pubsub/kafkapub: fix nil pointer dereference

[danielorbach]

ExampleOpenTopic (pubsub/kafkapubsub/example_test.go) show a simple call to OpenTopic without kafkapubsub.TopicOptions as passing a nil argument.
This is done by replacing a nil pointer with the appropriate zero-value. This behaviour exists in OpenSubscription too. However, PR #3163 introduced overriding batching options which inadvertently accesses a field of a nil pointer.

/cc @vangent I see you authored the relevant files, could you please take a look?

P.S> This is my first commit to this repository (read the contributing guidelines though) - so couple of questions:

1. Would you like me to report a concomitant bug and reference it from the commit's message?
2. Markdown in commit body - yes/no?

Local testing

$ go test ./...
?       gocloud.dev     [no test files]
ok      gocloud.dev/aws (cached)
ok      gocloud.dev/aws/awscloud        (cached) [no tests to run]
?       gocloud.dev/aws/rds     [no test files]
?       gocloud.dev/azure/azurecloud    [no test files]
?       gocloud.dev/azure/azuredb       [no test files]
ok      gocloud.dev/blob        (cached)
ok      gocloud.dev/blob/azureblob      (cached)
?       gocloud.dev/blob/driver [no test files]
?       gocloud.dev/blob/drivertest     [no test files]
ok      gocloud.dev/blob/fileblob       (cached)
ok      gocloud.dev/blob/gcsblob        (cached)
ok      gocloud.dev/blob/memblob        1.035s
ok      gocloud.dev/blob/s3blob (cached)
ok      gocloud.dev/docstore    (cached)
ok      gocloud.dev/docstore/awsdynamodb        (cached)
ok      gocloud.dev/docstore/driver     (cached)
?       gocloud.dev/docstore/drivertest [no test files]
ok      gocloud.dev/docstore/gcpfirestore       (cached)
ok      gocloud.dev/docstore/internal/fields    (cached)
ok      gocloud.dev/docstore/memdocstore        (cached)
ok      gocloud.dev/gcerrors    0.153s
ok      gocloud.dev/gcp (cached)
?       gocloud.dev/gcp/cloudsql        [no test files]
ok      gocloud.dev/gcp/gcpcloud        (cached) [no tests to run]
ok      gocloud.dev/internal/escape     (cached)
ok      gocloud.dev/internal/gcerr      (cached)
ok      gocloud.dev/internal/oc (cached)
ok      gocloud.dev/internal/openurl    (cached)
ok      gocloud.dev/internal/releasehelper      3.097s
ok      gocloud.dev/internal/retry      (cached)
?       gocloud.dev/internal/testing/octest     [no test files]
?       gocloud.dev/internal/testing/setup      [no test files]
?       gocloud.dev/internal/testing/terraform  [no test files]
ok      gocloud.dev/internal/testing/test-summary       (cached)
?       gocloud.dev/internal/useragent  [no test files]
ok      gocloud.dev/mysql       2.842s
ok      gocloud.dev/mysql/awsmysql      3.133s
ok      gocloud.dev/mysql/azuremysql    2.184s
ok      gocloud.dev/mysql/gcpmysql      0.158s
ok      gocloud.dev/postgres    (cached)
ok      gocloud.dev/postgres/awspostgres        2.520s
ok      gocloud.dev/postgres/gcppostgres        0.822s
ok      gocloud.dev/pubsub      (cached)
ok      gocloud.dev/pubsub/awssnssqs    (cached)
ok      gocloud.dev/pubsub/azuresb      (cached)
ok      gocloud.dev/pubsub/batcher      (cached)
?       gocloud.dev/pubsub/driver       [no test files]
?       gocloud.dev/pubsub/drivertest   [no test files]
ok      gocloud.dev/pubsub/gcppubsub    (cached)
ok      gocloud.dev/pubsub/mempubsub    (cached)
ok      gocloud.dev/runtimevar  (cached)
ok      gocloud.dev/runtimevar/awsparamstore    (cached)
ok      gocloud.dev/runtimevar/awssecretsmanager        (cached)
ok      gocloud.dev/runtimevar/blobvar  (cached)
ok      gocloud.dev/runtimevar/constantvar      (cached)
?       gocloud.dev/runtimevar/driver   [no test files]
?       gocloud.dev/runtimevar/drivertest       [no test files]
ok      gocloud.dev/runtimevar/filevar  (cached)
ok      gocloud.dev/runtimevar/gcpruntimeconfig (cached)
ok      gocloud.dev/runtimevar/gcpsecretmanager (cached)
ok      gocloud.dev/runtimevar/httpvar  (cached)
ok      gocloud.dev/secrets     (cached)
ok      gocloud.dev/secrets/awskms      (cached)
ok      gocloud.dev/secrets/azurekeyvault       (cached)
?       gocloud.dev/secrets/driver      [no test files]
?       gocloud.dev/secrets/drivertest  [no test files]
ok      gocloud.dev/secrets/gcpkms      (cached)
ok      gocloud.dev/secrets/localsecrets        (cached)
ok      gocloud.dev/server      (cached)
?       gocloud.dev/server/driver       [no test files]
ok      gocloud.dev/server/health       (cached)
ok      gocloud.dev/server/health/sqlhealth     (cached)
ok      gocloud.dev/server/requestlog   (cached)
?       gocloud.dev/server/sdserver     [no test files]
?       gocloud.dev/server/xrayserver   [no test files]

[vangent]

Thanks for the fix! Looks fine, no need for a bug.

[codecov]

Codecov Report

Merging #3211 (a5b7e6c) into master (664ae9b) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3211      +/-   ##
==========================================
- Coverage   72.99%   72.98%   -0.01%     
==========================================
  Files         112      112              
  Lines       14358    14361       +3     
==========================================
+ Hits        10480    10481       +1     
- Misses       3150     3152       +2     
  Partials      728      728              

Impacted Files	Coverage Δ
pubsub/kafkapubsub/kafka.go	84.74% <100.00%> (+0.15%)	⬆️
blob/s3blob/s3blob.go	89.43% <0.00%> (-0.28%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.