[core] Repurpose SRTO_GROUPMINSTABLETIMEO

[maxsharabayko]

SRTO_GROUPMINSTABLETIMEO is a socket option for SRT socket group. It does not do anything when set on an individual socket, but when set on a listener socket - it is used to derive the value for a group. Defines the minimum value of the stability timeout for a member socket to be considered unstable.

API Changes

• Renamed SRTO_GROUPSTABTIMEO to SRTO_GROUPMINSTABLETIMEO. Default: 60 ms.
• The SRTO_GROUPMINSTABLETIMEO option can be set on an individual socket and on a group socket.

Logic

• The option value is used in CUDTGroup::sendBackup_QualifyActiveState(..) instead of the constant value of 60 ms.

URI Scheme

• groupminstabletimeo=<int>

Tests

• Several Listeners in a Group, Sender.
• One Listener in a Group, Sender.
• A Group of Callers, Sender

Docs

• Update the docs

Fixes #1792
Fixes #2254 (a typo).

[maxsharabayko]

Test Cases

1. Several Listeners in a Group, Sender

Several listeners form a group. Confirm the groupminstabletimeo is applied.

srt-xtransmit generate "srt://:4201?groupminstabletimeo=70" srt://:4200 -v --sendrate 1Mbps --duration 5s

srt-xtransmit receive "srt://127.0.0.1:4200?grouptype=backup" srt://127.0.0.1:4201 -v

2. One Listener in a Group, Sender

One listener forms a group. Confirm the groupminstabletimeo is applied.

srt-xtransmit generate "srt://:4201?groupconnect=1&groupminstabletimeo=70" -v --sendrate 1Mbps \
    --duration 5s

srt-xtransmit receive "srt://127.0.0.1:4201?grouptype=backup" srt://127.0.0.1:4201 -v

3. A Group of Callers, Sender

Several callers form a group. Confirm the groupminstabletimeo is applied.

srt-xtransmit generate "srt://127.0.0.1:4200?grouptype=backup&groupminstabletimeo=70" srt://127.0.0.1:4201 \
    --sendrate 1Mbps --duration 5s -v

srt-xtransmit receive "srt://:4200" srt://:4201 -v

[stevomatthews]

Minor edits to API-socket-options.md