Subject: Re: [PATCH] mv: prevent mismatched data when ignoring errors.

From: Michael Haggerty <[email-masked]>
Date: Mon, 17 Mar 2014 16:07:46 +0100

On 03/17/2014 07:33 AM, Junio C Hamano wrote:
> Junio C Hamano <[email-masked]> writes:
> 
>> Would it make sense to go one step further to introduce two macros
>> to make this kind of screw-up less likely?
>> ...
>> After letting my eyes coast over hits from "git grep memmove", there
>> do seem to be some places that these would help readability, but not
>> very many.
> 
> I see quite a many hits that follow this pattern
> 
> 	memmove(array + pos, array + pos + 1, sizeof(*array) * (nr - pos))
> 
> to make a single slot in a middle of array available, which would be
> good candidates to use MOVE_DOWN().  Just to show a few:
> 
> builtin/mv.c:226:	memmove(source + i, source + i + 1,
> builtin/mv.c-227-		(argc - i) * sizeof(char *));
> builtin/mv.c:228:	memmove(destination + i,
> builtin/mv.c-229-		destination + i + 1,
> builtin/mv.c-230-		(argc - i) * sizeof(char *));
> cache-tree.c:92:	memmove(it->down + pos + 1,
> cache-tree.c-93-		it->down + pos,
> cache-tree.c-94-		sizeof(down) * (it->subtree_nr - pos - 1));
> 
> 
> Perhaps something like this patch to start off; I am not sure
> MOVE_DOWN_BOUNDED is needed, though.
> 
>  cache.h | 33 +++++++++++++++++++++++++++++++++
>  1 file changed, 33 insertions(+)
> 
> diff --git a/cache.h b/cache.h
> index b66cb49..b2615ab 100644
> --- a/cache.h
> +++ b/cache.h
> @@ -455,6 +455,39 @@ extern int daemonize(void);
>  		} \
>  	} while (0)
>  
> +/*
> + * With an array "array" that currently holds "nr" elements, move
> + * elements at "at" and later down by "count" elements to make room to
> + * add in new elements.  The caller is responsible for making sure
> + * that the array has enough room to hold "nr" + "count" slots.
> + */
> +#define MOVE_DOWN(array, nr, at, count)			\
> +	memmove((array) + (at) + (count),		\
> +		(array) + (at),				\
> +		sizeof((array)[0]) * ((nr) - (at)))
> +
> +/*
> + * With an array "array" that has enough memory to hold "alloc"
> + * elements allocated and currently holds "nr" elements, move elements
> + * at "at" and later down by "count" elements to make room to add in
> + * new elements.
> + */
> +#define MOVE_DOWN_BOUNDED(array, nr, at, count, alloc)		     \
> +	do {							     \
> +		if ((alloc) <= (nr) + (count))			     \
> +			BUG("MOVE_DOWN beyond the end of an array"); \
> +		MOVE_DOWN((array), (nr), (at), (count));	     \
> +	} while (0)
> +
> +/*
> + * With an array "array" that curently holds "nr" elements, move elements
> + * at "at" + "count" and later down by "count" elements, removing the
> + * elements between "at" and "at" + "count".
> + */
> +#define MOVE_UP(array, nr, at, count)				\
> +	memmove((array) + (at), (array) + (at) + (count),	\
> +		sizeof((array)[0]) * ((nr) - ((at) + (count))))
> +
>  /* Initialize and use the cache information */
>  extern int read_index(struct index_state *);
>  extern int read_index_preload(struct index_state *, const struct pathspec *pathspec);

I had recently been thinking along the same lines.  In many of the
potential callers that I noticed, ALLOC_GROW() was used immediately
before making space in the array for a new element.  So I suggest
something more like

+#define MOVE_DOWN(array, nr, at, count)	\
+	memmove((array) + (at) + (count),		\
+		(array) + (at),				\
+		sizeof((array)[0]) * ((nr) - (at)))
+#define ALLOC_INSERT_GAP(array, nr, at, count, alloc)		     \
+	do {							     \
+		ALLOC_GROW((array), (nr) + (count), (alloc));        \
+		MOVE_DOWN((array), (nr), (at), (count));	     \
+	} while (0)

Also, count==1 is so frequent that this special case might deserve its
own macro pair.

I'm not inspired by these macro names, though.

Michael

-- 
Michael Haggerty
[email-masked]
http://softwareswirl.blogspot.com/