release-26.3.1-rc: pkg/sql/partitioning/partitioning_test: TestValidIndexPartitionSetShowZones failed

pkg/sql/partitioning/partitioning_test.TestValidIndexPartitionSetShowZones [failed](https://mesolite.cluster.engflow.com/invocations/default/bf894504-ddb9-466f-a3ec-a3ce3807ede7?testReportRun=8&testReportShard=1&testReportAttempt=1#targets-Ly9wa2cvc3FsL3BhcnRpdGlvbmluZzpwYXJ0aXRpb25pbmdfdGVzdA==) on release-26.3.1-rc @ [3992bce5410f59c25c4bf2ef2354b861a58a3d2e](https://github.com/cockroachdb/cockroach/commits/3992bce5410f59c25c4bf2ef2354b861a58a3d2e):

Fatal error:

```
panic: runtime error: invalid memory address or nil pointer dereference [recovered, repanicked]
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x51b5e0d]

```
Stack:

```
goroutine 60776 [running]:
testing.tRunner.func1.2({0x5ada880, 0xb8a1020})
GOROOT/src/testing/testing.go:1974 +0x232
testing.tRunner.func1()
GOROOT/src/testing/testing.go:1977 +0x349
panic({0x5ada880?, 0xb8a1020?})
GOROOT/src/runtime/panic.go:860 +0x13a
github.com/cockroachdb/cockroach/pkg/util/leaktest.AfterTest.func2()
pkg/util/leaktest/leaktest.go:137 +0x3a9
panic({0x5ada880?, 0xb8a1020?})
GOROOT/src/runtime/panic.go:860 +0x13a
github.com/cockroachdb/cockroach/pkg/util/stop.(*Stopper).recover(0x48c305?, {0x699f2d0, 0xc02c6c0})
pkg/util/stop/stopper.go:226 +0x5e
panic({0x5ada880?, 0xb8a1020?})
GOROOT/src/runtime/panic.go:860 +0x13a
github.com/cockroachdb/cockroach/pkg/util/stop.(*Stopper).Stop(0x1404055375f0, {0x699f2d0, 0xc02c6c0})
pkg/util/stop/stopper.go:575 +0x155
panic({0x5ada880?, 0xb8a1020?})
GOROOT/src/runtime/panic.go:860 +0x13a
github.com/cockroachdb/cockroach/pkg/server.(*testTenant).DefaultZoneConfig(0x0?)
pkg/server/testserver.go:1316 +0x6d
github.com/cockroachdb/cockroach/pkg/testutils/serverutils.(*wrap).DefaultZoneConfig(0x0?)
bazel-out/k8-fastbuild/bin/pkg/testutils/serverutils/app_forwarder_generated.go:396 +0x86
pkg/sql/partitioning/partitioning_test_test.TestValidIndexPartitionSetShowZones(0x1404031d98c8)
pkg/sql/partitioning/zone_test.go:65 +0x1c9
testing.tRunner(0x1404031d98c8, 0x69125f8)
GOROOT/src/testing/testing.go:2036 +0xea
created by testing.(*T).Run in goroutine 1
GOROOT/src/testing/testing.go:2101 +0x4c5
```
Log preceding fatal error

```
=== RUN TestValidIndexPartitionSetShowZones
test_log_scope.go:172: test logs captured to: outputs.zip/logTestValidIndexPartitionSetShowZones1597659028
test_log_scope.go:82: use -show-logs to present logs inline
test_server_shim.go:168: automatically injected a shared process virtual cluster under test; see comment at top of test_server_shim.go for details.
stopper.go:226: -- test log scope end --

ERROR: a panic has occurred!
Details cannot be printed yet because we are still unwinding.
Hopefully the test harness prints the panic below, otherwise check the test logs.

test logs left over in: outputs.zip/logTestValidIndexPartitionSetShowZones1597659028
stopper.go:226: panic: runtime error: invalid memory address or nil pointer dereference
--- FAIL: TestValidIndexPartitionSetShowZones (1.51s)
```

Parameters:
- attempt=1
- run=8
- shard=1
Help

See also: [How To Investigate a Go Test Failure \(internal\)](https://cockroachlabs.atlassian.net/l/c/HgfXfJgM)

[This test on roachdash](https://roachdash.crdb.dev/?filter=status:open%20t:.*TestValidIndexPartitionSetShowZones.*&sort=title+created&display=lastcommented+project) | [Improve this report!](https://github.com/cockroachdb/cockroach/tree/master/pkg/cmd/bazci/githubpost/issues)

Jira issue: CRDB-66771

Start by running pkg/sql/partitioning/zone_test.go, focusing on TestValidIndexPartitionSetShowZones and its failure logs. Trace the DefaultZoneConfig call through pkg/server/testserver.go and the generated serverutils wrapper, then investigate the nil-pointer panic during stopper shutdown. Done means the targeted test completes without the panic and passes.

Written by the indexing model from the issue text.