chore(bigtable): migrate to std::optional

[colinmoy]

Replacing explicit usages of absl::optional with std::optional in the Bigtable Client library as part of broader modernization efforts

[gemini-code-assist]

Code Review

This pull request migrates the codebase from absl::optional to C++17's std::optional (along with corresponding changes to nullopt and make_optional) across multiple BigQuery and Bigtable files. Feedback on the changes highlights a safety issue in google/cloud/bigtable/internal/data_connection_impl.cc, where source_ is dereferenced without checking if the underlying StatusOr is OK, potentially leading to undefined behavior or assertion failures.

[gemini-code-assist]

Dereferencing source_ (which is a std::optional<StatusOr<...>>) when the underlying StatusOr is not OK results in undefined behavior or an assertion failure. Since Metadata() returns a std::optional, we should safely return std::nullopt if source_ is not OK.

  std::optional<google::bigtable::v2::ResultSetMetadata> Metadata() override {
    if (!source_.has_value() || !source_->ok()) return std::nullopt;
    return (**source_)->Metadata();
  }

References

1. Scrutinize Edge Cases: Always consider failure modes. Do not assume a perfect world. Also, never ignore Status types. ^(link)
2. Prefer defensive code, such as explicit ok() checks, even if they seem redundant based on the current implementation of a framework, as the framework's contract may change in the future.

[codecov]

Codecov Report

❌ Patch coverage is 96.71362% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.24%. Comparing base (f1a8e57) to head (046e57c).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
...le/cloud/bigtable/internal/data_connection_impl.cc	66.66%	2 Missing ⚠️
google/cloud/bigtable/internal/metrics.cc	66.66%	2 Missing ⚠️
...loud/bigtable/internal/partial_result_set_source.h	0.00%	2 Missing ⚠️
...ogle/cloud/bigtable/tests/data_integration_test.cc	50.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16215   +/-   ##
=======================================
  Coverage   92.23%   92.24%           
=======================================
  Files        2265     2265           
  Lines      210205   210210    +5     
=======================================
+ Hits       193888   193899   +11     
+ Misses      16317    16311    -6     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.