Re: [PATCH] for_each_string_list_item(): behave correctly for empty list

[Michael Haggerty <mhagger@alum.mit.edu>]

On 09/16/2017 01:51 PM, SZEDER Gábor wrote:
>>>> It would be a pain to have to change the signature of this macro, and
>>>> we'd prefer not to add overhead to each iteration of the loop. So
>>>> instead, whenever `list->items` is NULL, initialize `item` to point at
>>>> a dummy `string_list_item` created for the purpose.
>>>
>>> What signature change do you mean?  I don't understand what this
>>> paragraph is alluding to.
>>
>> I was thinking that one solution would be for the caller to provide a
>> `size_t` variable for the macro's use as a counter (since I don't see a
>> way for the macro to declare its own counter). The options are pretty
>> limited because whatever the macro expands to has to play the same
>> syntactic role as `for (...; ...; ...)`.
> 
> Another option to consider is to squeeze in an if-else before the for
> loop header to handle the empty list case like this:
> 
> diff --git a/string-list.h b/string-list.h
> index 29bfb7ae4..9eed47de0 100644
> --- a/string-list.h
> +++ b/string-list.h
> @@ -32,8 +32,11 @@ void string_list_clear_func(struct string_list *list, string_list_clear_func_t c
>  typedef int (*string_list_each_func_t)(struct string_list_item *, void *);
>  int for_each_string_list(struct string_list *list,
>  			 string_list_each_func_t, void *cb_data);
> -#define for_each_string_list_item(item,list) \
> -	for (item = (list)->items; item < (list)->items + (list)->nr; ++item)
> +#define for_each_string_list_item(item,list) 	\
> +	if ((list)->items == NULL) {		\
> +		/* empty list, do nothing */	\
> +	} else					\
> +		for (item = (list)->items; item < (list)->items + (list)->nr; ++item)
>  
>  /*
>   * Apply want to each item in list, retaining only the ones for which
> 
> This way there would be neither additional overhead in each iteration
> nor a new global.
> 
> Alas, there is a catch.  We can't use curly braces in the macro's else
> branch, because the macro would contain only the opening brace but not
> the closing one, which must come after the end of the loop's body.
> This means that the modified macro couldn't be used in if-else
> branches which themselves don't have curly braces, because it causes
> ambiguity:
> 
>   if (condition)
>       for_each_string_list_item(item, list)
>           a_simple_oneliner(item);

It's not ambiguous as far as the language standard is concerned. The
latter is clear that an `else` binds to the nearest `if`. The problem is
that this is a common programmer error, so compilers "helpfully" warn
about it even though it would do exactly what we want.

> Our coding guidelines encourage this style for one-liner loop bodies,
> and there is indeed one such place in our codebase, so the following
> hunk is needed as well:
> 
> diff --git a/send-pack.c b/send-pack.c
> index 11d6f3d98..00fa1622f 100644
> --- a/send-pack.c
> +++ b/send-pack.c
> @@ -295,9 +295,10 @@ static int generate_push_cert(struct strbuf *req_buf,
>  	}
>  	if (push_cert_nonce[0])
>  		strbuf_addf(&cert, "nonce %s\n", push_cert_nonce);
> -	if (args->push_options)
> +	if (args->push_options) {
>  		for_each_string_list_item(item, args->push_options)
>  			strbuf_addf(&cert, "push-option %s\n", item->string);
> +	}
>  	strbuf_addstr(&cert, "\n");
>  
>  	for (ref = remote_refs; ref; ref = ref->next) {
> 
> 
> Luckily, reasonably modern compilers warn about such ambiguity, so
> perhaps this is an acceptable compromise?

This change kindof goes *against* our coding guidelines, so it's not
ideal either, but I suppose we could probably live with it.

Michael