tests: Validate subchannel state transitions (#31971)

test/core/client_channel/lb_policy/lb_policy_test_lib.h

@@ -41,6 +41,7 @@
 #include "absl/types/optional.h"
 #include "absl/types/span.h"
 #include "absl/types/variant.h"
+#include "gmock/gmock.h"
 #include "gtest/gtest.h"
 
 #include <grpc/event_engine/event_engine.h>
@@ -160,10 +161,49 @@ class LoadBalancingPolicyTest : public ::testing::Test {
 
     const std::string& address() const { return address_; }
 
+    void AssertValidConnectivityStateTransition(
+        grpc_connectivity_state from_state, grpc_connectivity_state to_state,
+        SourceLocation location = SourceLocation()) {
+      switch (from_state) {
+        case GRPC_CHANNEL_IDLE:
+          ASSERT_EQ(to_state, GRPC_CHANNEL_CONNECTING)
+              << ConnectivityStateName(from_state) << "=>"
+              << ConnectivityStateName(to_state) << "\n"
+              << location.file() << ":" << location.line();
+          break;
+        case GRPC_CHANNEL_CONNECTING:
+          ASSERT_THAT(to_state,
+                      ::testing::AnyOf(GRPC_CHANNEL_READY,
+                                       GRPC_CHANNEL_TRANSIENT_FAILURE))
+              << ConnectivityStateName(from_state) << "=>"
+              << ConnectivityStateName(to_state) << "\n"
+              << location.file() << ":" << location.line();
+          break;
+        case GRPC_CHANNEL_READY:
+          ASSERT_EQ(to_state, GRPC_CHANNEL_IDLE)
+              << ConnectivityStateName(from_state) << "=>"
+              << ConnectivityStateName(to_state) << "\n"
+              << location.file() << ":" << location.line();
+          break;
+        case GRPC_CHANNEL_TRANSIENT_FAILURE:
+          ASSERT_EQ(to_state, GRPC_CHANNEL_IDLE)
+              << ConnectivityStateName(from_state) << "=>"
+              << ConnectivityStateName(to_state) << "\n"
+              << location.file() << ":" << location.line();
+          break;
+        default:
+          FAIL() << ConnectivityStateName(from_state) << "=>"
+                 << ConnectivityStateName(to_state) << "\n"
+                 << location.file() << ":" << location.line();
+          break;
+      }
+    }
+
     // Sets the connectivity state for this subchannel.  The updated state
     // will be reported to all associated SubchannelInterface objects.
     void SetConnectivityState(grpc_connectivity_state state,
-                              const absl::Status& status = absl::OkStatus()) {
+                              const absl::Status& status = absl::OkStatus(),
+                              SourceLocation location = SourceLocation()) {
       if (state == GRPC_CHANNEL_TRANSIENT_FAILURE) {
         EXPECT_FALSE(status.ok())
             << "bug in test: TRANSIENT_FAILURE must have non-OK status";
@@ -173,6 +213,8 @@ class LoadBalancingPolicyTest : public ::testing::Test {
             << " must have OK status: " << status;
       }
       MutexLock lock(&mu_);
+      AssertValidConnectivityStateTransition(state_tracker_.state(), state,
+                                             location);
       state_tracker_.SetState(state, status, "set from test");
     }