Fix producer panic by oldProducers

[wolfstudy]

Signed-off-by: xiaolongran xiaolongran@tencent.com

Motivation

panic: runtime error: index out of range [1] with length 1

goroutine 166288 [running]:
github.com/apache/pulsar-client-go/pulsar.(*producer).internalCreatePartitionsProducers(0xc0070aa6e0, 0x0, 0x0)
        /Users/lzhuang/scfproj/scf_trigger_v2/vendor/github.com/apache/pulsar-client-go/pulsar/producer_impl.go:194 +0x785
github.com/apache/pulsar-client-go/pulsar.(*producer).runBackgroundPartitionDiscovery.func1(0xc004167cd0, 0xc00559f5c0, 0xc006af6dc0, 0xc0070aa6e0)
        /Users/lzhuang/scfproj/scf_trigger_v2/vendor/github.com/apache/pulsar-client-go/pulsar/producer_impl.go:152 +0xce
created by github.com/apache/pulsar-client-go/pulsar.(*producer).runBackgroundPartitionDiscovery
        /Users/lzhuang/scfproj/scf_trigger_v2/vendor/github.com/apache/pulsar-client-go/pulsar/producer_impl.go:144 +0xcd

Modifications

indexing may panic because of 'nil' slice(oldProducers)

[freeznet]

LGTM

[wolfstudy]

==================
WARNING: DATA RACE
Read at 0x00c00071a038 by goroutine 332:
  github.com/apache/pulsar-client-go/pulsar.(*producer).getPartition()
      /pulsar-client-go/pulsar/producer_impl.go:281 +0xd6
  github.com/apache/pulsar-client-go/pulsar.(*producer).Send()
      /pulsar-client-go/pulsar/producer_impl.go:269 +0x61
  github.com/apache/pulsar-client-go/pulsar.TestConsumerAddTopicPartitions()
      /pulsar-client-go/pulsar/consumer_test.go:1463 +0x915
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1050 +0x1eb

Previous write at 0x00c00071a038 by goroutine 9:
  github.com/apache/pulsar-client-go/pulsar.(*producer).internalCreatePartitionsProducers()
      /pulsar-client-go/pulsar/producer_impl.go:179 +0x1e2
  github.com/apache/pulsar-client-go/pulsar.(*producer).runBackgroundPartitionDiscovery.func1()
      /pulsar-client-go/pulsar/producer_impl.go:153 +0xfa

Goroutine 332 (running) created at:
  testing.(*T).Run()
      /usr/local/go/src/testing/testing.go:1095 +0x537
  testing.runTests.func1()
      /usr/local/go/src/testing/testing.go:1339 +0xa6
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1050 +0x1eb
  testing.runTests()
      /usr/local/go/src/testing/testing.go:1337 +0x594
  testing.(*M).Run()
      /usr/local/go/src/testing/testing.go:1252 +0x2ff
  main.main()
      _testmain.go:432 +0x337

Goroutine 9 (running) created at:
  github.com/apache/pulsar-client-go/pulsar.(*producer).runBackgroundPartitionDiscovery()
      /pulsar-client-go/pulsar/producer_impl.go:145 +0x11b
  github.com/apache/pulsar-client-go/pulsar.newProducer()
      /pulsar-client-go/pulsar/producer_impl.go:133 +0x701
  github.com/apache/pulsar-client-go/pulsar.(*client).CreateProducer()
      /pulsar-client-go/pulsar/client_impl.go:150 +0xb4
  github.com/apache/pulsar-client-go/pulsar.TestConsumerAddTopicPartitions()
      /pulsar-client-go/pulsar/consumer_test.go:1433 +0x39a
  testing.tRunner()
      /usr/local/go/src/testing/testing.go:1050 +0x1eb
==================
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0xed0548]

goroutine 4816 [running]:
github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).getConsumerState(0x0, 0xc000000001)
	/pulsar-client-go/pulsar/consumer_partition.go:338 +0x48
github.com/apache/pulsar-client-go/pulsar.(*partitionConsumer).Close(0x0)
	/pulsar-client-go/pulsar/consumer_partition.go:347 +0x55
github.com/apache/pulsar-client-go/pulsar.(*consumer).Close.func1.1(0xc0000a4cc0, 0x0)
	/pulsar-client-go/pulsar/consumer_impl.go:519 +0x88
created by github.com/apache/pulsar-client-go/pulsar.(*consumer).Close.func1
	/pulsar-client-go/pulsar/consumer_impl.go:517 +0x1b8

[wolfstudy]

In this pr, a check for partition scale down is added to avoid the occurrence of the panic:

time="2021-08-24T21:18:27+08:00" level=info msg="[Changed number of partitions in topic]" new_partitions=1 old_partitions=20 topic="persistent://pulsar-ev297bm5n9j4/dogfood/1253970226_lam-1f67hbru"