Using --checkOnly causes "panic: runtime error: invalid memory address or nil pointer dereference"

[keithquinnpercona]

Using the command without --checkOnly works fine but when it is used the follow error is shown.

./migration_verifier   --srcURI mongodb://10.188.3.235:27018   --dstURI mongodb://10.188.3.136:27018   --metaURI "mongodb://127.0.0.1:27017/?replicaSet=mvmeta"   --srcNamespace "$NS"   --dstNamespace "$NS" --checkOnly   --clean   --start
2025-12-15T13:40:36Z INF migration-verifier started. buildTime="Mon Dec  1 17:37:18 UTC 2025" processID=2552 revision=v0.0.13
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x68 pc=0xbe60cb]

goroutine 1 [running]:
github.com/10gen/migration-verifier/internal/verifier.(*Verifier).WritesOff(0xc0002bafc0, {0x1021380, 0x16c6980})
	/home/runner/work/migration-verifier/migration-verifier/internal/verifier/migration_verifier.go:294 +0x14b
main.main.func3(0xc0002002c0)
	/home/runner/work/migration-verifier/migration-verifier/main/migration_verifier.go:216 +0x8a
github.com/urfave/cli.HandleAction({0xd65720?, 0xc00000ea38?}, 0xc0002bae00?)
	/home/runner/work/migration-verifier/migration-verifier/vendor/github.com/urfave/cli/app.go:524 +0x70
github.com/urfave/cli.(*App).Run(0xc0002bae00, {0xc0000001c0, 0xe, 0xe})
	/home/runner/work/migration-verifier/migration-verifier/vendor/github.com/urfave/cli/app.go:286 +0x75b
main.main()
	/home/runner/work/migration-verifier/migration-verifier/main/migration_verifier.go:232 +0x1a4b

[FGasper]

Thank you for the report.

--checkOnly has been broken for some time. We do want to fix it, but FYI it’s not a high priority currently.

As a workaround, you can poll /progress periodically and wait for generation > 0; if there are any tasks, then either a mismatch was found, or some change event arrived during generation 0 (which probably shouldn’t happen if you’re only wanting the initial scan).