[mtibben]

Synchronous REST requests cannot currently be cancelled while an SDK call is blocked. Checking an application cancellation flag between calls or disabling retries does not interrupt an active request, so an application can remain blocked waiting for the server or a timeout.

Add an experimental RestCancellation handle and RestCancellationOption for per-call options, including storage::Client operations and streams. Another thread can request cancellation, interrupting libcurl waits, preventing further retries, and reporting kCancelled through existing result or stream-error mechanisms. Streams retain cancellation state throughout their lifetime, and cancelled upload cleanup avoids finalizing buffered partial data.

Cancellable requests use bounded polling without accessing libcurl handles from another thread. Cancellable Storage reads bypass hedging so queued workers cannot delay cancellation. Credential acquisition already in progress remains outside this scope, and cancellation does not roll back operations the server has already committed.

[gemini-code-assist]

Code Review

This pull request introduces cooperative cancellation support for synchronous REST operations by adding a copyable RestCancellation handle and RestCancellationOption. It integrates cancellation checks into CurlImpl (using libcurl progress callbacks and a 50ms polling interval), the REST retry loop, backoff mechanisms, and Storage operations (bypassing read hedging). The reviewer feedback highlights opportunities to optimize performance and style by using get_optional on Options to avoid redundant map lookups in RestCancellationWait, IsRestCancelled, and CurlImpl, and suggests directly including the <optional> header in rest_cancellation.h.

[colinmoy]

/gcbrun

[codecov]

Codecov Report

❌ Patch coverage is 98.09028% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.63%. Comparing base (0e799e5) to head (831b82d).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
google/cloud/internal/rest_retry_loop_test.cc	92.10%	3 Missing ⚠️
google/cloud/internal/curl_impl.cc	93.54%	2 Missing ⚠️
google/cloud/storage/internal/connection_impl.cc	84.61%	2 Missing ⚠️
...cloud/storage/internal/retry_object_read_source.cc	90.00%	2 Missing ⚠️
google/cloud/internal/rest_cancellation.h	94.73%	1 Missing ⚠️
...e/cloud/storage/internal/rest/cancellation_test.cc	99.77%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16663    +/-   ##
========================================
  Coverage   92.62%   92.63%            
========================================
  Files        2271     2274     +3     
  Lines      224597   225163   +566     
========================================
+ Hits       208032   208573   +541     
- Misses      16565    16590    +25     

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