[v4,4/4] string-list: document iterator behavior on NULL input

Commit Message

Derrick Stolee Sept. 27, 2022, 1:57 p.m. UTC

From: Derrick Stolee <[e-mail]>

The for_each_string_list_item() macro takes a string_list and
automatically constructs a for loop to iterate over its contents. This
macro will segfault if the list is non-NULL.

We cannot change the macro to be careful around NULL values because
there are many callers that use the address of a local variable, which
will never be NULL and will cause compile errors with -Werror=address.

For now, leave a documentation comment to try to avoid mistakes in the
future where a caller does not check for a NULL list.

Signed-off-by: Derrick Stolee <[e-mail]>
---
 string-list.h | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

Comments

Junio C Hamano Sept. 27, 2022, 4:39 p.m. UTC | #1

"Derrick Stolee via GitGitGadget" <[e-mail]> writes:

> From: Derrick Stolee <[e-mail]>
>
> The for_each_string_list_item() macro takes a string_list and
> automatically constructs a for loop to iterate over its contents. This
> macro will segfault if the list is non-NULL.
>
> We cannot change the macro to be careful around NULL values because
> there are many callers that use the address of a local variable, which
> will never be NULL and will cause compile errors with -Werror=address.
>
> For now, leave a documentation comment to try to avoid mistakes in the
> future where a caller does not check for a NULL list.
>
> Signed-off-by: Derrick Stolee <[e-mail]>
> ---
>  string-list.h | 7 ++++++-
>  1 file changed, 6 insertions(+), 1 deletion(-)

For exactly the -Werror=address reason, any other way that checks
list for NULL-ness cannot be done here (other than with Peff's
inline helper that returns the first item or NULL), which is a bit
of shame but this is totally outside the topic, and an additional
comment is a good thing to have here.

Thanks.  All four patches look reasonable.  Will queue (but after I
make sure I can tag and push out -rc2 today).


> diff --git a/string-list.h b/string-list.h
> index d5a744e1438..c7b0d5d0008 100644
> --- a/string-list.h
> +++ b/string-list.h
> @@ -141,7 +141,12 @@ void string_list_clear_func(struct string_list *list, string_list_clear_func_t c
>  int for_each_string_list(struct string_list *list,
>  			 string_list_each_func_t func, void *cb_data);
>  
> -/** Iterate over each item, as a macro. */
> +/**
> + * Iterate over each item, as a macro.
> + *
> + * Be sure that 'list' is non-NULL. The macro cannot perform NULL
> + * checks due to -Werror=address errors.
> + */
>  #define for_each_string_list_item(item,list)            \
>  	for (item = (list)->items;                      \
>  	     item && item < (list)->items + (list)->nr; \

Patch

diff --git a/string-list.h b/string-list.h
index d5a744e1438..c7b0d5d0008 100644
--- a/string-list.h
+++ b/string-list.h
@@ -141,7 +141,12 @@  void string_list_clear_func(struct string_list *list, string_list_clear_func_t c
 int for_each_string_list(struct string_list *list,
 			 string_list_each_func_t func, void *cb_data);
 
-/** Iterate over each item, as a macro. */
+/**
+ * Iterate over each item, as a macro.
+ *
+ * Be sure that 'list' is non-NULL. The macro cannot perform NULL
+ * checks due to -Werror=address errors.
+ */
 #define for_each_string_list_item(item,list)            \
 	for (item = (list)->items;                      \
 	     item && item < (list)->items + (list)->nr; \