Use Java socket by default for RTMP

[mkrn]

Summary

Change the default RTMP socket implementation from Ktor to Java sockets. Ktor remains available via setSocketType(SocketType.KTOR), but new RtmpClient instances default to SocketType.JAVA.

Why

The Ktor RTMPS path can crash the app when the network is interrupted or the remote peer closes the connection while TLS is shutting down. We reproduced this in a production app by streaming to an rtmps:// endpoint, disabling Wi-Fi during the stream, and watching the process crash from Ktor's internal TLS closer coroutine.

Observed crash shape:

io.ktor.utils.io.ClosedWriteChannelException: Broken pipe
  at io.ktor.utils.io.ByteChannel.getWriteBuffer(ByteChannel.kt:54)
  at io.ktor.utils.io.ByteWriteChannelOperationsKt.writeByte(ByteWriteChannelOperations.kt:19)
  at io.ktor.network.tls.RenderKt.writeRecord(Render.kt:18)
  at io.ktor.network.tls.TLSClientHandshake...
Suppressed: [CoroutineName(cio-tls-closer), ... Dispatchers.IO]
Caused by: java.io.IOException: Broken pipe
  at sun.nio.ch.SocketChannelImpl.write(SocketChannelImpl.java:512)
  at io.ktor.network.sockets.CIOWriterKt...

This is consistent with #1856 and #2085. In #1856, Java sockets were suggested as the workaround and as a possible safer default. In our device test, switching RTMP to SocketType.JAVA stopped the fatal process crash while preserving reconnect behavior.

Verification

• ./gradlew :rtmp:assemble
• Android device test against an rtmps:// endpoint:
  • baseline stream packets: pass
  • zoom controls: pass
  • background/foreground: pass
  • Wi-Fi off/on during stream: pass, no fatal crash
  • Wi-Fi/mobile/Wi-Fi transition: pass, no fatal crash

Compatibility

This preserves the public API and keeps Ktor opt-in through setSocketType(SocketType.KTOR). The change only affects the default for RTMP clients.

Related: #1856, #2085

[pedroSG94]

Hello,

Thank you for the reports. I added 2 comments to the PR.
I'm fine with the idea to switch to java socket by default and I will do it but we should check the review comments to avoid problems