adapt to NULL callbacks

src/baseprotocol.c

@@ -260,6 +260,16 @@ PEP_STATUS try_base_prepare_message(
         message **result
     )
 {
+    /* Special case: if messageToSend is not defined there is no way to handle
+       passphrases: in that case just exit with PEP_SYNC_NO_CHANNEL.  This is
+       required for pEp4Thunderbird (P4TB-413) with the most recent
+       libpEpAdapter (master) and JSONServerAdapter (master) as of 2021-11-05:
+       JSONServerAdapter performs a temporary incomplete initialisation by
+       supplying some NULL callbacks, and initialises in a complete way only
+       later. */
+    if (session->messageToSend == NULL)
+        return PEP_SYNC_NO_CHANNEL;
+
     PEP_STATUS status = PEP_STATUS_OK;
 
     assert(session && session->messageToSend && session->notifyHandshake);

src/sync_api.c

@@ -12,6 +12,28 @@
 
 #include "KeySync_fsm.h"
 
+/* A dummy function performing no useful work, usable as a notifyHandshake
+   function.  Notice that this destroys its heap-allocated arguments, since the
+   callback function is supposed to take ownership of them. */
+static PEP_STATUS notifyHandshake_dummy(
+        pEp_identity *me,
+        pEp_identity *partner,
+        sync_handshake_signal signal
+    )
+{
+    free_identity(me);
+    free_identity(partner);
+}
+
+
+/* Like notifyHandshake_dummy , but for retrieve_next_sync_event_t . */
+static SYNC_EVENT retrieve_next_sync_event_dummy(void *management,
+        unsigned threshold)
+{
+    /* Do nothing. */
+}
+
+
 DYNAMIC_API PEP_STATUS register_sync_callbacks(
         PEP_SESSION session,
         void *management,
@@ -19,6 +41,14 @@ DYNAMIC_API PEP_STATUS register_sync_callbacks(
         retrieve_next_sync_event_t retrieve_next_sync_event
     )
 {
+    /* In case the callbacks are null pointers, replace them with dummy
+       functions.  This makes the code more robust elsewhere and handles object
+       ownership in a reasonable way. */
+    if (notifyHandshake == NULL)
+        notifyHandshake = notifyHandshake_dummy;
+    if (retrieve_next_sync_event == NULL)
+        retrieve_next_sync_event = retrieve_next_sync_event_dummy;
+
     assert(session && notifyHandshake && retrieve_next_sync_event);
     if (!(session && notifyHandshake && retrieve_next_sync_event))
         return PEP_ILLEGAL_VALUE;
@@ -46,8 +76,8 @@ DYNAMIC_API void unregister_sync_callbacks(PEP_SESSION session) {
 
     // unregister
     session->sync_management = NULL;
-    session->notifyHandshake = NULL;
-    session->retrieve_next_sync_event = NULL;
+    session->notifyHandshake = notifyHandshake_dummy;
+    session->retrieve_next_sync_event = retrieve_next_sync_event_dummy;
 }
 
 DYNAMIC_API PEP_STATUS deliverHandshakeResult(