Re: [PATCH 1/2] dir.c: fix bug in 'nd/exclusion-regression-fix' topic
- From
Junio C Hamano <gitster@pobox.com>
- Date
- Mar 18, 2016, 17:38 UTC
- Message-ID
- <xmqqegb7r8jn.fsf@gitster.mtv.corp.google.com>
- In-Reply-To
- <1458218744-15810-1-git-send-email-pclouds@gmail.com>
Nguyễn Thái Ngọc Duy <pclouds@gmail.com> writes:
Show 5 quoted lines
> The topic in question introduces "sticky" path list for this purpose: > given a path 'abc/def', if 'abc' already matches a pattern X, it's added > to X's sticky path list. When we need to check if 'abc/def' matches > pattern X and see that 'abc' is already in the list, we conclude right > away that 'abc/def' also matches X.
It took me two reads to realize that I can get what this paragraph wants to say if I ignore "given a path 'abc/def'" at the beginning. In short, if we already know that a directory 'abc' matches a pattern, the pattern remembers that fact in the list and further queries about anything in that directory, e.g. 'abc/def', is answered with "we already know abc matches, so it also does match".
"Sticky" is probably better understood if called "ancestor directory".
> The short reason (*) for sticky path list is to workaround limitations > of matching code that will return "not match" when we compare > 'abc/def' and pattern X.
If one asks about 'abc/def' first, you ask "does '*' match 'abc/def'?"--if there is a limitation with the matching code, this list would not work at all as a workaround. But we can safely assume that before asking about 'abc/def' we would always ask about 'abc', so it is OK. Is that what happens here?
Show 5 quoted lines
> The bug is in this code. Not only it does "when we need to check if > 'abc/def' matches...", it does an extra thing: if 'foo/bar' is _not_ in > the list, return 'not matched' by bypassing all matching code with the > "continue;" statement. It should let the remaining code decide match > status instead.
So once a pattern has a non-empty "sticky" list after examining 'abc' and if you ask about 'foo', because it is not in the list and instead of saying "it is not in the list, so we need to try matching the pattern against 'foo' to decide if it matches", it incorrectly says "'foo' does not match". Is that what happens here?
An abrupt appearance of 'foo/bar' in the description made me read the above three times and that is why I dropped '/bar' part in the above. The correctness of the 'sticky' hack seems to depend on the assumption that we would always ask about 'foo' before asking about 'foo/bar', so the scenario presented with 'foo/bar' is implausible; even when asking about 'foo/bar', we would have 'foo' in the list, no?
The remainder, assuming that I read the above correctly, made sense to me and justifies what the update code does.
Thanks.
Show 99 quoted lines
> This bug affects both .gitignore and sparse checkout, but it's reported
> as a sparse checkout bug, so let's examine how it happens. The
> sparse-checkout pattern has two rules
>
> /*
> !one/hideme
>
> and the worktree has three tracked files, one/hideme, one/showme and
> two/showme. What happens is this
>
> * check "one", it matches the first pattern -> positive -> keep
> examining.
>
> *1* "thanks" to 'nd/exclusion-regression-fix' we detect this pair of
> patterns, so we put "one" in the sticky list of pattern "/*"
>
> * enter "one", check "one/hideme", it matches the second pattern
> first (we search from bottom up) -> negative -> excluded
>
> * check "one/showme", it does not match the second pattern.
>
> *2* We then check it against the first pattern and notice the sticky list
> that includes "one", so we decide right away that "one/showme" is
> included.
>
> * leave "one", check "two" which does not match the second pattern.
>
> *3* then we check "two" against the first pattern and notice that this
> pattern has a non-empty sticky list, which contains "one", not "two".
> This bug kicks in and bypasses the true matching logic for pattern
> "/*". As a result, we exclude "two/showme".
>
> One may notice that the order of these steps matter. If *3* occurs
> before *1*, then the sticky list at that moment is empty and the bug
> does not kick in. Sparse checkout always examines entries in
> alphabetical order, so "abc/showme" would be examined before "one" and
> not hit this bug!
>
> The last remark is important when we move to .gitignore. We receive the
> list of entries with readdir() and cannot control the order of
> entries. Which means we can't write a test for .gitignore that will
> reliably fail without this fix. Which is why this patch only adds a test
> for sparse checkout, even though the same above steps happen to
> .gitignore.
>
> (*) The problem is known and will be fixed later and described in
> detail then. For this commit, it's sufficient to see the following
> link because the long reason is really long:
>
> http://article.gmane.org/gmane.comp.version-control.git/288479
>
> Reported-by: Durham Goode <durham@fb.com>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
> ---
> dir.c | 1 -
> t/t1011-read-tree-sparse-checkout.sh | 20 ++++++++++++++++++++
> 2 files changed, 20 insertions(+), 1 deletion(-)
>
> diff --git a/dir.c b/dir.c
> index 69e0be6..77f38a5 100644
> --- a/dir.c
> +++ b/dir.c
> @@ -1027,7 +1027,6 @@ static struct exclude *last_exclude_matching_from_list(const char *pathname,
> exc = x;
> break;
> }
> - continue;
> }
>
> if (x->flags & EXC_FLAG_MUSTBEDIR) {
> diff --git a/t/t1011-read-tree-sparse-checkout.sh b/t/t1011-read-tree-sparse-checkout.sh
> index 0c74bee..ecc5e93 100755
> --- a/t/t1011-read-tree-sparse-checkout.sh
> +++ b/t/t1011-read-tree-sparse-checkout.sh
> @@ -274,4 +274,24 @@ test_expect_success 'checkout with --ignore-skip-worktree-bits' '
> git diff --exit-code HEAD
> '
>
> +test_expect_success 'sparse checkout and dir.c sticky bits' '
> + git init sticky &&
> + (
> + cd sticky &&
> + mkdir one two &&
> + touch one/hideme one/showme two/showme &&
> + git add . &&
> + git commit -m initial &&
> + cat >.git/info/sparse-checkout <<-\EOF &&
> + /*
> + !one/hideme
> + EOF
> + git config core.sparsecheckout true &&
> + git checkout &&
> + test_path_is_missing one/hideme &&
> + test_path_is_file one/showme &&
> + test_path_is_file two/showme
> + )
> +'
> +
> test_done