CBG-4945: Fix default sync function dry run

[RIT3shSapata]

CBG-4945

Describe your PR here...

• Upstream PR- CBG-4412
• This PR fixes the behaviour of sync function dry run when the default sync function is used

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

• GSI=true,xattrs=true https://jenkins.sgwdev.com/job/SyncGatewayIntegration/206/

[bbrks]

This only works for the default collection

[bbrks]

This is only the default sync function for the default collection. Named collections have a slightly different default sync function.

I'd probably use the GetDefaultSyncFunction function instead and pull out the scope/collection name from db *DatabaseCollectionWithUser

For reference: The code inside getChannelsAndAccess is doing the job of "faking" a sync function when ChannelMapper is nil, by looking at specific properties in the doc body (around lines 3331-3344)

https://github.com/couchbase/sync_gateway/blob/main/db/crud.go#L3332-L3344

[Copilot]

Pull request overview

This PR fixes the behavior of the sync function dry run feature when using the default sync function in Sync Gateway. Previously, when no custom sync function was provided, the dry run would fail to properly apply the default sync function for the collection.

Key changes:

• Modified the conditional logic in SyncFnDryrun to properly handle cases where no custom sync function is defined
• Added test cases to verify the default sync function behavior in dry run scenarios

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
db/crud.go	Updated SyncFnDryrun method to correctly apply the default sync function when no custom sync function is provided
rest/diagnostic_doc_api_test.go	Added two new test cases to verify dry run behavior with the default sync function for both new and existing documents

[Copilot]

The phrase 'therefore the' appears twice in succession. The comment should be restructured for better readability. Consider: 'Since the tests run in named Scopes and Collections, the default sync function is: function(doc){channel("<collection_name>");}. Therefore, the channels returned will be named collections.'

Suggested change

	// Since the tests run in named Scopes and Collections therefore the
	// default sync function is:
	// function(doc){channel("<collection_name>");}
	// therefore the channels returned will be named collections
	// Since the tests run in named Scopes and Collections, the default sync function is:
	// function(doc){channel("<collection_name>");}. Therefore, the channels returned will be named collections.

[Copilot]

The logic flow has become less clear with the nested conditions. The outer condition checks syncFn == \"\" twice in different branches. Consider restructuring to: first check if syncFn is empty and set it to the default if needed, then proceed with a single conditional based on whether to use ChannelMapper or SyncRunner.