bugfix: fix nil pointer in vtgate on topo connection error

[olyazavr]

Description

We see vtgate container restarts whenever there's a problem connecting to zk:

2023-01-18T22:12:28.921215833Z E0118 22:12:28.921183       6 keyspace_events.go:352] error while watching keyspace "SyncStore": zk: could not connect to a server
2023-01-18T22:12:28.921252801Z E0118 22:12:28.921232       6 keyspace_events.go:352] error while watching keyspace "SyncStore": zk: could not connect to a server
2023-01-18T22:12:28.923405294Z panic: runtime error: invalid memory address or nil pointer dereference
2023-01-18T22:12:28.923409629Z [signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0xe4d153]
2023-01-18T22:12:28.923411603Z 
2023-01-18T22:12:28.923413422Z goroutine 113 [running]:
2023-01-18T22:12:28.923415237Z vitess.io/vitess/go/vt/topo/topoproto.SrvKeyspaceGetPartition(...)
2023-01-18T22:12:28.923417145Z 	vitess.io/vitess/go/vt/topo/topoproto/srvkeyspace.go:54
2023-01-18T22:12:28.923419039Z vitess.io/vitess/go/vt/discovery.(*keyspaceState).ensureConsistentLocked(0x1b9a040?)
2023-01-18T22:12:28.923421892Z 	vitess.io/vitess/go/vt/discovery/keyspace_events.go:222 +0x33
2023-01-18T22:12:28.923423694Z vitess.io/vitess/go/vt/discovery.(*keyspaceState).onHealthCheck(0xc029529d10, 0xc0295bbdb0)
2023-01-18T22:12:28.923437929Z 	vitess.io/vitess/go/vt/discovery/keyspace_events.go:328 +0x1ba
2023-01-18T22:12:28.923443219Z vitess.io/vitess/go/vt/discovery.(*KeyspaceEventWatcher).processHealthCheck(0xc0006b5fa8?, 0xc0295bbdb0)
2023-01-18T22:12:28.923445196Z 	vitess.io/vitess/go/vt/discovery/keyspace_events.go:403 +0x3c
2023-01-18T22:12:28.923450507Z vitess.io/vitess/go/vt/discovery.(*KeyspaceEventWatcher).run.func1()
2023-01-18T22:12:28.92345682Z 	vitess.io/vitess/go/vt/discovery/keyspace_events.go:194 +0x7b
2023-01-18T22:12:28.923459099Z created by vitess.io/vitess/go/vt/discovery.(*KeyspaceEventWatcher).run
2023-01-18T22:12:28.923460969Z 	vitess.io/vitess/go/vt/discovery/keyspace_events.go:183 +0xf8

This is not desirable, there should be an error, but it should not kill the whole vtgate process.

This is called from this healthcheck code, which does a nil check above, but then proceeds to this method without another nil check. If the return value of the method I have modified is nil, the healthcheck assumes the kss is in some sort of transient state and returns out, so this is safe to return nil from

The error we see above the nil pointer comes from this same file

Related Issue(s)

Checklist

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[vitess-bot]

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[harshit-gangal]

changes look good. But, a test would be needed here.

[olyazavr]

@harshit-gangal added test to ensure the error is not thrown if newKeyspace is nil. Also verified that without the fix, this test fails