Export GetCodecMap for efficient payload type lookups

[asnyatkov]

Description

Rename buildCodecMap to GetCodecMap so callers can build the codec map once and reuse it for multiple payload type lookups. CPU overhead manifests itself in thundering herd scenario when we're creating new peer connections at a very high TPS rate.

This is needed by pion/webrtc's codecsFromMediaDescription (sdp.go:1101), which currently calls GetCodecForPayloadType in a loop (sdp.go:1112), rebuilding the full codec map on every iteration. With GetCodecMap exported, webrtc can build the map once and do direct lookups instead.

There will be webrtc integration patch in webrtc. We should bump minor SDP version to avoid compatibility issues.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.43%. Comparing base (f66f2eb) to head (5014502).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #248   +/-   ##
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  Coverage   97.43%   97.43%           
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  Files          12       12           
  Lines        1405     1405           
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  Hits         1369     1369           
  Misses         19       19           
  Partials       17       17           

Flag	Coverage Δ
go	97.43% <100.00%> (ø)

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[JoTurk]

Can we cache codec map inside the library instead, I think this was the original plan when it got merged, I'm down for full api change if we have to and a major release but sdp users shouldn't have to implement the helpers themself.
I think it's a failure if they have to use a codec map api themself.

[asnyatkov]

With the way the code is structured right now, the only way to implement your suggestion is to add lazy caching to buildCodecMap. I was thinking that it can be done when SDP is parsed, but turns out webrtc doesn't do any parsing:

func codecsFromMediaDescription(mediaDescr *sdp.MediaDescription) (out []RTPCodecParameters, err error) {
        // vvvvv The descriptor is not parsed but constructed just to invoke GetCodecForPayloadType
	s := &sdp.SessionDescription{
		MediaDescriptions: []*sdp.MediaDescription{mediaDescr},
	}
        .......

Is this approach ok? That would also introduce private fields in sdp.SessionDescription.

[asnyatkov]

@JoTurk small nudge. This is a benign change, but it does make a difference in thundering herd scenarios.

[JoTurk]

What if we merge this now, and plan for a next major release where we control the constructors and the mutation calls so we can have internal map?