[mle] re-attach on FTD/MTD mode changes or from rx-on to sleepy (#8484)

This commit updates behavior when device mode gets changed
so to re-attach on switching between MTD/FTD modes and also on
switching from rx-on to sleepy (rx-off) mode (when attached as a
child).

This commit also updates `test-027-child-mode-change.py` to match
the new behavior.

src/core/thread/mle.cpp

@@ -815,30 +815,27 @@ Error Mle::SetDeviceMode(DeviceMode aDeviceMode)
 
     IgnoreError(Store());
 
-    switch (mRole)
-    {
-    case kRoleDisabled:
-        break;
+    // We need to re-attach on switching between MTD/FTD modes
+    // and also on switching from rx-on to sleepy (rx-off) mode.
 
-    case kRoleDetached:
+    if (IsAttached() && ((oldMode.IsFullThreadDevice() != mDeviceMode.IsFullThreadDevice()) ||
+                         (oldMode.IsRxOnWhenIdle() && !mDeviceMode.IsRxOnWhenIdle())))
+    {
+        mAttachCounter = 0;
+        IgnoreError(BecomeDetached());
+        ExitNow();
+    }
+
+    if (IsDetached())
+    {
         mAttachCounter = 0;
         SetStateDetached();
         Attach(kAnyPartition);
-        break;
-
-    case kRoleChild:
+    }
+    else if (IsChild())
+    {
         SetStateChild(GetRloc16());
         IgnoreError(SendChildUpdateRequest());
-        break;
-
-    case kRoleRouter:
-    case kRoleLeader:
-        if (oldMode.IsFullThreadDevice() && !mDeviceMode.IsFullThreadDevice())
-        {
-            IgnoreError(BecomeDetached());
-        }
-
-        break;
     }
 
 exit:

tests/scripts/expect/cli-router.exp

@@ -52,6 +52,7 @@ expect_line "Done"
 switch_node 2
 send "mode rdn\n"
 expect_line "Done"
+sleep 5
 send "state router\n"
 expect_line "Done"
 wait_for "router list" $router_id

tests/toranj/ncp/test-027-child-mode-change.py

@@ -127,8 +127,6 @@ child2_ml_address = child2.get(wpan.WPAN_IP6_MESH_LOCAL_ADDRESS)[1:-1]
 
 sender = parent.prepare_tx(parent_ml_address, child2_ml_address, 800, NUM_MSGS)
 
-child2_rx_ip_counter = int(child2.get(wpan.WPAN_NCP_COUNTER_RX_IP_SEC_TOTAL), 0)
-
 wpan.Node.perform_async_tx_rx()
 
 verify(sender.was_successful)
@@ -144,13 +142,6 @@ verify(int(child2.get(wpan.WPAN_THREAD_DEVICE_MODE), 0) == DEVICE_MODE_END_DEVIC
 # Verify that the child table on parent is also updated
 wpan.verify_within(check_child_table, WAIT_INTERVAL)
 
-
-def check_child2_received_msg():
-    verify(int(child2.get(wpan.WPAN_NCP_COUNTER_RX_IP_SEC_TOTAL), 0) >= child2_rx_ip_counter + NUM_MSGS)
-
-
-wpan.verify_within(check_child2_received_msg, WAIT_INTERVAL)
-
 # Reset parent and verify all children are recovered
 parent.reset()
 wpan.verify_within(check_child_table, WAIT_INTERVAL)