Re: [PATCH v18 3/4] qcow2: add zstd cluster compression

[Alberto Garcia]

On Thu 02 Apr 2020 08:36:44 AM CEST, Denis Plotnikov wrote:
> +static ssize_t qcow2_zstd_compress(void *dest, size_t dest_size,
> +                                   const void *src, size_t src_size)
> +{
> +    ssize_t ret;
> +    ZSTD_outBuffer output = { dest, dest_size, 0 };
> +    ZSTD_inBuffer input = { src, src_size, 0 };
> +    ZSTD_CCtx *cctx = ZSTD_createCCtx();
> +
> +    if (!cctx) {
> +        return -EIO;
> +    }
> +    /*
> +     * Use the zstd streamed interface for symmetry with decompression,
> +     * where streaming is essential since we don't record the exact
> +     * compressed size.
> +     *
> +     * In the loop, we try to compress all the data into one zstd frame.
> +     * ZSTD_compressStream2 potentially can finish a frame earlier
> +     * than the full input data is consumed. That's why we are looping
> +     * until all the input data is consumed.
> +     */
> +    while (input.pos < input.size) {
> +        size_t zstd_ret;
> +        /*
> +         * ZSTD spec: "You must continue calling ZSTD_compressStream2()
> +         * with ZSTD_e_end until it returns 0, at which point you are
> +         * free to start a new frame". We assume that "start a new frame"
> +         * means call ZSTD_compressStream2 in the very beginning or when
> +         * ZSTD_compressStream2 has returned with 0.
> +         */
> +        do {
> +            zstd_ret = ZSTD_compressStream2(cctx, &output, &input, 
> ZSTD_e_end);
> +
> +            if (ZSTD_isError(zstd_ret)) {
> +                ret = -EIO;
> +                goto out;
> +            }
> +            /* Dest buffer isn't big enough to store compressed content */
> +            if (zstd_ret > output.size - output.pos) {
> +                ret = -ENOMEM;
> +                goto out;
> +            }
> +        } while (zstd_ret);
> +    }
> +    /* make sure we can safely return compressed buffer size with ssize_t */
> +    assert(output.pos <= SSIZE_MAX);

The patch looks good to me, but why don't you assert this at the
beginning of the function? You already know the buffer sizes...

Either way,

Reviewed-by: Alberto Garcia <address@hidden>

Berto