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Chris LuandGitHub
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* pb: carry a volume digest on the heartbeat The full volume list is the only way a master notices a volume that vanished without a delta, so it cannot simply be dropped. A digest gives the same guarantee without the list, and a way back to the list when they disagree. The digest has explicit presence: a server holding no volumes reports 0, which has to stay distinguishable from a server that does not compute one at all. * volume: report a digest of the volumes each heartbeat carries Digests exactly what goes on the wire: volumes skipped as quarantined, phantom or expired are absent from both the list and the digest, so the master compares against the same set the server meant to report. Runs the master's own hash over the master's own conversion of the message, so the two ends cannot drift into disagreeing about a field. * master: check the reported volume digest and ask for the list on a mismatch Compared after everything the heartbeat carried has been applied, so agreement means the master is current rather than that nothing changed. Servers reporting no digest are untouched, and a mismatch on a heartbeat that already carried the full list is reported rather than answered: there is nothing further to ask for, so asking again would loop. Nodes reporting one volume id twice are skipped for the same reason. * rust: report the heartbeat volume digest Mirrors the Go volume server. The master compares this against a digest it computes itself, so the hash has to agree byte for byte across the two implementations, not merely be a hash of the same fields: report_hash_vectors pins it against values generated by the Go side, and the ttl and replica placement narrowing the master applies when it decodes a message is applied here too rather than assumed away. A drift there would not corrupt anything, but every volume server on this implementation would report a digest the master can never match and fall back to sending its whole volume list forever, which is the cost the digest exists to avoid. * master: pin what the digest check does to each kind of report The upgrade story rests on these: a server that reports no digest is never asked for anything, so the two sides can be upgraded in either order, and a disagreement that resending cannot fix is reported rather than re-asked, so it cannot loop. * topology: enumerate the digest coverage test from the message The list of fields was written out by hand, so a field added to VolumeInformationMessage later would fall outside the digest while the test went on passing, and a change to it would never reach the master. Walk the message descriptor instead. Some fields are narrowed or normalised on the way into VolumeInfo, so the smallest change to the wire value can land back on the stored one; the test offers several values per field and asks only that some change is visible.