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

[Junio C Hamano <gitster@pobox.com>]

Junio C Hamano <gitster@pobox.com> 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);