source.require(byteCount.toLong()) // Eagerly read the frame before firing client thread.
    source.read(buffer, byteCount.toLong())
    pushQueue.execute("$connectionName[$streamId] onData") {
      ignoreIoExceptions {
        val cancel = pushObserver.onData(streamId, buffer, byteCount, inFinished)
        if (cancel) writer.rstStream(streamId, ErrorCode.CANCEL)
        if (cancel || inFinished) {
          this.withLock {