[test] avoid FRIEND_TEST which is discouraged (#346)

This commit removes usage of the discouraged FRIEND_TEST and use a test
peer instead to access private methods and variables.

Author: bukepo

src/library/commissioner_impl.hpp

@@ -32,7 +32,6 @@
 #include <chrono>
 #include <cstdint>
 #include <functional>
-#include <gtest/gtest_prod.h>
 #include <map>
 #include <string>
 #include <vector>
@@ -57,6 +56,8 @@ namespace ot {
 
 namespace commissioner {
 
+class CommissionerTestPeer;
+
 // This is the implementation of the Commissioner interface with
 // event-driven model. The commissioner is not running unless the
 // the event loop has been started. Starting the event loop will
@@ -75,6 +76,7 @@ namespace commissioner {
 class CommissionerImpl : public Commissioner
 {
     friend class JoinerSession;
+    friend class CommissionerTestPeer;
 
 public:
     explicit CommissionerImpl(CommissionerHandler &aHandler, struct event_base *aEventBase);
@@ -299,8 +301,6 @@ class CommissionerImpl : public Commissioner
 
     coap::Resource mResourceDiagAns;
     NetDiagData    mDiagAnsTlvs;
-
-    FRIEND_TEST(CommissionerSafeTestProxyMode, ShouldBeAbleToSendToJoinerIfJoinerSessionExists);
 };
 
 /*

src/library/commissioner_safe.hpp

@@ -36,7 +36,6 @@
 
 #include <event2/event.h>
 #include <event2/util.h>
-#include <gtest/gtest_prod.h>
 #include <stdint.h>
 
 #include <functional>
@@ -58,6 +57,8 @@ namespace ot {
 
 namespace commissioner {
 
+class CommissionerTestPeer;
+
 /**
  * This class implements the Commissioner interface.
  *
@@ -71,6 +72,8 @@ namespace commissioner {
  */
 class CommissionerSafe : public Commissioner
 {
+    friend class CommissionerTestPeer;
+
 public:
     explicit CommissionerSafe(CommissionerHandler &aHandler)
         : mHandler(aHandler)
@@ -241,9 +244,6 @@ class CommissionerSafe : public Commissioner
 
     // The even loop thread running in background.
     std::thread mEventThread;
-
-    FRIEND_TEST(CommissionerSafeTestProxyMode, ShouldBeAbleToReceiveJoinerMessage);
-    FRIEND_TEST(CommissionerSafeTestProxyMode, ShouldBeAbleToSendToJoinerIfJoinerSessionExists);
 };
 
 } // namespace commissioner

src/library/commissioner_safe_test.cpp

@@ -47,6 +47,7 @@
 #include "commissioner/error.hpp"
 #include "common/address.hpp"
 #include "library/commissioner_safe.hpp"
+#include "library/commissioner_test_peer.hpp"
 #include "library/joiner_session.hpp"
 
 using testing::ContainerEq;
@@ -99,7 +100,8 @@ TEST(CommissionerSafeTestProxyMode, ShouldBeAbleToReceiveJoinerMessage)
 
     EXPECT_CALL(mockHandler, OnJoinerMessage(ContainerEq(joinerId), 5400, ContainerEq(payload))).Times(1);
 
-    JoinerSession session(*static_cast<CommissionerSafe &>(*commissioner).mImpl, joinerId, std::string(), 12345, 0x1200,
+    auto         &commissionerSafe = static_cast<CommissionerSafe &>(*commissioner);
+    JoinerSession session(*CommissionerTestPeer::GetImpl(commissionerSafe), joinerId, std::string(), 12345, 0x1200,
                           Address(), 12344, Address(), 12343);
     session.RecvJoinerDtlsRecords(payload, 5400);
     sleep(1);
@@ -124,8 +126,9 @@ TEST(CommissionerSafeTestProxyMode, ShouldBeAbleToSendToJoinerIfJoinerSessionExi
     ByteArray joinerId{1, 2, 3, 4, 5, 6, 7, 8};
     ByteArray payload{1, 2, 3};
 
-    auto impl = static_cast<CommissionerSafe &>(*commissioner).mImpl;
-    impl->mJoinerSessions.emplace(
+    auto &commissionerSafe = static_cast<CommissionerSafe &>(*commissioner);
+    auto  impl             = CommissionerTestPeer::GetImpl(commissionerSafe);
+    CommissionerTestPeer::GetJoinerSessions(*impl).emplace(
         std::piecewise_construct, std::forward_as_tuple(joinerId),
         std::forward_as_tuple(*impl, joinerId, std::string(), 12345, 0x1200, Address(), 12344, Address(), 12343));
 

src/library/commissioner_test_peer.hpp

@@ -0,0 +1,62 @@
+/*
+ *    Copyright (c) 2026, The OpenThread Commissioner Authors.
+ *    All rights reserved.
+ *
+ *    Redistribution and use in source and binary forms, with or without
+ *    modification, are permitted provided that the following conditions are met:
+ *    1. Redistributions of source code must retain the above copyright
+ *       notice, this list of conditions and the following disclaimer.
+ *    2. Redistributions in binary form must reproduce the above copyright
+ *       notice, this list of conditions and the following disclaimer in the
+ *       documentation and/or other materials provided with the distribution.
+ *    3. Neither the name of the copyright holder nor the
+ *       names of its contributors may be used to endorse or promote products
+ *       derived from this software without specific prior written permission.
+ *
+ *    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ *    AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ *    IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
+ *    ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE
+ *    LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+ *    CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ *    SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ *    INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ *    CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ *    ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ *    POSSIBILITY OF SUCH DAMAGE.
+ */
+
+/**
+ * @file
+ *   This file defines a test peer for Commissioner classes to access private members in tests.
+ */
+
+#ifndef OT_COMM_LIBRARY_COMMISSIONER_TEST_PEER_HPP_
+#define OT_COMM_LIBRARY_COMMISSIONER_TEST_PEER_HPP_
+
+#include <map>
+#include <memory>
+
+#include "library/commissioner_impl.hpp"
+#include "library/commissioner_safe.hpp"
+
+namespace ot {
+
+namespace commissioner {
+
+class CommissionerTestPeer
+{
+public:
+    static std::shared_ptr<CommissionerImpl> &GetImpl(CommissionerSafe &aSafe) { return aSafe.mImpl; }
+
+    static std::map<ByteArray, JoinerSession> &GetJoinerSessions(CommissionerImpl &aImpl)
+    {
+        return aImpl.mJoinerSessions;
+    }
+};
+
+} // namespace commissioner
+
+} // namespace ot
+
+#endif // OT_COMM_LIBRARY_COMMISSIONER_TEST_PEER_HPP_