [LOCKED m_GlobControlLock]
+             locatePeer -- > [LOCKED m_GlobControlLock]
+             [IF failure, LOCK m_AcceptLock]
+             generateSocketID --> [LOCKED m_IDLock]
+             CUDT::open  -- > [LOCKED m_ConnectionLock]
+             CUDT::updateListenerMux  -- > [LOCKED m_GlobControlLock]
+             CUDT::acceptAndRespond --> [LOCKED m_ConnectionLock]
+                 CUDT::interpretSrtHandshake ->
+                 [IF group extension found]
+                 CUDT::interpretGroup
+                 {
+                     [SCOPE LOCK m_GlobControlLock]
+                     [IF Responder]
+                     {
+                         CUDT::makeMePeerOf
+                             [LOCKS m_GroupLock]
+                                 CUDTGroup::syncWithSocket
+                             CUDTGroup::find --> [LOCKED m_GroupLock]
+                     }
+                     debugGroup -- > [LOCKED m_GroupLock]
+                 }
+                 {
+                     [SCOPE LOCK m_GlobControlLock]
+                     CUDT::synchronizeWithGroup -- > [LOCKED m_GroupLock]
+                 }
+                 CRcvQueue::setNewEntry -- > [LOCKED CRcvQueue::m_IDLock]
+                 {
+                     [SCOPE LOCK m_GlobControlLock]
+                     {
+                         [SCOPE LOCK m_GroupLock]
+                     }
+                 }
+             {
+                 [SCOPE LOCK m_AcceptLock]
+                     CEPoll::update_events
+             }
+             [IF Rollback]
+             CUDT::closeInternal
+                 [LOCKING m_EPollLock]
+                 {
+                     [SCOPE LOCK m_ConnectionLock]
+                         [SCOPE LOCK m_SendLock]
+                             [SCOPE LOCK m_RecvLock]
+                                 [LOCKING m_RcvBufferLock]
+                 }
+             {
+                 [SCOPE LOCK m_GlobControlLock]
+                 CUDT::removeFromGroup --> [LOCKED m_GroupLock]
+             }
+
+         CEPoll::update_events
+}
+
+-- CUDT::bstats: TRT-LOCKED m_ConnectionLock 
+
+-- CUDT::packData
+
+CSndQueue::worker
+ CSndUList::pop -- > [LOCKED m_ListLock]
+     CUDT::packData 
+
+```
+