Re: [PATCH] refs: run copy and rename through transactions

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> writes:

>> +struct files_copy_or_rename_transaction_data {
>> +	struct ref_lock *lock;
>> +	struct object_id orig_oid;
>> +	struct object_id destination_oid;
>> +	char *destination_target;
>> +	int logmoved;
>> +	int destination_exists;
>> +	int destination_log_backed_up;
>> +};
>
> Good to have a type that can be used to hold pieces of information
> specific to the operation.  Can't we do without rename/copy specific
> addition to the generic ref_transaction struct by following the same
> principle?
>
> The comment above the members does make it understandable, but ...
>
>> @@ -240,6 +253,21 @@ struct ref_transaction {
>>  	void *backend_data;
>>  	unsigned int flags;
>>  	uint64_t max_index;
>> +
>> +	/*
>> +	 * Rename and copy operations need backend-specific reflog handling.
>> +	 * Their logical updates still live in `updates`, so hooks see the
>> +	 * operation like any other reference transaction. The fields below
>> +	 * retain the state that backends verify after taking their locks.
>> +	 */
>> +	enum ref_transaction_type type;
>> +	char *old_refname;
>> +	char *new_refname;
>> +	char *logmsg;
>> +	struct object_id source_oid;
>> +	struct object_id destination_oid;
>> +	char *destination_target;
>> +	unsigned int destination_exists:1;
>>  };
>
> ... is it the best we can do to contaminate a rather generic data
> structure for such a details relevant only to one specific
> operation?

More importantly, this structure suggests to me that you can have a
single rename (or copy) from one source to one destination in a
single transaction.  Is that correct or am I misunderstanding the
way this data structure is used?  How would one rename A, B and C
to X, Y and Z in a single transaction?  Or perhaps rename A to B
and copy C to D in a single transaction?