Migrate from NGINX-RTMP, support exec

[winlinvip]

https://github.com/winlinvip/simple-rtmp-server/issues/367

[winlinvip]

This wiki discusses the handling of the exec signal SIGTERM.
https://github.com/arut/nginx-rtmp-module/wiki/Exec-wrapper-in-bash

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[winlinvip]

This mentions executing "exec_record_done" when the DVR ends, adding metadata to the FLV file.
https://github.com/arut/nginx-rtmp-module/wiki/FAQ#seek-does-not-work-with-flv-files-recorded-by-the-module

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[winlinvip]

exec is mainly introduced in this wiki: https://github.com/arut/nginx-rtmp-module/wiki/Directives#exec

'
Please make sure to maintain the markdown structure.

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[winlinvip]

https://github.com/arut/nginx-rtmp-module/wiki/Directives#exec_push

exec_push command arg*

Specifies external command with arguments to be executed on every stream published. When publishing stops the process is terminated.

NGINX-RTMP is a program that is executed during streaming and sends a quit signal to the program when the streaming ends.

The available variables include:

$name - stream name
$app - application name
$addr - client address
$flashver - client flash version
$swfurl - client swf url
$tcurl - client tc url
$pageurl - client page url

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[winlinvip]

https://github.com/arut/nginx-rtmp-module/wiki/Directives#exec_pull

exec_pull command arg*

Specifies external command with arguments to be executed on play event. The command is executed when first client connects to the stream and is killed when the last one disconnects. This directive makes it possible to pull remote stream in any format for local clients.

When the first client starts playing, the process is initiated. The program ends when the last client exits.

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[winlinvip]

https://github.com/arut/nginx-rtmp-module/wiki/Directives#exec_static
Ingest can be considered as an implementation of this feature.

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[winlinvip]

https://github.com/arut/nginx-rtmp-module/wiki/Directives#respawn
NGINX-RTMP will restart the process when the process exits while the stream is still active.

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[winlinvip]

https://github.com/arut/nginx-rtmp-module/wiki/Directives#exec_record_done
Command executed upon completion of the DVR.

recorder - recorder name
path - recorded file path (/tmp/rec/mystream-1389499351.flv)
filename - path with directory omitted (mystream-1389499351.flv)
basename - file name with extension omitted (mystream-1389499351)
dirname - directory path (/tmp/rec)

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[winlinvip]

It seems that these exec commands only need to support two options:

1. exec: Start a process when the streaming starts.
2. exec_on_dvr: Also known as exec_record_done, start a program when the recording is completed.

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[winlinvip]

https://github.com/arut/nginx-rtmp-module/wiki/Directives#on_record_done
NGINX-RTMP also provides HTTP callbacks, similar to SRS's on_dvr.

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[winlinvip]

I haven't decided whether to support exec yet, so let's put this feature on hold.

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