[FFmpeg-devel,1/2] lavf: Add general API for IO buffer synchtonization.

This commit adds a new set of functions to avio and url subsystems, which
allow users to invoke IO buffer synchronization with the underlying media.
The most obvious target for this extension if the filesystem streams. Invoking
IO synchronization allows user applications to ensure that all written content
has reached the filesystem on the media and can be observed by other processes.

The public API for this is avio_sync() function, which can be called on
AVIOContext. The internal, lower layer API is ffurl_sync(), which works
directly on the underlying URLContext. URLContext backends can add support for
this new API by setting the sync handler to the new url_sync member of
URLProtocol. When no handler is set then the sync operation is a no-op.
---
 libavformat/avio.c    |  7 +++++++
 libavformat/avio.h    | 16 ++++++++++++++++
 libavformat/aviobuf.c | 17 +++++++++++++++++
 libavformat/url.h     | 12 ++++++++++++
 4 files changed, 52 insertions(+)

Andrey Semashev (2018-12-03):
> This commit adds a new set of functions to avio and url subsystems, which
> allow users to invoke IO buffer synchronization with the underlying media.
> The most obvious target for this extension if the filesystem streams. Invoking
> IO synchronization allows user applications to ensure that all written content
> has reached the filesystem on the media and can be observed by other processes.
> 
> The public API for this is avio_sync() function, which can be called on
> AVIOContext. The internal, lower layer API is ffurl_sync(), which works
> directly on the underlying URLContext. URLContext backends can add support for
> this new API by setting the sync handler to the new url_sync member of
> URLProtocol. When no handler is set then the sync operation is a no-op.
> ---
>  libavformat/avio.c    |  7 +++++++
>  libavformat/avio.h    | 16 ++++++++++++++++
>  libavformat/aviobuf.c | 17 +++++++++++++++++
>  libavformat/url.h     | 12 ++++++++++++
>  4 files changed, 52 insertions(+)
> 
> diff --git a/libavformat/avio.c b/libavformat/avio.c
> index 663789ec02..19413ac586 100644
> --- a/libavformat/avio.c
> +++ b/libavformat/avio.c
> @@ -623,6 +623,13 @@ int64_t ffurl_size(URLContext *h)
>      return size;
>  }
>  
> +int ffurl_sync(URLContext *h)
> +{
> +    if (h->prot->url_sync)
> +        return h->prot->url_sync(h);

> +    return 0;

I think it should be an error, probably ENOSUP. Otherwise, the
application will assume it was done.

The doc needs to be changed accordingly of course.

> +}
> +
>  int ffurl_get_file_handle(URLContext *h)
>  {
>      if (!h || !h->prot || !h->prot->url_get_file_handle)
> diff --git a/libavformat/avio.h b/libavformat/avio.h
> index 75912ce6be..784cd1b509 100644
> --- a/libavformat/avio.h
> +++ b/libavformat/avio.h
> @@ -349,6 +349,11 @@ typedef struct AVIOContext {
>       * Try to buffer at least this amount of data before flushing it
>       */
>      int min_packet_size;
> +
> +    /**
> +     * A callback for synchronizing buffers with the media state.
> +     */
> +    int (*sync)(void *opaque);
>  } AVIOContext;
>  
>  /**
> @@ -586,6 +591,17 @@ int avio_printf(AVIOContext *s, const char *fmt, ...) av_printf_format(2, 3);
>   */
>  void avio_flush(AVIOContext *s);
>  
> +/**
> + * Flush internal buffers and ensure the synchronized state of the
> + * resource associated with the context s. This call may be expensive.
> + * Not all resources support syncing, this operation is a no-op
> + * if sync is not supported or needed.
> + * This function can only be used if s was opened by avio_open().
> + *
> + * @return 0 on success, an AVERROR < 0 on error.
> + */
> +int avio_sync(AVIOContext *s);
> +
>  /**
>   * Read size bytes from AVIOContext into buf.
>   * @return number of bytes read or AVERROR
> diff --git a/libavformat/aviobuf.c b/libavformat/aviobuf.c
> index 5a33f82950..10be372bce 100644
> --- a/libavformat/aviobuf.c
> +++ b/libavformat/aviobuf.c
> @@ -243,6 +243,14 @@ void avio_flush(AVIOContext *s)
>          avio_seek(s, seekback, SEEK_CUR);
>  }
>  
> +int avio_sync(AVIOContext *s)
> +{
> +    avio_flush(s);
> +    if (s->sync)
> +        return s->sync(s->opaque);
> +    return 0;
> +}
> +
>  int64_t avio_seek(AVIOContext *s, int64_t offset, int whence)
>  {
>      int64_t offset1;
> @@ -978,6 +986,12 @@ static int64_t io_read_seek(void *opaque, int stream_index, int64_t timestamp, i
>      return internal->h->prot->url_read_seek(internal->h, stream_index, timestamp, flags);
>  }
>  
> +static int io_sync(void *opaque)
> +{
> +    AVIOInternal *internal = opaque;
> +    return ffurl_sync(internal->h);
> +}
> +
>  int ffio_fdopen(AVIOContext **s, URLContext *h)
>  {
>      AVIOInternal *internal = NULL;
> @@ -1026,6 +1040,9 @@ int ffio_fdopen(AVIOContext **s, URLContext *h)
>  
>          if (h->prot->url_read_seek)
>              (*s)->seekable |= AVIO_SEEKABLE_TIME;
> +
> +        if (h->prot->url_sync)
> +            (*s)->sync = io_sync;
>      }
>      (*s)->short_seek_get = io_short_seek;
>      (*s)->av_class = &ff_avio_class;
> diff --git a/libavformat/url.h b/libavformat/url.h
> index 4750bfff82..26658695b0 100644
> --- a/libavformat/url.h
> +++ b/libavformat/url.h
> @@ -97,6 +97,7 @@ typedef struct URLProtocol {
>      int (*url_delete)(URLContext *h);
>      int (*url_move)(URLContext *h_src, URLContext *h_dst);
>      const char *default_whitelist;
> +    int (*url_sync)(URLContext *h);
>  } URLProtocol;
>  
>  /**
> @@ -228,6 +229,17 @@ int64_t ffurl_seek(URLContext *h, int64_t pos, int whence);
>  int ffurl_closep(URLContext **h);
>  int ffurl_close(URLContext *h);
>  
> +/**
> + * Flush any buffered data and synchronize the resource accessed
> + * by the URLContext h. This call may be expensive.
> + * Not all types of resources support syncing, the call is a no-op
> + * if sync is not supported or needed.
> + *
> + * @return a negative value if an error condition occurred, 0
> + * otherwise
> + */
> +int ffurl_sync(URLContext *h);
> +
>  /**
>   * Return the filesize of the resource accessed by h, AVERROR(ENOSYS)
>   * if the operation is not supported by h, or another negative value

Regards,

On 12/3/18 3:43 PM, Nicolas George wrote:
> Andrey Semashev (2018-12-03):
>> This commit adds a new set of functions to avio and url subsystems, which
>> allow users to invoke IO buffer synchronization with the underlying media.
>> The most obvious target for this extension if the filesystem streams. Invoking
>> IO synchronization allows user applications to ensure that all written content
>> has reached the filesystem on the media and can be observed by other processes.
>>
>> The public API for this is avio_sync() function, which can be called on
>> AVIOContext. The internal, lower layer API is ffurl_sync(), which works
>> directly on the underlying URLContext. URLContext backends can add support for
>> this new API by setting the sync handler to the new url_sync member of
>> URLProtocol. When no handler is set then the sync operation is a no-op.
>> ---
>>   libavformat/avio.c    |  7 +++++++
>>   libavformat/avio.h    | 16 ++++++++++++++++
>>   libavformat/aviobuf.c | 17 +++++++++++++++++
>>   libavformat/url.h     | 12 ++++++++++++
>>   4 files changed, 52 insertions(+)
>>
>> diff --git a/libavformat/avio.c b/libavformat/avio.c
>> index 663789ec02..19413ac586 100644
>> --- a/libavformat/avio.c
>> +++ b/libavformat/avio.c
>> @@ -623,6 +623,13 @@ int64_t ffurl_size(URLContext *h)
>>       return size;
>>   }
>>   
>> +int ffurl_sync(URLContext *h)
>> +{
>> +    if (h->prot->url_sync)
>> +        return h->prot->url_sync(h);
> 
>> +    return 0;
> 
> I think it should be an error, probably ENOSUP. Otherwise, the
> application will assume it was done.

I don't think this would be useful to users. I mean, the application can 
already check whether the operations is supported or not by inspecting 
url_sync. When the application doesn't care (which, I think, will be the 
majority of cases), they will treat ENOTSUP the same way as a success, 
so it only adds more burden to test for this error code.

I could use some separate "success" code for the "not supported" result, 
if there is one. Is there?