William Duclot <william.duclot@xxxxxxxxxxxxxxxxxxxxxxx> writes:
> multiple threads. Those limitations prevent strbuf to be used in
prevent strbuf from being used ...
> API ENHANCEMENT
> ---------------
>
> All functions of the API can still be reliably called without
> knowledge of the initialization (normal/preallocated/fixed) with the
> exception that strbuf_grow() may die() if the string try to overflow a
s/try/tries/
> @@ -20,16 +28,37 @@ char strbuf_slopbuf[1];
>
> void strbuf_init(struct strbuf *sb, size_t hint)
> {
> + sb->flags = 0;
> sb->alloc = sb->len = 0;
> sb->buf = strbuf_slopbuf;
> if (hint)
> strbuf_grow(sb, hint);
> }
If you set flags = 0 here, existing callers will have all flags off,
including OWNS_MEMORY.
I *think* this is OK, as sb->buf is currently pointing to
strbuf_slopbuf, which the the strbuf doesn't own. But that is too subtle
to go without an explanatory comment IMHO.
Also, doesn't this make the "new_buf" case useless in strbuf_grow?
With your patch, the code looks like:
void strbuf_grow(struct strbuf *sb, size_t extra)
{
int new_buf = !sb->alloc;
...
if (sb->flags & STRBUF_OWNS_MEMORY) {
if (new_buf) // <---------------------------------------- (1)
sb->buf = NULL;
ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
} else {
/*
* The strbuf doesn't own the buffer: to avoid to realloc it,
* the strbuf needs to use a new buffer without freeing the old
*/
if (sb->len + extra + 1 > sb->alloc) {
size_t new_alloc = MAX(sb->len + extra + 1, alloc_nr(sb->alloc));
char *buf = xmalloc(new_alloc);
memcpy(buf, sb->buf, sb->alloc);
sb->buf = buf;
sb->alloc = new_alloc;
sb->flags |= STRBUF_OWNS_MEMORY;
}
}
if (new_buf) // <---------------------------------------- (2)
sb->buf[0] = '\0';
}
I think (1) is now dead code, since sb->alloc == 0 implies that
STRBUF_OWNS_MEMORY is set. (2) seems redundant since you've just
memcpy-ed the existing '\0' into the buffer.
> +void strbuf_wrap_preallocated(struct strbuf *sb, char *path_buf,
> + size_t path_buf_len, size_t alloc_len)
> +{
> + if (!path_buf)
> + die("you try to use a NULL buffer to initialize a strbuf");
> +
> + strbuf_init(sb, 0);
> + strbuf_attach(sb, path_buf, path_buf_len, alloc_len);
> + sb->flags &= ~STRBUF_OWNS_MEMORY;
> + sb->flags &= ~STRBUF_FIXED_MEMORY;
> +}
strbuf_wrap_preallocated seem very close to strbuf_attach. I'd rather
see a symmetric code sharing like
void strbuf_attach_internal(struct strbuf *sb, ..., unsigned int flags)
and then strbuf_attach() and strbuf_wrap_preallocated() become
straightforward wrappers around it.
This would avoid setting and then unsetting STRBUF_OWNS_MEMORY (the
performance impact is probably small, but it looks weird to set the flag
and then unset it right away).
After your patch, there are differences between
strbuf_wrap_preallocated() which I think are inconsistencies:
* strbuf_attach() does not check for NULL buffer, but it doesn't accept
them either if I read correctly. It would make sense to add the check
to strbuf_attach(), but it's weird to have the performance-critical
oriented function do the expensive stuff that the
non-performance-critical one doesn't.
* strbuf_attach() calls strbuf_release(), which allows reusing an
existing strbuf. strbuf_wrap_preallocated() calls strbuf_init which
would override silently any previous content. I think strbuf_attach()
does the right thing here.
(And I'm probably the one who misguided you to do this)
In any case, you probably want to include calls to strbuf_attach() and
strbuf_wrap_*() functions on existing non-empty strbufs.
> +void strbuf_wrap_fixed(struct strbuf *sb, char *path_buf,
> + size_t path_buf_len, size_t alloc_len)
> +{
> + strbuf_wrap_preallocated(sb, path_buf, path_buf_len, alloc_len);
> + sb->flags |= STRBUF_FIXED_MEMORY;
> +}
And this could become a 3rd caller of strbuf_attach_internal().
> @@ -61,9 +96,32 @@ void strbuf_grow(struct strbuf *sb, size_t extra)
> if (unsigned_add_overflows(extra, 1) ||
> unsigned_add_overflows(sb->len, extra + 1))
> die("you want to use way too much memory");
> - if (new_buf)
> - sb->buf = NULL;
> - ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
> + if ((sb->flags & STRBUF_FIXED_MEMORY) && sb->len + extra + 1 > sb->alloc)
> + die("you try to make a string overflow the buffer of a fixed strbuf");
> +
> + /*
> + * ALLOC_GROW may do a realloc() if needed, so we must not use it on
> + * a buffer the strbuf doesn't own
> + */
> + if (sb->flags & STRBUF_OWNS_MEMORY) {
> + if (new_buf)
> + sb->buf = NULL;
> + ALLOC_GROW(sb->buf, sb->len + extra + 1, sb->alloc);
> + } else {
> + /*
> + * The strbuf doesn't own the buffer: to avoid to realloc it,
> + * the strbuf needs to use a new buffer without freeing the old
> + */
> + if (sb->len + extra + 1 > sb->alloc) {
> + size_t new_alloc = MAX(sb->len + extra + 1, alloc_nr(sb->alloc));
> + char *buf = xmalloc(new_alloc);
> + memcpy(buf, sb->buf, sb->alloc);
I think you want to memcpy only sb->len + 1 bytes. Here, you're
memcpy-ing the allocated-but-not-initialized part of the array.
xmemdupz can probably simplify the code too (either you include the '\0'
in what memcpy copies, or you let xmemdupz add it).
> +/**
> + * Allow the caller to give a pre-allocated piece of memory for the strbuf
> + * to use. It is possible then to strbuf_grow() the string past the size of the
> + * pre-allocated buffer: a new buffer will be allocated. The pre-allocated
To make it clearer: "a new buffer will then be allocated"?
> +/**
> + * Allow the caller to give a pre-allocated piece of memory for the strbuf
> + * to use and indicate that the strbuf must use exclusively this buffer,
> + * never realloc() it or allocate a new one. It means that the string can
> + * be manipulated but cannot overflow the pre-allocated buffer. The
> + * pre-allocated buffer will never be freed.
> + */
Perhaps say explicitly that although the allocated buffer has a fixed
size, the string itself can grow as long as it does not overflow the
buffer?
> @@ -91,6 +116,8 @@ extern void strbuf_release(struct strbuf *);
> * Detach the string from the strbuf and returns it; you now own the
> * storage the string occupies and it is your responsibility from then on
> * to release it with `free(3)` when you are done with it.
> + * Must allocate a copy of the buffer in case of a preallocated/fixed buffer.
> + * Performance-critical operations have to be aware of this.
Better than just warn about performance, you can give the alternative.
--
Matthieu Moy
http://www-verimag.imag.fr/~moy/
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