Add stream details for ready to start hook

[ditogam]

Describe the feature

It would be nice to have stream info for runOnReady, so we can construct propper command based on input info. For example: If source video codec is already H264, but audio codec is AAC, we can construct ffmpeg like: ffmpeg -i "srt://localhost:8890?streamid=read:$RTSP_PATH" -c:v copy -c:a libopus -b:a 256k -f mpegts "srt://localhost:8890?streamid=publish:webrtc_$RTSP_PATH"

If source video codec is already H264, and audio codec is already Opus, we can construct ffmpeg like: ffmpeg -i "srt://localhost:8890?streamid=read:$RTSP_PATH" -c copy -f mpegts "srt://localhost:8890?streamid=publish:webrtc_$RTSP_PATH"

Or dynamically construct gstreamer pipeline

Here is little idea. Putting codec info by format and RTPMap/FMTPMap for additional info like SampleRate, ChannelCount and so on

Subject: [PATCH] mediamtx
---
Index: internal/core/path.go
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
diff --git a/internal/core/path.go b/internal/core/path.go
--- a/internal/core/path.go (revision a1dc9f45f5e2a03084274bea139ecaa2e805d659)
+++ b/internal/core/path.go (date 1719673626662)
@@ -2,6 +2,8 @@

 import (
    "context"
+   "encoding/base64"
+   "encoding/json"
    "fmt"
    "net"
    "strconv"
@@ -623,6 +625,39 @@
    return pa.conf
 }

+func (pa *path) ExternalCmdEnvFromSession(desc *description.Session) externalcmd.Environment {
+   env := pa.ExternalCmdEnv()
+
+   if desc != nil {
+       if desc.BaseURL != nil {
+           env["MTX_BASE_URL"] = desc.BaseURL.String()
+       }
+
+       env["MTX_TITLE"] = desc.Title
+
+       for _, media := range desc.Medias {
+           envNamePrefix := "MTX_MEDIA_TYPE_" + strings.ToUpper(string(media.Type))
+           if len(media.Formats) == 1 {
+               format := media.Formats[0]
+               env[envNamePrefix+"_CODEC"] = format.Codec()
+               env[envNamePrefix+"_CLOCK_RATE"] = strconv.Itoa(format.ClockRate())
+               env[envNamePrefix+"_RTP_MAP"] = format.RTPMap()
+
+               fmtp := format.FMTP()
+               if fmtp != nil {
+                   jsonData, err := json.Marshal(fmtp)
+                   if err == nil {
+                       env[envNamePrefix+"_FMTP_MAP_JSON_BASE64"] = base64.StdEncoding.EncodeToString(jsonData)
+                   }
+               }
+           }
+
+       }
+   }
+
+   return env
+}
+
 func (pa *path) ExternalCmdEnv() externalcmd.Environment {
    _, port, _ := net.SplitHostPort(pa.rtspAddress)
    env := externalcmd.Environment{
@@ -731,7 +766,7 @@
        Logger:          pa,
        ExternalCmdPool: pa.externalCmdPool,
        Conf:            pa.conf,
-       ExternalCmdEnv:  pa.ExternalCmdEnv(),
+       ExternalCmdEnv:  pa.ExternalCmdEnvFromSession(desc),
        Desc:            pa.source.APISourceDescribe(),
        Query:           pa.publisherQuery,
    })