[tamirms]

In a parallel SortedBuilder or UnsortedBuilder build, a block that fails to build cancels the workers' context and reaches the writer, which reports the failure. The goroutine dispatching blocks waits on both, and when it saw the context first it returned context.Canceled instead of the failure. A build with a duplicate key in its first block returned context.Canceled about one time in eight, so a caller checking for ErrDuplicateKey missed it.

• The dispatcher, in dispatchBlockWork and in the unsorted builder's parallel finish, now returns the context's cause: the error of the worker or writer that failed, or the build context's error when the caller canceled the build.
• The writer now cancels the workers with its error as the cause. Its own errors need that, and so does a block's error when the writer's cancel lands before the failed worker's, which 9 of 14,000 sorted builds did under stress.

The teardown fuzz harness's duplicate-key scenario now requires ErrDuplicateKey and no longer accepts context.Canceled. The old code failed it 15 times in 3 runs.

🤖 Generated with Claude Code