lazy: Fix data race in TestGoodUpdateThenResolverError

[arjan-bal]

The test verifies that lazy's child get the latest resolver state followed by the latest resolver error. The test waits for an RPC to succeed before verifying the child has received the resolver error. The RPC can complete before the child get the resolver error. This PR uses a grpcsync.Event to wait till the child receives the resolver error.

Failing test run: https://github.com/grpc/grpc-go/actions/runs/13314475263/attempts/2?pr=8086

Failure stack trace

WARNING: DATA RACE Read at 0x00c00019c57f by goroutine 191: google.golang.org/grpc/balancer/lazy_test.s.TestGoodUpdateThenResolverError() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy_ext_test.go:289 +0x13b7 google.golang.org/grpc/balancer/lazy_test.(*s).TestGoodUpdateThenResolverError() <autogenerated>:1 +0x3e runtime.call16() /opt/hostedtoolcache/go/1.23.6/x[64](https://github.com/grpc/grpc-go/actions/runs/13314475263/job/37208920450?pr=8086#step:8:65)/src/runtime/asm_amd64.s:775 +0x42 reflect.callMethod() <autogenerated>:1 +0x56 reflect.methodValueCall() /opt/hostedtoolcache/go/1.23.6/x64/src/reflect/asm_amd64.s:76 +0x6d testing.tRunner() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1690 +0x226 testing.(*T).Run.gowrap1() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1743 +0x44 Previous write at 0x00c00019c57f by goroutine 200: google.golang.org/grpc/balancer/lazy_test.s.TestGoodUpdateThenResolverError.func3() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy_ext_test.go:216 +0xa4 google.golang.org/grpc/internal/balancer/stub.(*bal).ResolverError() /home/runner/work/grpc-go/grpc-go/internal/balancer/stub/stub.go:70 +0x94 google.golang.org/grpc/balancer/lazy.(*lazyBalancer).ExitIdle() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy.go:145 +0x586 google.golang.org/grpc/balancer/lazy.NewBalancer.func1.gowrap1() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy.go:[65](https://github.com/grpc/grpc-go/actions/runs/13314475263/job/37208920450?pr=8086#step:8:66) +0x33 Goroutine 191 (running) created at: testing.(*T).Run() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1743 +0x825 google.golang.org/grpc/internal/grpctest.RunSubTests() /home/runner/work/grpc-go/grpc-go/internal/grpctest/grpctest.go:114 +0x351 google.golang.org/grpc/balancer/lazy_test.Test() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy_ext_test.go:60 +0x34 testing.tRunner() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1690 +0x226 testing.(*T).Run.gowrap1() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1743 +0x44 Goroutine 200 (finished) created at: google.golang.org/grpc/balancer/lazy.NewBalancer.func1() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy.go:65 +0x8d sync.OnceFunc.func1() /opt/hostedtoolcache/go/1.23.6/x64/src/sync/oncefunc.go:27 +0x7b sync.(*Once).doSlow() /opt/hostedtoolcache/go/1.23.6/x64/src/sync/once.go:76 +0xe1 sync.(*Once).Do() /opt/hostedtoolcache/go/1.23.6/x64/src/sync/once.go:67 +0x44 sync.OnceFunc.func2() /opt/hostedtoolcache/go/1.23.6/x64/src/sync/oncefunc.go:32 +0x4a google.golang.org/grpc/balancer/lazy.(*idlePicker).Pick() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy.go:157 +0x35 google.golang.org/grpc.(*pickerWrapper).pick() /home/runner/work/grpc-go/grpc-go/picker_wrapper.go:156 +0x948 google.golang.org/grpc.(*ClientConn).getTransport() /home/runner/work/grpc-go/grpc-go/clientconn.go:1051 +0xae google.golang.org/grpc.(*csAttempt).getTransport() /home/runner/work/grpc-go/grpc-go/stream.go:470 +0x144 google.golang.org/grpc.newClientStreamWithParams.func2() /home/runner/work/grpc-go/grpc-go/stream.go:353 +0x33 google.golang.org/grpc.(*clientStream).withRetry() /home/runner/work/grpc-go/grpc-go/stream.go:790 +0x197 google.golang.org/grpc.newClientStreamWithParams() /home/runner/work/grpc-go/grpc-go/stream.go:365 +0x1797 google.golang.org/grpc.newClientStream.func3() /home/runner/work/grpc-go/grpc-go/stream.go:222 +0x191 google.golang.org/grpc.newClientStream() /home/runner/work/grpc-go/grpc-go/stream.go:257 +0xc69 google.golang.org/grpc.invoke() /home/runner/work/grpc-go/grpc-go/call.go:[66](https://github.com/grpc/grpc-go/actions/runs/13314475263/job/37208920450?pr=8086#step:8:67) +0xc6 google.golang.org/grpc.(*ClientConn).Invoke() /home/runner/work/grpc-go/grpc-go/call.go:37 +0x2f0 google.golang.org/grpc/interop/grpc_testing.(*testServiceClient).EmptyCall() /home/runner/work/grpc-go/grpc-go/interop/grpc_testing/test_grpc.pb.go:95 +0x1af google.golang.org/grpc/balancer/lazy_test.s.TestGoodUpdateThenResolverError() /home/runner/work/grpc-go/grpc-go/balancer/lazy/lazy_ext_test.go:281 +0x12dc google.golang.org/grpc/balancer/lazy_test.(*s).TestGoodUpdateThenResolverError() <autogenerated>:1 +0x3e runtime.call16() /opt/hostedtoolcache/go/1.23.6/x64/src/runtime/asm_amd64.s:775 +0x42 reflect.callMethod() <autogenerated>:1 +0x56 reflect.methodValueCall() /opt/hostedtoolcache/go/1.23.6/x64/src/reflect/asm_amd64.s:76 +0x6d testing.tRunner() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1[69](https://github.com/grpc/grpc-go/actions/runs/13314475263/job/37208920450?pr=8086#step:8:70)0 +0x226 testing.(*T).Run.gowrap1() /opt/hostedtoolcache/go/1.23.6/x64/src/testing/testing.go:1743 +0x44 

RELEASE NOTES: N/A

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.18%. Comparing base (fabe274) to head (87a41ab).
Report is 5 commits behind head on master.

Additional details and impacted files

@@ Coverage Diff @@ ## master #8088 +/- ## ========================================== - Coverage 82.20% 82.18% -0.02%  ========================================== Files 387 387 Lines 38928 38943 +15 ========================================== + Hits 31999 32005 +6  - Misses 5606 5615 +9  Partials 1323 1323 

see 23 files with indirect coverage changes