Input: Allow Restartable sources to be lazy

This change is made with queue users in mind. Since sources
of this kind *know* how to (re)create themselves, they can
avoid being created at all until needed.

This also adds machinery to preload tracks *before* they are
needed, for gapless playback on queues and so on. Queues
make use of the event system to do this.

src/input/ytdl_src.rs

@@ -11,7 +11,7 @@ use std::{
     io::{BufRead, BufReader, Read},
     process::{Command, Stdio},
 };
-use tokio::task;
+use tokio::{process::Command as TokioCommand, task};
 use tracing::trace;
 
 const YOUTUBE_DL_COMMAND: &str = if cfg!(feature = "youtube-dlc") {
@@ -66,6 +66,7 @@ pub(crate) async fn _ytdl(uri: &str, pre_args: &[&str]) -> Result<Input> {
         .stdout(Stdio::piped())
         .spawn()?;
 
+    // This rigmarole is required due to the inner synchronous reading context.
     let stderr = youtube_dl.stderr.take();
     let (returned_stderr, value) = task::spawn_blocking(move || {
         let mut s = stderr.unwrap();
@@ -113,6 +114,45 @@ pub(crate) async fn _ytdl(uri: &str, pre_args: &[&str]) -> Result<Input> {
     ))
 }
 
+pub(crate) async fn _ytdl_metadata(uri: &str) -> Result<Metadata> {
+    // Most of these flags are likely unused, but we want identical search
+    // and/or selection as the above functions.
+    let ytdl_args = [
+        "-j",
+        "-f",
+        "webm[abr>0]/bestaudio/best",
+        "-R",
+        "infinite",
+        "--no-playlist",
+        "--ignore-config",
+        uri,
+        "-o",
+        "-",
+    ];
+
+    let youtube_dl_output = TokioCommand::new(YOUTUBE_DL_COMMAND)
+        .args(&ytdl_args)
+        .stdin(Stdio::null())
+        .output()
+        .await?;
+
+    let o_vec = youtube_dl_output.stderr;
+
+    let end = (&o_vec)
+        .iter()
+        .position(|el| *el == 0xA)
+        .unwrap_or_else(|| o_vec.len());
+
+    let value = serde_json::from_slice(&o_vec[..end]).map_err(|err| Error::Json {
+        error: err,
+        parsed_text: std::str::from_utf8(&o_vec).unwrap_or_default().to_string(),
+    })?;
+
+    let metadata = Metadata::from_ytdl_output(value);
+
+    Ok(metadata)
+}
+
 /// Creates a streamed audio source from YouTube search results with `youtube-dl(c)`,`ffmpeg`, and `ytsearch`.
 /// Takes the first video listed from the YouTube search.
 ///