[mle] add return error to otThreadSetRouterEligible (#4363)

When trying to set router-eligible and the device is not capable of
becoming a router, return OT_ERROR_NOT_CAPABLE.

include/openthread/thread_ftd.h

@@ -142,8 +142,11 @@ bool otThreadIsRouterEligible(otInstance *aInstance);
  * @param[in]  aInstance  A pointer to an OpenThread instance.
  * @param[in]  aEligible  TRUE to configure the device as router-eligible, FALSE otherwise.
  *
+ * @retval OT_ERROR_NONE         Successfully set the router-eligible configuration.
+ * @retval OT_ERROR_NOT_CAPABLE  The device is not capable of becoming a router.
+ *
  */
-void otThreadSetRouterEligible(otInstance *aInstance, bool aEligible);
+otError otThreadSetRouterEligible(otInstance *aInstance, bool aEligible);
 
 /**
  * Set the preferred Router Id.

src/cli/cli.cpp

@@ -2777,11 +2777,11 @@ void Interpreter::ProcessRouterEligible(int argc, char *argv[])
     }
     else if (strcmp(argv[0], "enable") == 0)
     {
-        otThreadSetRouterEligible(mInstance, true);
+        error = otThreadSetRouterEligible(mInstance, true);
     }
     else if (strcmp(argv[0], "disable") == 0)
     {
-        otThreadSetRouterEligible(mInstance, false);
+        error = otThreadSetRouterEligible(mInstance, false);
     }
     else
     {

src/core/api/thread_ftd_api.cpp

@@ -65,11 +65,11 @@ bool otThreadIsRouterEligible(otInstance *aInstance)
     return instance.Get<Mle::MleRouter>().IsRouterEligible();
 }
 
-void otThreadSetRouterEligible(otInstance *aInstance, bool aEligible)
+otError otThreadSetRouterEligible(otInstance *aInstance, bool aEligible)
 {
     Instance &instance = *static_cast<Instance *>(aInstance);
 
-    instance.Get<Mle::MleRouter>().SetRouterEligible(aEligible);
+    return instance.Get<Mle::MleRouter>().SetRouterEligible(aEligible);
 }
 
 otError otThreadSetPreferredRouterId(otInstance *aInstance, uint8_t aRouterId)

src/core/thread/mle_router.cpp

@@ -102,8 +102,12 @@ bool MleRouter::IsRouterEligible(void) const
     return mRouterEligible && IsFullThreadDevice();
 }
 
-void MleRouter::SetRouterEligible(bool aEligible)
+otError MleRouter::SetRouterEligible(bool aEligible)
 {
+    otError error = OT_ERROR_NONE;
+
+    VerifyOrExit(IsFullThreadDevice(), error = OT_ERROR_NOT_CAPABLE);
+
     mRouterEligible = aEligible;
 
     switch (mRole)
@@ -125,6 +129,9 @@ void MleRouter::SetRouterEligible(bool aEligible)
 
         break;
     }
+
+exit:
+    return error;
 }
 
 otError MleRouter::BecomeRouter(ThreadStatusTlv::Status aStatus)

src/core/thread/mle_router_ftd.hpp

@@ -102,8 +102,11 @@ public:
      *
      * @param[in]  aEligible  TRUE to configure device router-eligible, FALSE otherwise.
      *
+     * @retval OT_ERROR_NONE         Successfully set the router-eligible configuration.
+     * @retval OT_ERROR_NOT_CAPABLE  The device is not capable of becoming a router.
+     *
      */
-    void SetRouterEligible(bool aEligible);
+    otError SetRouterEligible(bool aEligible);
 
     /**
      * This method indicates whether a node is the only router on the network.

src/ncp/ncp_base_ftd.cpp

@@ -304,7 +304,7 @@ template <> otError NcpBase::HandlePropertySet<SPINEL_PROP_THREAD_ROUTER_ROLE_EN
 
     SuccessOrExit(error = mDecoder.ReadBool(eligible));
 
-    otThreadSetRouterEligible(mInstance, eligible);
+    error = otThreadSetRouterEligible(mInstance, eligible);
 
 exit:
     return error;