refactor(bigtable): reorganize query public types and files

[scotthart]

renames results.h to result_source_interface.h
collects all the RowStream, TupleStream, and associated iterators into a single header file row_stream.h
moves tests around appropriately

[gemini-code-assist]

Summary of Changes

Hello @scotthart, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request undertakes a significant refactoring effort to improve the organization and clarity of query-related types within the Bigtable and Spanner client libraries. By renaming a generic "results" header to a more specific "result source interface" and consolidating all stream-related classes and iterators into new "row stream" files, the changes aim to create a more intuitive and maintainable codebase. This reorganization helps delineate responsibilities and makes it easier to locate and understand the components involved in processing query results.

Highlights

• Renamed results.h to result_source_interface.h: This change provides a more descriptive name for the interface defining how query results are sourced, improving clarity on its purpose.
• Consolidated Stream Types into row_stream.h and row_stream.cc: Classes like RowStream, TupleStream, and their associated iterators (RowStreamIterator, TupleStreamIterator) have been moved from query_row.h and the old results.h into new, dedicated row_stream files. This centralizes stream-related functionalities.
• Updated Build Configurations and Dependencies: CMakeLists.txt and google_cloud_cpp_bigtable.bzl were updated to reflect the new file structure, and various source/header files now include the correct new headers, ensuring the project builds correctly with the refactored files.
• Refactored Test Files: Existing tests for the moved stream functionalities were relocated to a new row_stream_test.cc file, and query_row_test.cc was cleaned up by removing the moved test cases.
• Spanner Module Alignment: The google/cloud/spanner module also saw a corresponding rename of results.cc to result_source_interface.cc for consistency across the codebase.

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[gemini-code-assist]

Code Review

This pull request refactors the Bigtable client library by reorganizing query-related public types. results.h is renamed to result_source_interface.h, and various stream and iterator classes are consolidated into new row_stream.h and row_stream.cc files. The changes are well-structured and improve the organization of the code.

My review includes a few points:

• A critical fix for missing header includes in the new row_stream.h file, which would cause compilation errors.
• A suggestion to remove unused headers in row_stream.cc.
• A high-severity finding regarding a likely accidental and inconsistent file rename in the spanner component, which is out of scope for this PR.

Overall, this is a good refactoring. Addressing these points will ensure the correctness and consistency of the codebase.

[gemini-code-assist]

This header is missing some includes that were present in query_row.h and are still needed for the moved code to compile:

• google/cloud/internal/make_status.h: for GetSingularRow.
• <utility>: for std::move used in GetSingularRow and TupleStreamIterator.
• <functional>: for std::function used in RowStreamIterator::Source.
• <iterator>: for std::input_iterator_tag used in RowStreamIterator and TupleStreamIterator.

#include <memory>
#include "google/cloud/internal/make_status.h"
#include <functional>
#include <iterator>
#include <utility>

[gemini-code-assist]

These headers are not used in this file and can be removed to reduce dependencies and improve compilation times.

[codecov]

Codecov Report

❌ Patch coverage is 99.27361% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.04%. Comparing base (601c5b8) to head (f76ad26).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/bigtable/row_stream.h	96.22%	2 Missing ⚠️
google/cloud/bigtable/row_stream.cc	95.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15744   +/-   ##
=======================================
  Coverage   93.03%   93.04%           
=======================================
  Files        2445     2448    +3     
  Lines      226036   226036           
=======================================
+ Hits       210301   210307    +6     
+ Misses      15735    15729    -6     

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