This logs Twitch's ad breaks and their respective durations, which is especially useful if --twitch-disable-ads is set, so users will know how long the output will be paused for.
Everyone's invited to give this a test:
https://github.com/streamlink/streamlink/blob/master/CONTRIBUTING.md#pull-request-feedback
This implementation caches the ad-break's X-TV-TWITCH-AD-COMMERCIAL-ID metadata in order to avoid logging multiple parts of the same ad break. This is because multiple EXT-X-DATERANGE tags can be inserted into the HLS playlists one after another, e.g. when multiple "ad-break-in-progress" placeholder loops are being shown in a row, or distinct "actual ads" are shown if enabled via --twitch-access-token-param=playerType=web (we default to "embed" so we only see the placeholders). Those ad-part date-ranges always share the ID of the whole ad break and always include the full duration via the X-TV-TWITCH-AD-POD-FILLED-DURATION metadata, so when the full ad break is 90s, it only logs it once instead of logging "30s" three times when there are three EXT-X-DATERANGE tags. There's also metadata for the number of parts in a break and the current part number, but this is irrelevant since we're only interested in the full duration which we only want to log once.
Prerolls + midrolls
[plugins.twitch][info] Will skip ad segments
[plugins.twitch][info] Low latency streaming (HLS live edge: 2)
[plugins.twitch][info] Waiting for pre-roll ads to finish, be patient
[plugins.twitch][info] Detected advertisement break of 15 seconds
[stream.hls][info] Filtering out segments and pausing stream output
[stream.hls][info] Resuming stream output
[plugins.twitch][info] Detected advertisement break of 90 seconds
[stream.hls][info] Filtering out segments and pausing stream output
[stream.hls][warning] Encountered a stream discontinuity. This is unsupported and will result in incoherent output data.
[stream.hls][info] Resuming stream output
No prerolls
[plugins.twitch][info] Will skip ad segments
[plugins.twitch][info] Low latency streaming (HLS live edge: 2)
[plugins.twitch][info] Detected advertisement break of 120 seconds
[stream.hls][info] Filtering out segments and pausing stream output
[stream.hls][warning] Encountered a stream discontinuity. This is unsupported and will result in incoherent output data.
[stream.hls][info] Resuming stream output
[plugins.twitch][info] Detected advertisement break of 120 seconds
[stream.hls][info] Filtering out segments and pausing stream output
[stream.hls][warning] Encountered a stream discontinuity. This is unsupported and will result in incoherent output data.
[stream.hls][info] Resuming stream output