[Patch] flv: add support for time offset

[Jianyong Chen]

> On Aug 12, 2019, at 7:06 PM, winshining <winshining at 163.com> wrote:
> 
> Hi, balus
> I think the second parameter of ngx_http_flv_read in ngx_http_flv_read_metadata and ngx_http_flv_read_tags is incorrect, it should be NGX_FLV_TAG_HEADER_SIZE.
> Besides, 'keyframes' is a non-standard object in metadata, there is no description about it in official tech document, you can refer to https://www.adobe.com/devnet/f4v.html <https://www.adobe.com/devnet/f4v.html> for details. Therefore, if a FLV file without the 'keyframes' object in metadata is played, the member 'times' of structure ngx_http_flv_file_t will be NULL, ngx_http_flv_timestamp_to_offset will crash then.
> 
> 
>  
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Thanks for reading my code!

The first problem is a little confusing indeed. Notice that
NGX_FLV_HEADER_SIZE == NGX_FLV_TAG_HEADER_SIZE + NGX_FLV_PREV_TAG_SIZE，
and when I read a tag, I actually read tag_header + tag_data + prev_tag_size,
thus NGX_FLV_HEADER_SIZE is just what I need. Anyway, this is my fault, I
should have unified the two macros.

The second probelm has been carefully considered in the code. If there is no
‘keyframes’ object found in metadata, ngx_http_flv_parse_metadata() will return
NGX_DECLINED, which means to send the whole file, and therefore
ngx_http_flv_timestamp_to_offset() won’t be called and thus won’t crash.

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