rtsp source: add rtspScale option to inject RFC 2326 Scale header on PLAY

[Adiel-Sharabi]

Summary

Adds a new per-path option rtspScale for the RTSP static source. When set, the value is sent as the Scale header in the PLAY request, per RFC 2326 §12.34.

• Negative values play in reverse: -1.0 = realtime reverse, -2.0 = 2x reverse
• Values > 1 fast-forward; values 0 < x < 1 play in slow motion
• Default empty = header omitted = current behavior (backward-compatible)

Why

I'm integrating MediaMTX as the broker between a Windows access-control system and a Nx Meta VMS server. The VMS honors Scale: -1.0 to play archive recordings in reverse, which the customer needs for incident review (operator scrubs backwards through footage).

gortsplib.Client.Play(*headers.Range) doesn't expose a Scale parameter and doPlay() is unexported, so the cleanest way to inject the header is through the existing OnRequest hook on the static-source Client. When the request method is PLAY and rtspScale is non-empty, we set the header before the request leaves the client.

Other VMS servers (Pelco, Genetec, some Milestone deployments) follow the same RFC and would benefit too.

What's in the change

File	Change
internal/conf/path.go	New RTSPScale string field on Path (json: rtspScale)
internal/staticsources/rtsp/source.go	OnRequest hook injects Scale header on PLAY when RTSPScale != "" (10 lines)
internal/staticsources/rtsp/source_test.go	TestScale — verifies the header lands on the server side
mediamtx.yml	Documents the new field next to rtspRangeStart

Total diff: 4 files, 107 insertions, 0 deletions. No deprecations, no behavior change for users who don't set the field.

Example

paths:
  reverse-archive:
    source: rtsp://user:pass@vms-server:7001/camera-uuid?pos=2026-05-25T12:00:00.000Z
    sourceOnDemand: yes
    rtspScale: "-1.0"   # play backwards in realtime

Verification

• TestScale passes (go test ./internal/staticsources/rtsp/ -run TestScale)
• Verified end-to-end against a real Nx Meta 6.1.1 server + Axis M5013 camera: server returns 200 OK to the PLAY with Scale: -1.0 and streams the (server-side-interpreted) reverse playback.
• Backward compatibility: omitted/empty rtspScale produces identical RTSP traffic to current behavior.

Open question for maintainers

Naming: I went with rtspScale to match the existing rtspRange{Type,Start} family. Happy to rename to rtspPlayScale or similar if you prefer.

Test plan

• go test ./...
• Manual: configure a path with rtspScale: "-1.0" against any RTSP server that supports Scale; observe the header in the PLAY exchange (e.g. via tcpdump -A or the server's debug log)
• Confirm empty/omitted field produces identical traffic to current behavior

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 62.59%. Comparing base (5367afa) to head (2b38dd4).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/staticsources/rtsp/source.go	60.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5800   +/-   ##
=======================================
  Coverage   62.59%   62.59%           
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  Files         220      220           
  Lines       18779    18783    +4     
=======================================
+ Hits        11754    11758    +4     
- Misses       6060     6061    +1     
+ Partials      965      964    -1     

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