[FFmpeg-devel,09/35] lavu/fifo: add a flag for automatically growing the FIFO as needed

This will not increase the FIFO beyond 1MB, unless the caller explicitly
specifies otherwise.
---
 doc/APIchanges   |  3 ++-
 libavutil/fifo.c | 41 +++++++++++++++++++++++++++++++++++++++--
 libavutil/fifo.h | 15 ++++++++++++++-
 3 files changed, 55 insertions(+), 4 deletions(-)

Anton Khirnov:
> This will not increase the FIFO beyond 1MB, unless the caller explicitly
> specifies otherwise.
> ---
>  doc/APIchanges   |  3 ++-
>  libavutil/fifo.c | 41 +++++++++++++++++++++++++++++++++++++++--
>  libavutil/fifo.h | 15 ++++++++++++++-
>  3 files changed, 55 insertions(+), 4 deletions(-)
> 
> diff --git a/doc/APIchanges b/doc/APIchanges
> index f64759d69d..52b42762ea 100644
> --- a/doc/APIchanges
> +++ b/doc/APIchanges
> @@ -20,7 +20,8 @@ API changes, most recent first:
>    rather than bytes.
>    Add av_fifo_elem_size(), av_fifo_can_read(), av_fifo_can_write(),
>    av_fifo_grow2(), av_fifo_drain2(), av_fifo_write(), av_fifo_write_from_cb(),
> -  av_fifo_read(), av_fifo_read_to_cb(), av_fifo_peek(), av_fifo_peek_to_cb().
> +  av_fifo_read(), av_fifo_read_to_cb(), av_fifo_peek(), av_fifo_peek_to_cb(),
> +  av_fifo_auto_grow_limit().
>  
>  2022-01-xx - xxxxxxxxxx - lavu fifo.h
>    Access to all AVFifoBuffer members is deprecated. The struct will
> diff --git a/libavutil/fifo.c b/libavutil/fifo.c
> index ea944bc936..2c15df5d5c 100644
> --- a/libavutil/fifo.c
> +++ b/libavutil/fifo.c
> @@ -34,6 +34,9 @@
>  # define CTX_STRUCT_NAME AVFifoBuffer
>  #endif
>  
> +// by default the FIFO can be auto-grown to 1MB
> +#define AUTO_GROW_DEFAULT_BYTES (1024 * 1024)
> +
>  typedef struct CTX_STRUCT_NAME {
>      // These fields must match then contents of AVFifoBuffer in fifo.h
>      // until FF_API_FIFO_PUBLIC is removed
> @@ -48,6 +51,9 @@ typedef struct CTX_STRUCT_NAME {
>      size_t offset_r, offset_w;
>      // distinguishes the ambigous situation offset_r == offset_w
>      int    is_empty;
> +
> +    unsigned int flags;
> +    size_t       auto_grow_limit;
>  } FifoBuffer;
>  
>  AVFifoBuffer *av_fifo_alloc2(size_t nb_elems, size_t elem_size,
> @@ -75,10 +81,19 @@ AVFifoBuffer *av_fifo_alloc2(size_t nb_elems, size_t elem_size,
>      f->nb_elems  = nb_elems;
>      f->elem_size = elem_size;
>  
> +    f->flags           = flags;
> +    f->auto_grow_limit = FFMAX(AUTO_GROW_DEFAULT_BYTES / elem_size, 1);
> +
>      av_fifo_reset((AVFifoBuffer*)f);
>      return (AVFifoBuffer*)f;
>  }
>  
> +void av_fifo_auto_grow_limit(AVFifoBuffer *f, size_t max_elems)
> +{
> +    FifoBuffer *fb = (FifoBuffer*)f;
> +    fb->auto_grow_limit = max_elems;
> +}
> +
>  AVFifoBuffer *av_fifo_alloc_array(size_t nmemb, size_t size)
>  {
>      if (nmemb > SIZE_MAX / size)
> @@ -207,6 +222,27 @@ void av_fifo_drain2(AVFifoBuffer *f, size_t size)
>  #endif
>  }
>  
> +static int fifo_check_space(AVFifoBuffer *f, size_t to_write)
> +{
> +    FifoBuffer         *fb = (FifoBuffer*)f;
> +    const size_t can_write = av_fifo_can_write(f);
> +    const size_t need_grow = to_write > can_write ? to_write - can_write : 0;
> +    size_t can_grow;
> +
> +    if (!need_grow)
> +        return 0;
> +
> +    can_grow = fb->auto_grow_limit > fb->nb_elems ?
> +               fb->auto_grow_limit - fb->nb_elems : 0;
> +    if ((fb->flags & AV_FIFO_FLAG_AUTO_GROW) && need_grow <= can_grow) {
> +        // allocate a bit more than necessary, if we can
> +        const size_t inc = (need_grow < can_grow / 2 ) ? need_grow * 2 : can_grow;

This is still linear if one always writes a fixed amount of elements.
Using av_fast_realloc_array() would actually fit very well in here.
Shall I resend a size_t version?

> +        return av_fifo_grow2(f, inc);
> +    }
> +
> +    return AVERROR(ENOSPC);
> +}
> +
>  static int fifo_write_common(AVFifoBuffer *f, const uint8_t *buf, size_t *nb_elems,
>                               AVFifoCB read_cb, void *opaque)
>  {
> @@ -218,8 +254,9 @@ static int fifo_write_common(AVFifoBuffer *f, const uint8_t *buf, size_t *nb_ele
>      uint32_t wndx= f->wndx;
>  #endif
>  
> -    if (to_write > av_fifo_can_write(f))
> -        return AVERROR(ENOSPC);
> +    ret = fifo_check_space(f, to_write);
> +    if (ret < 0)
> +        return ret;
>  
>      do {
>          size_t    len = FFMIN(fb->nb_elems - offset_w, to_write);
> diff --git a/libavutil/fifo.h b/libavutil/fifo.h
> index c7be5e8f7d..11eb36944a 100644
> --- a/libavutil/fifo.h
> +++ b/libavutil/fifo.h
> @@ -83,6 +83,13 @@ AVFifoBuffer *av_fifo_alloc(unsigned int size);
>   */
>  AVFifoBuffer *av_fifo_alloc_array(size_t nmemb, size_t size);
>  
> +/**
> + * Automatically resize the FIFO on writes, so that the data fits. This
> + * automatic resizing happens up to a limit that can be modified with
> + * av_fifo_auto_grow_limit().
> + */
> +#define AV_FIFO_FLAG_AUTO_GROW      (1 << 0)
> +
>  /**
>   * Allocate and initialize an AVFifoBuffer with a given element size.
>   *
> @@ -90,7 +97,7 @@ AVFifoBuffer *av_fifo_alloc_array(size_t nmemb, size_t size);
>   * @param nb_elems initial number of elements that can be stored in the FIFO
>   * @param elem_size Size in bytes of a single element. Further operations on
>   *                  the returned FIFO will implicitly use this element size.
> - * @param flags currently unused, must be 0
> + * @param flags a combination of AV_FIFO_FLAG_*
>   *
>   * @return newly-allocated AVFifoBuffer on success, a negative error code on failure
>   */
> @@ -115,6 +122,12 @@ void av_fifo_freep(AVFifoBuffer **f);
>   */
>  size_t av_fifo_elem_size(const AVFifoBuffer *f);
>  
> +/**
> + * Set the maximum size (in elements) to which the FIFO can be resized
> + * automatically. Has no effect unless AV_FIFO_FLAG_AUTO_GROW is used.
> + */
> +void av_fifo_auto_grow_limit(AVFifoBuffer *f, size_t max_elems);
> +
>  /**
>   * Reset the AVFifoBuffer to the state right after av_fifo_alloc, in particular it is emptied.
>   * @param f AVFifoBuffer to reset
>