Save registry inside of scan transaction

[HebaruSan]

Problem

If you run a ckan scan command and Ctrl-C out of it at just the right time (towards the end, I had to add a WriteLine to get the timing correct), you can end up with a truncated registry.json file:

-rw-r--r-- 1 16933954 May  7 17:13  registry.json
-rw-r--r-- 1  9207808 May  7 17:11  registry.json_BROKEN

Anything you try to do after this will throw exceptions and generally not work.

Cause

RegistryManager.Save is transaction-aware:

CKAN/Core/Registry/RegistryManager.cs

Lines 413 to 455 in f2209e2

	public void Save(bool enforce_consistency = true)
	{
	TxFileManager file_transaction = new TxFileManager();
	log.InfoFormat("Saving CKAN registry at {0}", path);
	if (enforce_consistency)
	{
	// No saving the registry unless it's in a sane state.
	registry.CheckSanity();
	}
	string directoryPath = Path.GetDirectoryName(path);
	if (directoryPath == null)
	{
	log.ErrorFormat("Failed to save registry, invalid path: {0}", path);
	throw new DirectoryNotFoundKraken(path, "Can't find a directory in " + path);
	}
	if (!Directory.Exists(directoryPath))
	{
	Directory.CreateDirectory(directoryPath);
	}
	file_transaction.WriteAllText(path, Serialize());
	ExportInstalled(
	Path.Combine(directoryPath, $"installed-{ksp.Name}.ckan"),
	false, true
	);
	if (!Directory.Exists(ksp.InstallHistoryDir()))
	{
	Directory.CreateDirectory(ksp.InstallHistoryDir());
	}
	ExportInstalled(
	Path.Combine(
	ksp.InstallHistoryDir(),
	$"installed-{ksp.Name}-{DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss")}.ckan"
	),
	false, true
	);
	}

... but KSP.ScanGameData calls it outside of its transaction:

CKAN/Core/KSP.cs

Lines 409 to 445 in f2209e2

	public void ScanGameData()
	{
	var manager = RegistryManager.Instance(this);
	using (TransactionScope tx = CkanTransaction.CreateTransactionScope())
	{
	manager.registry.ClearDlls();
	// TODO: It would be great to optimise this to skip .git directories and the like.
	// Yes, I keep my GameData in git.
	// Alas, EnumerateFiles is *case-sensitive* in its pattern, which causes
	// DLL files to be missed under Linux; we have to pick .dll, .DLL, or scanning
	// GameData *twice*.
	//
	// The least evil is to walk it once, and filter it ourselves.
	IEnumerable<string> files = Directory.EnumerateFiles(
	GameData(),
	"*",
	SearchOption.AllDirectories
	);
	files = files.Where(file => Regex.IsMatch(file, @"\.dll$", RegexOptions.IgnoreCase));
	foreach (string dll in files.Select(KSPPathUtils.NormalizePath))
	{
	var relativePath = ToRelativeGameDir(dll);
	if (!DllIgnoreList.Contains(relativePath))
	manager.registry.RegisterDll(this, dll);
	}
	manager.ScanDlc();
	tx.Complete();
	}
	manager.Save(enforce_consistency: false);
	}

When a TxFileManager is used outside of a transaction, it falls back to making live updates to the file system. So if you kill CKAN while it's doing that, it'll just stop after whatever byte happened to be most recently written.

Changes

Now KSP.ScanGameData saves the registry inside of its transaction. This way if you abort the operation, the original registry.json file will remain intact.

Fixes #2754.