Fix parallelSync getting stuck when all workers fail

[ChrisSchinnerl]

If for some reason all workers in parallelSync fail, it will never exit.

Result of failing reproduction test here: https://github.com/SiaFoundation/coreutils/actions/runs/22136567973/job/63989707212?pr=399

[Copilot]

Pull request overview

Fixes a deadlock scenario in the syncer’s parallelSync orchestration where syncing could stall indefinitely if every worker peer fails during block fetching, and adds a regression test plus a changeset entry.

Changes:

• Track active parallelSync workers and abort with an error if no workers remain while requests are still incomplete.
• Add TestParallelSyncStall to reproduce the “all workers fail” scenario using a peer that serves headers but always fails block RPCs.
• Add a knope changeset documenting the patch.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
syncer/parallel_sync.go	Adds active-worker tracking and an early exit to prevent indefinite blocking when no peers can make progress.
syncer/syncer_test.go	Adds a regression test that simulates a peer stalling during block fetch and verifies the sync loop continues.
.changeset/fix_parallelsync_stalling_when_all_workers_fail.md	Documents the bugfix as a patch changeset.

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[chatgpt-codex-connector]

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[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated no new comments.

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