Fix old-epoch backup worker cleanup after backup completion

[tclinkenbeard-oai]

Summary

A partitioned backup can finish after recovery has selected backup workers for an older transaction-log generation but before those workers read the backup-start key. When the last active backup clears that key, the old-generation workers wait for a future backup instead of acknowledging completion. Their obsolete transaction-log generation remains retained, preventing full cluster recovery.

Allow only workers with a finite old-generation range to exit when no backup is active. This uses the existing acknowledged worker-completion path so the obsolete generation can be retired. Current-generation workers continue waiting for future backups, and active and paused backups retain their existing behavior.

Validation

• fdbserver_backupworker_test.
• tests/slow/BackupAndRestore.toml with fault injection and buggification.
• tests/slow/BackupCorrectnessPartitioned.toml with fault injection and buggification.

[foundationdb-ci]

Result of foundationdb-pr-clang-ide on Linux RHEL 9

• Commit ID: 43bc191
• Duration 0:30:39
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-clang-arm on Linux RHEL 9

• Commit ID: 43bc191
• Duration 0:47:55
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr on Linux RHEL 9

• Commit ID: 43bc191
• Duration 1:08:14
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
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[foundationdb-ci]

Result of foundationdb-pr-clang on Linux RHEL 9

• Commit ID: 43bc191
• Duration 1:10:20
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
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[foundationdb-ci]

Result of foundationdb-pr-cluster-tests on Linux RHEL 9

• Commit ID: 43bc191
• Duration 1:30:37
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
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• Cluster Test Logs zip file of the test logs (available for 30 days)

[foundationdb-ci]

Result of foundationdb-pr-macos-m1 on macOS 14.x

• Commit ID: 43bc191
• Duration 2:50:35
• Result: ✅ SUCCEEDED
• Error: N/A
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[foundationdb-ci]

Result of foundationdb-pr-macos on macOS 14.x

• Commit ID: 43bc191
• Duration 4:37:20
• Result: ✅ SUCCEEDED
• Error: N/A
• Build Log terminal output (available for 30 days)
• Build Workspace zip file of the working directory (available for 30 days)