Re: [PATCH v3 4/8] unpack-trees: fix nested sparse-dir search

["René Scharfe" <l.s.r@web.de>]

Am 20.08.21 um 21:35 schrieb René Scharfe:
> Am 20.08.21 um 17:18 schrieb Derrick Stolee:
>> On 8/19/2021 4:01 AM, Johannes Schindelin wrote:
>>> Hi Stolee,
>>>
>>> On Tue, 17 Aug 2021, Derrick Stolee via GitGitGadget wrote:
>>>
>>>> From: Derrick Stolee <dstolee@microsoft.com>
>>>>
>>>> The iterated search in find_cache_entry() was recently modified to
>>>> include a loop that searches backwards for a sparse directory entry that
>>>> matches the given traverse_info and name_entry. However, the string
>>>> comparison failed to actually concatenate those two strings, so this
>>>> failed to find a sparse directory when it was not a top-level directory.
>>>>
>>>> This caused some errors in rare cases where a 'git checkout' spanned a
>>>> diff that modified files within the sparse directory entry, but we could
>>>> not correctly find the entry.
>>>
>>> Good explanation.
>>>
>>> I wonder a bit about the performance impact. How "hot" is this function?
>>> I.e. how often is it called, on average?
>>>
>>> I ask because I see opportunities to optimize in both directions: it could
>>> be written more concisely (if speed does not matter as much), and it could
>>> be made faster (if speed matters a lot). See below for more.
>>
>> I would definitely optimize for speed here. This can be a very hot path,
>> I believe.
>>
>>>> +	strbuf_addstr(&full_path, info->traverse_path);
>>>> +	strbuf_add(&full_path, p->path, p->pathlen);
>>>> +	strbuf_addch(&full_path, '/');
>>>
>>> This could be reduced to:
>>>
>>> 	strbuf_addf(&full_path, "%s%.*s/",
>>> 		    info->traverse_path, (int)p->pathlen, p->path);
>>
>> We should definitely avoid formatted strings here, if possible.
>>
>>> But if speed matters, we probably need something more like this:
>>>
>>> 	size_t full_path_len;
>>> 	const char *full_path;
>>> 	char *full_path_1 = NULL;
>>>
>>> 	if (!*info->traverse_path) {
>>> 		full_path = p->path;
>>> 		full_path_len = p->pathlen;
>>> 	} else {
>>> 		size_t len = strlen(info->traverse_path);
>>>
>>> 		full_path_len = len + p->pathlen + 1;
>>> 		full_path = full_path_1 = xmalloc(full_path_len + 1);
>>> 		memcpy(full_path_1, info->traverse_path, len);
>>> 		memcpy(full_path_1 + len, p->path, p->pathlen);
>>> 		full_path_1[full_path_len - 1] = '/';
>>> 		full_path_1[full_path_len] = '\0';
>>> 	}
>>
>> The critical benefit here is that we do not need to allocate a
>> buffer if the traverse_path does not exist. That might be a
>> worthwhile investment. That leads to justifying the use of
>> bare 'char *'s instead of 'struct strbuf'.
>>
>> If the traverse_path is usually non-null, then we could continue using
>> strbufs as a helper and get the planned performance gains by using
>> strbuf_grow(&full_path, full_path_len + 1) followed by strbuf_add()
>> (instead of strbuf_addstr()). That would make this code a bit less
>> ugly with the only real overhead being the extra insertions of '\0'
>> characters as we add the strings to the strbuf().
>
> You create full_path only to compare it to another string.  You can
> compare the pieces directly, without allocating and copying:
>
> 	const char *path;
>
> 	if (!skip_prefix(ce->name, info->traverse_path, &path) ||
> 	    strncmp(path, p->path, p->pathlen) ||
> 	    strcmp(path + p->pathlen, "/"))

The strcmp line is wrong (should be path[p->pathlen] != '/'), but
you get the idea..

> 		return NULL;
>
> A test would be nice to demonstrate the fixed issue.
>
> René
>