Re: [RFC v1] Add virtual file system settings and hook proc

[Ben Peart <peartben@gmail.com>]

On 11/4/2018 7:02 PM, Junio C Hamano wrote:
> Ben Peart <peartben@gmail.com> writes:
> 
>>>> +	if (*dtype == DT_UNKNOWN)
>>>> +		*dtype = get_dtype(NULL, istate, pathname, pathlen);
>>>
>>> We try to defer paying cost to determine unknown *dtype as late as
>>> possible by having this call in last_exclude_matching_from_list(),
>>> and not here.  If we are doing this, we probably should update the
>>> callpaths that call last_exclude_matching_from_list() to make the
>>> caller responsible for doing get_dtype() and drop the lazy finding
>>> of dtype from the callee.  Alternatively, the new "is excluded from
>>> vfs" helper can learn to do the lazy get_dtype() just like the
>>> existing last_exclude_matching_from_list() does.  I suspect the
>>> latter may be simpler.
>>
>> In is_excluded_from_virtualfilesystem() dtype can't be lazy because it
>> is always needed (which is why I test and die if it isn't known).
> 
> You make a call to that function even when virtual-file-system hook
> is not in use, i.e. instead of the caller saying
> 
> 	if (is_vfs_in_use()) {
> 		*dtype = get_dtype(...);
>                  if (is_excluded_from_vfs(...) > 0)
> 			return 1;
> 	}
> 
> your caller makes an unconditional call to is_excluded_from_vfs().
> Isn't that the only reason why you break the laziness of determining
> dtype?
> 
> You can keep the caller simple by making an unconditional call, but
> maintain the laziness by updating the callig convention to pass
> dtype (not *dtype) to the function, e.g..
> 
> 	if (is_excluded_from_vfs(pathname, pathlen, dtype) > 0)
> 		return 1;
> 
> and then at the beginning of the helper
> 
> 	if (is_vfs_in_use())
> 		return -1; /* undetermined */
> 	*dtype = get_dtype(...);
> 	... whatever logic it has now ...
> 
> no?
> 

Oops!  You're right, I docall get_dtype() even if the vfs isn't in use. 
I'll add an additional test to avoid doing that.  Thank you.

I did look into moving the delay load logic into is_excluded_from_vfs() 
but get_dtype() is static to dir.c and I'd prefer to keep it that way if 
possible.

>> Your comments are all feedback on the code - how it was implemented,
>> style, etc.  Any thoughts on whether this is something we could/should
>> merge into master (after any necessary cleanup)?  Would anyone else
>> find this interesting/helpful?
> 
> I am pretty much neutral.  Not strongly opposed to it, but not all
> that interested until seeing its integration with the "userland" to
> see how the whole thing works ;-)
>