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

Am 20.08.21 um 17:18 schrieb Derrick Stolee:

On 8/19/2021 4:01 AM, Johannes Schindelin wrote:

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Hi Stolee,

On Tue, 17 Aug 2021, Derrick Stolee via GitGitGadget wrote:

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From: Derrick Stolee <redacted>

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.

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+	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.

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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, "/"))
		return NULL;

A test would be nice to demonstrate the fixed issue.

René