CBG-5106: Incorrect history being sent for legacy revision cases in delta sync

[gregns1]

CBG-5106

• Move rev message history builder into a shared function to be called from multiple places in the code
• Fixes scenario where empty history is sent for a SGW rev that has CV but a client rev that is legacy rev.
• Introduces some lower level tests to assert on this behaviour.

QE tests now show history being sent correctly for both test cases in the linked ticket:

2026-05-28T10:50:40.398+01:00 [DBG] Sync+: c:[65248013] db:upgrade col:_default Sending rev "<ud>nonconflict_3</ud>" 18b3b1dcf40d0000@Qmce34epjCesjaS8myHE+Q as delta. DeltaSrc:2-509dc0903869f6b103fd1266b169e73876b80d27
2026-05-28T10:50:40.398+01:00 [TRC] SyncMsg+: c:[65248013] db:upgrade Sent Req MSG#3~: Body: '<ud>{"updated_by":"delta_sync_history_test"}</ud>' Properties: <ud>map[Content-Type:application/json Profile:rev collection:0 deltaSrc:2-509dc0903869f6b103fd1266b169e73876b80d27 history:3-3a1c6412b5433949e8ed130d996300a7,2-509dc0903869f6b103fd1266b169e73876b80d27 id:nonconflict_3 rev:18b3b1dcf40d0000@Qmce34epjCesjaS8myHE+Q sequence:34]</ud>
...
2026-05-28T10:51:31.840+01:00 [DBG] Sync+: c:[78356265] db:upgrade col:_default Sending rev "<ud>nonconflict_2</ud>" 2-36a2cfe8544956ad3c5b6491cbfc7d94 as delta. DeltaSrc:1-e643a00fb9cad8cf87ee4d2c4e2383f6ecf118c0
2026-05-28T10:51:31.840+01:00 [TRC] SyncMsg+: c:[78356265] db:upgrade Sent Req MSG#3~: Body: '<ud>{"number":2}</ud>' Properties: <ud>map[Content-Type:application/json Profile:rev collection:0 deltaSrc:1-e643a00fb9cad8cf87ee4d2c4e2383f6ecf118c0 history:1-e643a00fb9cad8cf87ee4d2c4e2383f6ecf118c0 id:nonconflict_2 rev:2-36a2cfe8544956ad3c5b6491cbfc7d94 sequence:6]

Pre-review checklist

• Removed debug logging (fmt.Print, log.Print, ...)
• Logging sensitive data? Make sure it's tagged (e.g. base.UD(docID), base.MD(dbName))
• Updated relevant information in the API specifications (such as endpoint descriptions, schemas, ...) in docs/api

Integration Tests

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/710/
• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/713/ (test fix)

[Copilot]

Pull request overview

This PR fixes CBG-5106, where delta-sync rev messages sent to a legacy (pre-HLV) client could end up with an empty/incorrect history property, breaking conflict detection on the receiver. The fix centralizes the previously duplicated history-construction logic across sendRevision, sendDelta, and sendRevAsDelta (redacted path), and propagates the remoteIsLegacyRev flag through sendRevAsDelta into sendDelta so the delta path can produce the same history as the full-revision path.

Changes:

• Introduce buildRevHistory/revHistoryInput to centralize rev-message history construction for the 5 local/remote legacy-vs-HLV scenarios, with extensive scenario documentation.
• Plumb remoteIsLegacyRev through sendRevAsDelta → sendDelta (and fallback sendRevision calls) so deltas to legacy clients include [hlvHistory, revID, revTreeHistory...].
• Add two new lower-level BLIP tests for the delta + legacy-client case and unskip the previously skipped TestActiveReplicatorDeltaSyncWhenBothSidesLegacy.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
db/blip_sync_context.go	Adds buildRevHistory helper with scenario docs; sendDelta now takes remoteIsLegacyRev and uses the helper; sendRevision refactored to use the helper.
db/blip_handler.go	sendRevAsDelta takes remoteIsLegacyRev, propagates it to fallback sendRevision and sendDelta calls, and uses buildRevHistory for the redacted-rev path.
rest/blip_legacy_revid_test.go	Adds two tests verifying delta history sent to legacy clients (with and without HLV pv history).
rest/replicatortest/replicator_test_legacy_rev_test.go	Removes t.Skip from TestActiveReplicatorDeltaSyncWhenBothSidesLegacy now that the fix lands.