VReplication: Properly handle target shards w/o a primary in Reshard

[mattlord]

Description

In the vtctldclient Reshard command, we were not checking that the target shard had a primary tablet in the shard record before we attempted to access it. This resulted in a client panic when doing a Reshard and a target shard did not have a primary tablet. The Reshard would have failed anyway, but obviously we should not panic.

The new test case fails on main this way:

❯ go test -v -timeout 30s -run ^TestReshardCreate$ vitess.io/vitess/go/vt/vtctl/workflow
=== RUN   TestReshardCreate
=== RUN   TestReshardCreate/basic
W0627 13:20:10.626498   71963 server.go:1753] Streams will not be started since --auto-start is set to false
    resharder_test.go:158: ReshardCreate response: shard_streams:{key:"targetks/-80" value:{streams:{id:1 tablet:{cell:"cell" uid:200} source_shard:"targetks/0" position:"9d10e6ec-07a0-11ee-ae73-8e53f4cf3083:1-97" status:"Running" info:"VStream Lag: 0s"}}} shard_streams:{key:"targetks/0" value:{streams:{id:1 tablet:{cell:"cell" uid:100} source_shard:"targetks/0" position:"9d10e6ec-07a0-11ee-ae73-8e53f4cf3083:1-97" status:"Running" info:"VStream Lag: 0s"}}} shard_streams:{key:"targetks/80-" value:{streams:{id:1 tablet:{cell:"cell" uid:210} source_shard:"targetks/0" position:"9d10e6ec-07a0-11ee-ae73-8e53f4cf3083:1-97" status:"Running" info:"VStream Lag: 0s"}}} traffic_state:"Reads Not Switched. Writes Not Switched"
=== RUN   TestReshardCreate/no_primary
--- FAIL: TestReshardCreate (0.01s)
    --- PASS: TestReshardCreate/basic (0.00s)
    --- FAIL: TestReshardCreate/no_primary (0.00s)
panic: runtime error: invalid memory address or nil pointer dereference [recovered]
        panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x28 pc=0x1053cb16c]

goroutine 71 [running]:
testing.tRunner.func1.2({0x105fb71a0, 0x106edd380})
        /Users/matt/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1631 +0x1c4
testing.tRunner.func1()
        /Users/matt/go/pkg/mod/golang.org/[email protected]/src/testing/testing.go:1634 +0x33c
panic({0x105fb71a0?, 0x106edd380?})
        /Users/matt/go/pkg/mod/golang.org/[email protected]/src/runtime/panic.go:770 +0x124
vitess.io/vitess/go/vt/topo.(*Server).GetTablet(0x0?, {0x106241c38, 0x140002eb0a0}, 0x0)
        /Users/matt/git/vitess/go/vt/topo/tablet.go:177 +0x3c
vitess.io/vitess/go/vt/vtctl/workflow.(*Server).buildResharder(0x1400082cdb0, {0x106241c38, 0x140002eb0a0}, {0x1058b4995, 0x8}, {0x1058ae754, 0x3}, {0x14000431020, 0x1, 0x1076c0608?}, ...)

This is a pretty simple fix, so I think that we should backport this to v18 — when the vtctldclient Reshard command command was first added.

Related Issue(s)

• Fixes: Bug Report: vtctldclient Reshard panic when target shard has no PRIMARY tablet #16284

Checklist

• "Backport to:" labels have been added if this change should be back-ported to release branches
• If this change is to be back-ported to previous releases, a justification is included in the PR description
• Tests were added or are not required
• Did the new or modified tests pass consistently locally and on CI?
• Documentation was added or is not required

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[codecov]

Codecov Report

Attention: Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.

Project coverage is 68.68%. Comparing base (96f9c3d) to head (ec43b47).
Report is 2 commits behind head on main.

Files	Patch %	Lines
go/vt/vtctl/workflow/server.go	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16283      +/-   ##
==========================================
+ Coverage   68.60%   68.68%   +0.07%     
==========================================
  Files        1544     1544              
  Lines      198018   198068      +50     
==========================================
+ Hits       135859   136040     +181     
+ Misses      62159    62028     -131     

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