Enhancements required for linkfield migration

[GuySartorelli]

Commit 1: Adds a test utility. Required for a test in silverstripe/silverstripe-linkfield#244
Commit 2: Adds JOIN support for SQLUpdate. Required for migrating owners in silverstripe/silverstripe-linkfield#244

Issue

• Streamline migration from v2/v3 to v4 silverstripe-linkfield#228

[kinglozzer]

This seems a bit odd - introducing a property with no way to change its value, then using reflection in the linked pull request. Why not add a setter for this? If it’s just for semver reasons, I’d rather we broke that convention than hacked in code like this. If we can’t, then could we introduce a SS_FORCE_IS_CLI environment variable instead of a property/reflection?

[GuySartorelli]

Why not add a setter for this?

This is literally only for use in unit tests. There is no valid use case for using this outside of unit tests, and by not adding a setter usage outside of tests is discouraged.

A setter also becomes public API, which means we have to maintain BC for it afterward, which is not what we want for something that is only for use in testing. Same with environment variables.

[emteknetnz]

Suggested change

	/**
	* Used by unit tests to override `isCli()`
	* @internal
	*/
	private static ?bool $cliOverride = null;
	/**
	* Used by unit tests to override `isCli()`
	* This is not config, so use reflection to change the value
	* @internal
	*/
	private static ?bool $isCliOverride = null;

[GuySartorelli]

Done

[emteknetnz]

Suggested change

	if (self::$cliOverride !== null) {
	return self::$cliOverride;
	if (self::$isCliOverride !== null) {
	return self::$isCliOverride;

[GuySartorelli]

Done

[GuySartorelli]

I have added JOIN support for SQLUpdate so I can do an optimisation in the linkfield migration when setting the owner. Since this is targetting the next minor it's safe to do.
It also means others can now use this functionality, which is well supported in SQL and really useful.
It also will save me at least half a day trying to get kinda-gross workarounds working.