AllocateBlockVolumeResponse used bs.ListenAddr() to derive replica
addresses. When the VS binds to ":port" (no explicit IP), host
resolved to empty string, producing ":dataPort" as the replica
address. This ":port" propagated through master assignments to both
primary and replica sides.
Now canonicalizes empty/wildcard host using PreferredOutboundIP()
before constructing replication addresses. Also exported
PreferredOutboundIP for use by the server package.
This is the source fix — all downstream paths (heartbeat, API
response, assignment) inherit the canonical address.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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