patchsparse-checkout: allow one-character directories in cone mode

> From: Derrick Stolee <dstolee@microsoft.com>
>
> In 9e6d3e64 (sparse-checkout: detect short patterns, 2020-01-24), a
> condition on the minimum length of a cone-mode pattern was introduced.
> However, this condition was off-by-one.
>
> If we have a directory with a single character, say "b", then the
> command
>
> 	git sparse-checkout set b
>
> will correctly add the pattern "/b/" to the sparse-checkout file. When
> this is interpeted in dir.c, the pattern is "/b" with the
> PATTERN_FLAG_MUSTBEDIR flag. This string has length two, which satisfies
> our inclusive inequality (<= 2).
>
> The reason for this inequality is that we will start to read the pattern
> string character-by-character using three char pointers: prev, cur,
> next. In particular, next is set to the current pattern plus two. The
> mistake was that next will still be a valid pointer when the pattern
> length is two, since the string is null-terminated.
>
> Make this inequality strict so these patterns work.
>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>     sparse-checkout: allow one-character directories in cone mode
>     
>     This is based on ds/sparse-add.
>     
>     I discovered this while taking v2.25.1 and ds/sparse-add into our fork
>     of Git and testing it with Scalar.
>     
>     Off-by-one errors are tricky, sometimes.

>     Thanks, -Stolee
>
> Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-558%2Fderrickstolee%2Fsparse-short-pattern-v1
> Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-558/derrickstolee/sparse-short-pattern-v1
> Pull-Request: https://github.com/gitgitgadget/git/pull/558
>
>  dir.c                              |  2 +-
>  t/t1091-sparse-checkout-builtin.sh | 12 +++++++++++-
>  2 files changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/dir.c b/dir.c
> index 7ac0920b713..a87900d43a2 100644
> --- a/dir.c
> +++ b/dir.c
> @@ -682,7 +682,7 @@ static void add_pattern_to_hashsets(struct pattern_list *pl, struct path_pattern
>  		return;
>  	}
>  
> -	if (given->patternlen <= 2 ||
> +	if (given->patternlen < 2 ||
>  	    *given->pattern == '*' ||
>  	    strstr(given->pattern, "**")) {
>  		/* Not a cone pattern. */
> diff --git a/t/t1091-sparse-checkout-builtin.sh b/t/t1091-sparse-checkout-builtin.sh
> index c35cbdef454..b4c9c32a037 100755
> --- a/t/t1091-sparse-checkout-builtin.sh
> +++ b/t/t1091-sparse-checkout-builtin.sh
> @@ -417,10 +417,20 @@ test_expect_success 'pattern-checks: too short' '
>  	cat >repo/.git/info/sparse-checkout <<-\EOF &&
>  	/*
>  	!/*/
> -	/a
> +	/
>  	EOF
>  	check_read_tree_errors repo "a" "disabling cone pattern matching"
>  '
> +test_expect_success 'pattern-checks: not too short' '
> +	cat >repo/.git/info/sparse-checkout <<-\EOF &&
> +	/*
> +	!/*/
> +	/b/
> +	EOF
> +	git -C repo read-tree -mu HEAD 2>err &&
> +	test_must_be_empty err &&
> +	check_files repo a
> +'
>  
>  test_expect_success 'pattern-checks: trailing "*"' '
>  	cat >repo/.git/info/sparse-checkout <<-\EOF &&
>
> base-commit: ef07659926f64d70e8cb41025c3d7456eecb962e