fix(storage): add extra validation for type in object_metadata_parser

[nidhiii-27]

object_metadata_parser was triggering a process crash (abort) when encountering a type mismatch in JSON fields, specifically the "id" field. When json.value("id", "") is called on a field containing a non-string type, such as an integer, the library triggers a process abort instead of returning a Status error.

This change introduces a robust internal::ParseStringField helper that returns a google::cloud::Status on type mismatch. Updated the object_metadata_parser to use this helper for string fields and added type verification.

Fixes b/472347315

[gemini-code-assist]

Summary of Changes

Hello @nidhiii-27, 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 resolves a critical issue where the object_metadata_parser could cause a process crash when encountering a type mismatch for string fields in JSON, such as an integer being provided where a string was expected. The changes introduce a new, robust helper function, internal::ParseStringField, which gracefully handles these type mismatches by returning a google::cloud::Status error. This helper is then integrated into the object_metadata_parser for all relevant string fields, significantly improving the stability and error reporting of the metadata parsing logic.

Highlights

• Robust String Field Parsing: Introduced internal::ParseStringField to safely parse string fields from JSON, returning a Status on type mismatch instead of causing a process crash.
• Enhanced Object Metadata Parsing: Updated object_metadata_parser to utilize the new ParseStringField helper for various string fields, significantly improving error handling and preventing crashes due to type mismatches.
• Metadata Value Validation: Added explicit validation within the ParseMetadata function to ensure that all values within the 'metadata' map are indeed strings, returning an InvalidArgumentError if not.
• Comprehensive Unit Tests: Included new unit tests for ParseStringField to cover successful parsing, handling of missing fields, and proper error reporting for invalid field types (e.g., integers or arrays where strings are expected).

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[gemini-code-assist]

Code Review

This pull request introduces validation for string fields during JSON parsing to prevent a process crash on type mismatch, which is a great improvement. This is achieved by adding a ParseStringField helper and using it for several fields in ObjectMetadataParser. The changes are a good step towards making the parser more robust. My review includes a suggestion to improve an error message for better diagnostics and points out that several string fields were missed and should also be updated to use the new safe parsing logic to fully address the underlying issue.

[codecov]

Codecov Report

❌ Patch coverage is 89.55224% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.94%. Comparing base (1986715) to head (9e4920a).

Files with missing lines	Patch %	Lines
...e/cloud/storage/internal/object_metadata_parser.cc	77.41%	7 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #15864   +/-   ##
=======================================
  Coverage   92.94%   92.94%           
=======================================
  Files        2458     2458           
  Lines      227590   227624   +34     
=======================================
+ Hits       211527   211571   +44     
+ Misses      16063    16053   -10     

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