[Audio] Connect to lavalink in the background (#2335)

Also:
- restart and reconnect if connection settings change
  - shutdown and restart if not configured to use external
- show a message in [p]play et al. when the connection hasn't been made
- move the JAR download to manager so audio.py can access it
- only start if no process exists

Resolves #2306

redbot/cogs/audio/manager.py

@@ -8,6 +8,10 @@ import re
 from subprocess import Popen, DEVNULL
 from typing import Optional, Tuple
 
+from aiohttp import ClientSession
+
+import redbot.core
+
 _JavaVersion = Tuple[int, int]
 
 log = logging.getLogger("red.audio.manager")
@@ -111,6 +115,10 @@ async def start_lavalink_server(loop):
     start_cmd = "java {} -jar {}".format(extra_flags, LAVALINK_JAR_FILE.resolve())
 
     global proc
+
+    if proc and proc.poll() is None:
+        return  # already running
+
     proc = Popen(
         shlex.split(start_cmd, posix=os.name == "posix"),
         cwd=str(LAVALINK_DOWNLOAD_DIR),
@@ -126,11 +134,39 @@ async def start_lavalink_server(loop):
 
 
 def shutdown_lavalink_server():
-    log.info("Shutting down lavalink server.")
     global shutdown
     shutdown = True
     global proc
     if proc is not None:
+        log.info("Shutting down lavalink server.")
         proc.terminate()
         proc.wait()
         proc = None
+
+
+async def download_lavalink(session):
+    from . import LAVALINK_DOWNLOAD_URL, LAVALINK_JAR_FILE
+
+    with LAVALINK_JAR_FILE.open(mode="wb") as f:
+        async with session.get(LAVALINK_DOWNLOAD_URL) as resp:
+            while True:
+                chunk = await resp.content.read(512)
+                if not chunk:
+                    break
+                f.write(chunk)
+
+
+async def maybe_download_lavalink(loop, cog):
+    from . import LAVALINK_DOWNLOAD_DIR, LAVALINK_JAR_FILE, BUNDLED_APP_YML_FILE, APP_YML_FILE
+
+    jar_exists = LAVALINK_JAR_FILE.exists()
+    current_build = redbot.VersionInfo.from_json(await cog.config.current_version())
+
+    if not jar_exists or current_build < redbot.core.version_info:
+        log.info("Downloading Lavalink.jar")
+        LAVALINK_DOWNLOAD_DIR.mkdir(parents=True, exist_ok=True)
+        async with ClientSession(loop=loop) as session:
+            await download_lavalink(session)
+        await cog.config.current_version.set(redbot.core.version_info.to_json())
+
+    shutil.copyfile(str(BUNDLED_APP_YML_FILE), str(APP_YML_FILE))