Re: [PATCH 5/8] odb: remove mutual recursion when parsing alternates

> When adding an alternative object database source we not only have to
> consider the added source itself, but we also have to add _its_ sources
> to our database. We implement this via mutual recursion:
> 
>   1. We first call `link_alt_odb_entries()`.
> 
>   2. `link_alt_odb_entries()` calls `parse_alternates()`.
> 
>   3. We then add each parsed alternate via `odb_add_source()`.
> 
>   4. `odb_add_source()` calls `link_alt_odb_entries()` again.
> 
> This flow is somewhat hard to follow, but more importantly it means that
> parsing of alternates is somewhat tied to the recursive behaviour.
> 
> Refactor the function to remove the mutual recursion between adding
> sources and parsing alternates. The parsing step thus becomes completely
> oblivious to the fact that there is recursive behaviour going on at all.
> Instead, the recursion is handled exclusively by `odb_add_source()`,
> which now recurses with itself.
> 
> This refactoring allows us to move parsing of alternates into object
> database sources in a subsequent step.
> 
> Signed-off-by: Patrick Steinhardt <ps@pks.im>
> ---
>  odb.c | 60 +++++++++++++++++++++++++++---------------------------------
>  1 file changed, 27 insertions(+), 33 deletions(-)
> 
> diff --git a/odb.c b/odb.c
> index 94cff19221..27f3c8e263 100644
> --- a/odb.c
> +++ b/odb.c
> @@ -147,9 +147,8 @@ static bool odb_is_source_usable(struct object_database *o, const char *path)
>   * of the object ID, an extra slash for the first level indirection, and
>   * the terminating NUL.
>   */
> -static void read_info_alternates(struct object_database *odb,
> -				 const char *relative_base,
> -				 int depth);
> +static void read_info_alternates(const char *relative_base,
> +				 struct strvec *out);
>  
>  static struct odb_source *odb_source_new(struct object_database *odb,
>  					 const char *path,
> @@ -171,6 +170,7 @@ static struct odb_source *odb_add_source(struct object_database *odb,
>  					 int depth)
>  {
>  	struct odb_source *alternate = NULL;
> +	struct strvec sources = STRVEC_INIT;
>  	khiter_t pos;
>  	int ret;
>  
> @@ -189,9 +189,17 @@ static struct odb_source *odb_add_source(struct object_database *odb,
>  	kh_value(odb->source_by_path, pos) = alternate;
>  
>  	/* recursively add alternates */
> -	read_info_alternates(odb, alternate->path, depth + 1);
> +	read_info_alternates(alternate->path, &sources);
> +	if (sources.nr && depth + 1 > 5) {
> +		error(_("%s: ignoring alternate object stores, nesting too deep"),
> +		      source);
> +	} else {
> +		for (size_t i = 0; i < sources.nr; i++)
> +			odb_add_source(odb, sources.v[i], depth + 1);
> +	}

>  
>   error:
> +	strvec_clear(&sources);
>  	return alternate;
>  }
>  

> @@ -622,13 +610,19 @@ int odb_for_each_alternate(struct object_database *odb,
>  
>  void odb_prepare_alternates(struct object_database *odb)
>  {
> +	struct strvec sources = STRVEC_INIT;
> +
>  	if (odb->loaded_alternates)
>  		return;
>  
> -	link_alt_odb_entries(odb, odb->alternate_db, PATH_SEP, NULL, 0);
> +	parse_alternates(odb->alternate_db, PATH_SEP, NULL, &sources);
> +	read_info_alternates(odb->sources->path, &sources);
> +	for (size_t i = 0; i < sources.nr; i++)
> +		odb_add_source(odb, sources.v[i], 0);