Re: [PATCH 04/10] commit-slab: add a function to deep free entries on the slab

From: René Scharfe <l.s.r@web.de>
Date: Jun 18, 2020, 20:59 UTC
Message-ID: <1c982d57-a446-b655-af95-1e4aeabf30ef@web.de>
In-Reply-To: <32a6f11cc475f5292075093dcdc3b2a5c6d02ed0.1591362033.git.gitgitgadget@gmail.com>

Am 05.06.20 um 15:00 schrieb SZEDER Gábor via GitGitGadget:

Show 20 quoted lines

> From: =?UTF-8?q?SZEDER=20G=C3=A1bor?= <szeder.dev@gmail.com>
>
> clear_##slabname() frees only the memory allocated for a commit slab
> itself, but entries in the commit slab might own additional memory
> outside the slab that should be freed as well.  We already have (at
> least) one such commit slab, and this patch series is about to add one
> more.
>
> To free all additional memory owned by entries on the commit slab the
> user of such a slab could iterate over all commits it knows about,
> peek whether there is a valid entry associated with each commit, and
> free the additional memory, if any.  Or it could rely on intimate
> knowledge about the internals of the commit slab implementation, and
> could itself iterate directly through all entries in the slab, and
> free the additional memory.  Or it could just leak the additional
> memory...
>
> Introduce deep_clear_##slabname() to allow releasing memory owned by
> commit slab entries by invoking the 'void free_fn(elemtype *ptr)'
> function specified as parameter for each entry in the slab.

Adding a new function instead of extending the existing ones makes sense, as this is a rare requirement.

Show 44 quoted lines

>
> Use it in get_shallow_commits() in 'shallow.c' to replace an
> open-coded iteration over a commit slab's entries.
>
> Signed-off-by: SZEDER Gábor <szeder.dev@gmail.com>
> Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
> ---
>  commit-slab-decl.h |  1 +
>  commit-slab-impl.h | 13 +++++++++++++
>  commit-slab.h      | 10 ++++++++++
>  shallow.c          | 14 +++++---------
>  4 files changed, 29 insertions(+), 9 deletions(-)
>
> diff --git a/commit-slab-decl.h b/commit-slab-decl.h
> index adc7b46c83b..286164b7e27 100644
> --- a/commit-slab-decl.h
> +++ b/commit-slab-decl.h
> @@ -32,6 +32,7 @@ struct slabname {							\
>  void init_ ##slabname## _with_stride(struct slabname *s, unsigned stride); \
>  void init_ ##slabname(struct slabname *s);				\
>  void clear_ ##slabname(struct slabname *s);				\
> +void deep_clear_ ##slabname(struct slabname *s, void (*free_fn)(elemtype *ptr)); \
>  elemtype *slabname## _at_peek(struct slabname *s, const struct commit *c, int add_if_missing); \
>  elemtype *slabname## _at(struct slabname *s, const struct commit *c);	\
>  elemtype *slabname## _peek(struct slabname *s, const struct commit *c)
> diff --git a/commit-slab-impl.h b/commit-slab-impl.h
> index 5c0eb91a5d1..557738df271 100644
> --- a/commit-slab-impl.h
> +++ b/commit-slab-impl.h
> @@ -38,6 +38,19 @@ scope void clear_ ##slabname(struct slabname *s)			\
>  	FREE_AND_NULL(s->slab);						\
>  }									\
>  									\
> +scope void deep_clear_ ##slabname(struct slabname *s, void (*free_fn)(elemtype *)) \
> +{									\
> +	unsigned int i;							\
> +	for (i = 0; i < s->slab_count; i++) {				\
> +		unsigned int j;						\
> +		if (!s->slab[i])					\
> +			continue;					\
> +		for (j = 0; j < s->slab_size; j++)			\
> +			free_fn(&s->slab[i][j * s->stride]);		\
> +	}								\
> +	clear_ ##slabname(s);						> +}									\

Why pass an elemtype pointer to the callback function instead of a plain elemtype? Because it matches the return type of _at() and _peek(). Consistency, good. Handing it a pointer allows the callback to pass it on to free(), though, which would be bad, since we do that in clear_() as well. Hmm.

Show 37 quoted lines

> +									\
>  scope elemtype *slabname## _at_peek(struct slabname *s,			\
>  						  const struct commit *c, \
>  						  int add_if_missing)   \
> diff --git a/commit-slab.h b/commit-slab.h
> index 05b3f2804e7..8e72a305365 100644
> --- a/commit-slab.h
> +++ b/commit-slab.h
> @@ -47,6 +47,16 @@
>   *
>   *   Call this function before the slab falls out of scope to avoid
>   *   leaking memory.
> + *
> + * - void deep_clear_indegree(struct indegree *, void (*free_fn)(int*))
> + *
> + *   Empties the slab, similar to clear_indegree(), but in addition it
> + *   calls the given 'free_fn' for each slab entry to release any
> + *   additional memory that might be owned by the entry (but not the
> + *   entry itself!).
> + *   Note that 'free_fn' might be called even for entries for which no
> + *   indegree_at() call has been made; in this case 'free_fn' is invoked
> + *   with a pointer to a zero-initialized location.
>   */
>
>  #define define_commit_slab(slabname, elemtype) \
> diff --git a/shallow.c b/shallow.c
> index 7fd04afed19..c4ac8a73273 100644
> --- a/shallow.c
> +++ b/shallow.c
> @@ -84,6 +84,10 @@ int is_repository_shallow(struct repository *r)
>   * supports a "valid" flag.
>   */
>  define_commit_slab(commit_depth, int *);
> +static void free_depth_in_slab(int **ptr)
> +{
> +	FREE_AND_NULL(*ptr);
> +}

Why FREE_AND_NULL? The original loop below called free(). The slabs are all released by deep_clear_() immediately after the callbacks are done anyway, so what's the point in zeroing these pointers?

Show 17 quoted lines

>  struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
>  		int shallow_flag, int not_shallow_flag)
>  {
> @@ -150,15 +154,7 @@ struct commit_list *get_shallow_commits(struct object_array *heads, int depth,
>  			}
>  		}
>  	}
> -	for (i = 0; i < depths.slab_count; i++) {
> -		int j;
> -
> -		if (!depths.slab[i])
> -			continue;
> -		for (j = 0; j < depths.slab_size; j++)
> -			free(depths.slab[i][j]);
> -	}
> -	clear_commit_depth(&depths);
> +	deep_clear_commit_depth(&depths, free_depth_in_slab);

What a relief!

>
>  	return result;
>  }
>