CBG-5366: set sync info to binary document when cluster compat v4.1+

[gregns1]

CBG-5366

• Introduces a versioned syncInfo encoding: at cluster compat ≥ 4.1, marshalSyncInfo prepends a SyncInfoTypeV1 discriminator byte; older clusters keep writing bare JSON so a not-yet-upgraded peer can still read the doc. DecodeSyncInfo handles both forms.
• Plumbs *ClusterCompatVersion through InitSyncInfo / SetSyncInfoMetadataID / SetSyncInfoMetaVersion and their callers (bootstrap, server context, attachment migration, resync DCP) via closure on dbcontext
• Replaces DataStore.Update/Add in the syncInfo write path with a GetRaw + WriteCas CAS-retry loop. Update/Add route through gocb's RawJSONTranscoder / SGJSONTranscoder, which reject or mis-tag the non-JSON V1 prefix byte; the new path uses syncInfoWriteOpts to pick the datatype tag from the payload (binary for V1, JSON for bare JSON), preserving the older-peer read path. Retry semantics match the existing Update (unbounded loop, retry on IsCasMismatch).
• Adds a WriteCasCallback hook to LeakyDataStore and migrates TestInitSyncInfoErrors to it, since AddCallback no longer fires on the new write path.

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

Dependencies (if applicable)

• Link upstream PRs
• Update Go module dependencies when merged

Integration Tests

• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/667/
• https://jenkins.sgwdev.com/job/SyncGatewayIntegration/671/ (run base package to show above failures are fixed)

[Copilot]

Pull request overview

This PR updates Sync Gateway’s _sync:syncInfo document encoding and write path to support a rolling-upgrade safe transition to a binary-prefixed (V1) format once the cluster-wide minimum Sync Gateway version reaches 4.1+. It also threads cluster compatibility version resolution through DatabaseContextOptions so long-lived components observe changes during upgrades.

Changes:

• Introduces a versioned syncInfo encoding (legacy bare JSON vs. V1 discriminator byte + JSON) with DecodeSyncInfo handling both.
• Switches syncInfo writes from Update/Add to a GetRaw + WriteCas CAS-retry loop to correctly set KV datatype (JSON vs binary) based on payload.
• Plumbs cluster compat version resolution into DB context options and key call sites (bootstrap, resync DCP, attachment migration), with tests covering upgrade gating and both encodings.

Reviewed changes

Copilot reviewed 18 out of 18 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
rest/server_context.go	Passes cluster compat version into syncInfo init and stores a closure on DB context options for dynamic re-resolution during rolling upgrades.
rest/config_manager.go	Switches syncInfo read during metadataID computation to raw fetch + DecodeSyncInfo to support V1.
rest/config_manager_test.go	Adds coverage for binary/V1 syncInfo decoding in computeMetadataID and corrupt-doc fallback behavior.
rest/cluster_compat_test.go	Adds a rolling-upgrade gate test ensuring syncInfo stays legacy with a 4.0 peer and flips to V1 at 4.1+.
db/util_testing.go	Updates syncInfo meta version assertion helper to use raw fetch + DecodeSyncInfo.
db/database.go	Adds ClusterCompatVersion func() *base.ClusterCompatVersion to DatabaseContextOptions.
db/database_test.go	Updates tests for new syncInfo function signatures (passing cluster compat version).
db/background_mgr_resync_dcp.go	Passes resolved cluster compat version into syncInfo update after resync completion.
db/background_mgr_resync_dcp_test.go	Adds end-to-end test verifying resync writes V1 syncInfo when ccv>=4.1.
db/background_mgr_attachment_migration.go	Passes resolved cluster compat version into syncInfo meta version update after migration.
db/background_mgr_attachment_migration_test.go	Adds end-to-end test verifying attachment migration writes V1 syncInfo when ccv>=4.1.
base/rosmar_cluster.go	Fixes GetDocument exists return semantics and adds GetRawDocument implementation for bootstrap reads.
base/leaky_datastore.go	Adds WriteCasCallback injection seam for CAS write path testing.
base/leaky_bucket.go	Adds WriteCasCallback config option to support new syncInfo write path testing.
base/constants_syncdocs.go	Implements versioned syncInfo encoding/decoding and CAS-based write/update logic using raw bytes + datatype control.
base/constants_syncdocs_test.go	Updates error-injection tests for new write path and adds comprehensive tests for V1/legacy encodings and decode behavior.
base/bootstrap.go	Extends BootstrapConnection with GetRawDocument and adds Couchbase Server implementation using raw transcoder.
base/bootstrap_test.go	Adds tests for GetRawDocument and for correct GetDocument exists behavior across Rosmar/Couchbase implementations.

[gregns1]

Changed to syncInfoEncodingType

[bbrks]

LGTM - I had some comments but they were all on the cluster compat version gate and we can just follow-up with some refactoring there to tidy up. Filed CBG-5406 to track given the minimal usage in 4.1

[bbrks]

This pattern feels like there should just be a function on ServerContext that returns cluster compat version or nil and is doing all of its own nil checking, etc. inside.

[bbrks]

CCV (or the function to get it) is better suited to be stored on DatabaseContext than in the options.

[bbrks]

likewise helper function on DatabaseContext that returns ccv or nil and does its own nil checking will help keep code dependent on CCV cleaner.

[bbrks]

Could maybe improve the ergonomics of this API by pushing the nil check down into the AtLeast method itself (returning false is fine for that case)

A typed nil won't panic if the method is defined on the pointer and lets us avoid nil checks at usage time.