diff --git a/src/core/builtins/builtins_source.ml b/src/core/builtins/builtins_source.ml
index 1497693389..4bd5ad1f18 100644
--- a/src/core/builtins/builtins_source.ml
+++ b/src/core/builtins/builtins_source.ml
@@ -41,6 +41,15 @@ let _ =
       s#set_name n;
       Lang.unit)
 
+let _ =
+  Lang.add_builtin ~base:source "last_metadata" ~category:(`Source `Liquidsoap)
+    ~descr:"Return the last metadata from the source."
+    [("", Lang.source_t (Lang.univ_t ()), None, None)]
+    (Lang.nullable_t Lang.metadata_t)
+    (fun p ->
+      let s = Lang.to_source (List.assoc "" p) in
+      match s#last_metadata with None -> Lang.null | Some m -> Lang.metadata m)
+
 let _ =
   Lang.add_builtin ~base:source "skip" ~category:(`Source `Liquidsoap)
     ~descr:"Skip to the next track."
diff --git a/src/core/operators/dyn_op.ml b/src/core/operators/dyn_op.ml
index 426f65f874..d4358cb5bc 100644
--- a/src/core/operators/dyn_op.ml
+++ b/src/core/operators/dyn_op.ml
@@ -20,7 +20,8 @@
 
  *****************************************************************************)
 
-class dyn ~init ~track_sensitive ~infallible ~resurection_time ~self_sync f =
+class dyn ~init ~track_sensitive ~infallible ~resurection_time ~self_sync
+  next_fn =
   object (self)
     inherit Source.source ~name:"source.dynamic" ()
 
@@ -33,46 +34,62 @@ class dyn ~init ~track_sensitive ~infallible ~resurection_time ~self_sync f =
     val mutable activation = []
     val source : Source.source option Atomic.t = Atomic.make init
     val mutable last_select = Unix.gettimeofday ()
-    val proposed = Atomic.make None
-    method propose s = Atomic.set proposed (Some s)
 
-    method private prepare s =
+    method private no_source =
+      if infallible then
+        Lang.raise_error ~pos:[]
+          ~message:
+            (Printf.sprintf
+               "Infallible source.dynamic %s was not able to prepare a source \
+                in time! Make sure to eithe define infallible sources in the \
+                source's dynamic function or mark the source as fallible.."
+               self#id)
+          "failure";
+      None
+
+    method prepare s =
       Typing.(s#frame_type <: self#frame_type);
       Clock.unify ~pos:self#pos s#clock self#clock;
-      s#wake_up;
+      s#wake_up
+
+    method private exchange s =
+      self#prepare s;
       (match Atomic.exchange source (Some s) with
         | Some s -> s#sleep
         | None -> ());
-      if s#is_ready then Some s else None
+      if s#is_ready then Some s else self#no_source
 
     method private get_next reselect =
       self#mutexify
         (fun () ->
-          match Atomic.exchange proposed None with
-            | Some s -> self#prepare s
+          last_select <- Unix.gettimeofday ();
+          let s =
+            Lang.apply next_fn
+              [
+                ( "",
+                  match Atomic.get source with
+                    | None -> Lang.null
+                    | Some s -> Lang.source s );
+              ]
+            |> Lang.to_option |> Option.map Lang.to_source
+          in
+          match s with
             | None -> (
-                last_select <- Unix.gettimeofday ();
-                let s =
-                  Lang.apply f [] |> Lang.to_option |> Option.map Lang.to_source
-                in
-                match s with
-                  | None -> (
-                      match Atomic.get source with
-                        | Some s
-                          when self#can_reselect
-                                 ~reselect:
-                                   (match reselect with
-                                     | `Force -> `Ok
-                                     | v -> v)
-                                 s ->
-                            Some s
-                        | _ -> None)
-                  | Some s -> self#prepare s))
+                match Atomic.get source with
+                  | Some s
+                    when self#can_reselect
+                           ~reselect:
+                             (match reselect with `Force -> `Ok | v -> v)
+                           s ->
+                      Some s
+                  | _ -> self#no_source)
+            | Some s -> self#exchange s)
         ()
 
     method private get_source ~reselect () =
       match (Atomic.get source, reselect) with
-        | None, _ | _, `Force -> self#get_next reselect
+        | None, _ | _, `Force | Some _, `After_position _ ->
+            self#get_next reselect
         | Some s, _ when self#can_reselect ~reselect s -> Some s
         | Some _, _ when Unix.gettimeofday () -. last_select < resurection_time
           ->
@@ -83,7 +100,7 @@ class dyn ~init ~track_sensitive ~infallible ~resurection_time ~self_sync f =
       self#on_wake_up (fun () ->
           Lang.iter_sources
             (fun s -> Typing.(s#frame_type <: self#frame_type))
-            f;
+            next_fn;
           ignore (self#get_source ~reselect:`Force ()));
       self#on_sleep (fun () ->
           match Atomic.exchange source None with
@@ -141,28 +158,30 @@ let _ =
            should wait before trying to update source again (`null` means \
            never)." );
       ( "",
-        Lang.fun_t [] (Lang.nullable_t (Lang.source_t frame_t)),
-        Some (Lang.val_fun [] (fun _ -> Lang.null)),
+        Lang.fun_t
+          [(false, "", Lang.nullable_t (Lang.source_t frame_t))]
+          (Lang.nullable_t (Lang.source_t frame_t)),
+        None,
         Some
           "Function returning the source to be used, `null` means keep current \
-           source." );
+           source. Argument is the currently set source." );
     ]
     ~return_t:frame_t
     ~descr:
       "Dynamically change the underlying source: it can either be changed by \
        the function given as argument, which returns the source to be played, \
        or by calling the `set` method."
-    ~category:`Track ~flags:[`Experimental]
+    ~category:`Track
     ~meth:
       [
-        ( "set",
+        ( "prepare",
           ([], Lang.fun_t [(false, "", Lang.source_t frame_t)] Lang.unit_t),
-          "Set the source.",
+          "Prepare a source that will be returned later.",
           fun s ->
             Lang.val_fun
               [("", "x", None)]
               (fun p ->
-                s#propose (List.assoc "x" p |> Lang.to_source);
+                s#prepare (List.assoc "x" p |> Lang.to_source);
                 Lang.unit) );
       ]
     (fun p ->
diff --git a/src/libs/extra/native.liq b/src/libs/extra/native.liq
index 4cfdfa5f86..af3ab4d7a4 100644
--- a/src/libs/extra/native.liq
+++ b/src/libs/extra/native.liq
@@ -19,7 +19,7 @@ end
 def native.fallback(~id=null(), ~track_sensitive=true, sources) =
   fail = (source.fail() : source)
 
-  def s() =
+  def s(_) =
     list.find(default=fail, source.is_ready, getter.get(sources))
   end
 
@@ -44,7 +44,7 @@ def native.sequence(~id=null(), sources) =
   n = ref(0)
   fail = source.fail()
 
-  def rec s() =
+  def rec s(v) =
     sn = list.nth(default=list.last(default=fail, sources), sources, n())
     if
       source.is_ready(sn) or n() >= len - 1
@@ -52,7 +52,7 @@ def native.sequence(~id=null(), sources) =
       sn
     else
       ref.incr(n)
-      s()
+      s(v)
     end
   end
 
@@ -173,7 +173,7 @@ def native.request.dynamic(%argsof(request.dynamic), f) =
   thread.run(queue=thread_queue, every=retry_delay, fill)
 
   # Source
-  def s() =
+  def s(_) =
     if
       not list.is_empty(queue())
     then
diff --git a/src/libs/request.liq b/src/libs/request.liq
index 511d312b5b..6381fe2361 100644
--- a/src/libs/request.liq
+++ b/src/libs/request.liq
@@ -223,7 +223,7 @@ def request.once(
   done = ref(false)
   fail = fallback([])
 
-  def next() =
+  def next(_) =
     if
       done()
     then
@@ -330,7 +330,7 @@ def request.single(
     static_request := null()
   end
 
-  def next() =
+  def next(_) =
     if
       done()
     then
@@ -423,7 +423,11 @@ def request.player(~simultaneous=true) =
       l := request.once(r)::l()
     end
 
-    source.dynamic({add(normalize=false, l())}).{
+    def next(_) =
+      add(normalize=false, l())
+    end
+
+    source.dynamic(next).{
       play=play,
       length=
         {
@@ -432,10 +436,20 @@ def request.player(~simultaneous=true) =
         }
     }
   else
-    s = source.dynamic()
+    next_source = ref(null())
+
+    def next(_) =
+      s = next_source()
+      next_source := null()
+      s
+    end
+
+    s = source.dynamic(next)
 
     def play(r) =
-      s.set(request.once(r))
+      r = request.once(r)
+      s.prepare(s)
+      next_source := r
     end
 
     s.{play=play, length={1}}
diff --git a/src/libs/source.liq b/src/libs/source.liq
index 24526c93ea..bb92a214a4 100644
--- a/src/libs/source.liq
+++ b/src/libs/source.liq
@@ -190,7 +190,7 @@ def append(~id=null("append"), ~insert_missing=true, ~merge=false, s, f) =
   s = if insert_missing then source.on_track(s, f) else s end
   s = source.on_metadata(s, f)
 
-  def next() =
+  def next(_) =
     p = pending()
     pending := null()
     last_meta := null()
@@ -248,7 +248,7 @@ def prepend(~id=null("prepend"), ~merge=false, s, f) =
   s = source.on_track(s, fun (m) -> m == [] ? last_meta := null() : on_meta(m) )
   s = source.on_metadata(s, on_meta)
 
-  def next() =
+  def next(_) =
     if
       null.defined(last_meta)
     then
diff --git a/src/libs/video.liq b/src/libs/video.liq
index 9797af4d58..3677c5c518 100644
--- a/src/libs/video.liq
+++ b/src/libs/video.liq
@@ -34,27 +34,24 @@ def request.image(
   ~y=getter(0),
   req
 ) =
-  s = source.dynamic()
-
   last_req = ref(null())
 
-  def set_req() =
+  def next(_) =
     req = (getter.get(req) : request)
 
     if
       req != last_req()
     then
+      last_req := req
       image = request.single(id=id, fallible=fallible, req)
       image = video.crop(image)
-      image = video.resize(id=id, x=x, y=y, width=width, height=height, image)
-      s.set(image)
-      last_req := req
+      video.resize(id=id, x=x, y=y, width=width, height=height, image)
+    else
+      null()
     end
   end
 
-  s.on_wake_up(set_req)
-
-  source.on_frame(s, set_req)
+  source.dynamic(id=id, next)
 end
 
 # Generate a source from an image file.
@@ -197,34 +194,45 @@ end
 # @category Source / Video processing
 # @param s Audio source whose metadata contain cover-art.
 def video.cover(s) =
-  video = source.dynamic()
+  last_filename = ref(null())
+  fail = (source.fail() : source)
+
+  def next(_) =
+    m = source.last_metadata(s ?? fail) ?? []
 
-  def read_cover(m) =
     filename = m["filename"]
-    cover =
-      if file.exists(filename) then file.cover(filename) else "".{mime=""} end
 
     if
-      null.defined(cover)
+      filename != last_filename()
     then
-      cover = null.get(cover)
-      ext = if cover.mime == "image/png" then ".png" else ".jpg" end
-      f = file.temp("cover", ext)
-      log.debug(
-        "Found cover for #{filename}."
-      )
-      file.write(data=cover, f)
-      video.set(request.once(request.create(temporary=true, f)))
+      last_filename := filename
+
+      cover =
+        if file.exists(filename) then file.cover(filename) else "".{mime=""} end
+
+      if
+        null.defined(cover)
+      then
+        cover = null.get(cover)
+        ext = if cover.mime == "image/png" then ".png" else ".jpg" end
+        f = file.temp("cover", ext)
+        log.debug(
+          "Found cover for #{filename}."
+        )
+        file.write(data=cover, f)
+        request.once(request.create(temporary=true, f))
+      else
+        log.debug(
+          "No cover for #{filename}."
+        )
+	fail
+      end
     else
-      log.debug(
-        "No cover for #{filename}."
-      )
-      video.set(source.fail())
+      null()
     end
   end
 
-  s.on_track(read_cover)
-  (video : source(video=canvas))
+  source.dynamic(track_sensitive=false, next)
 end
 
 let output.youtube = ()
@@ -652,7 +660,16 @@ def video.slideshow(
   id = string.id.default(default="video.slideshow", id)
   l = ref(l)
   n = ref(-1)
-  s = source.dynamic()
+
+  next_source = ref(null())
+
+  def next(_) =
+    s = next_source()
+    next_source := null()
+    s
+  end
+
+  s = source.dynamic(next)
 
   def current() =
     list.nth(l(), n())
@@ -664,7 +681,9 @@ def video.slideshow(
       0 <= n' and n' < list.length(l()) and n' != n()
     then
       n := n'
-      s.set(request.once(request.create(current())))
+      new_source = request.once(request.create(current()))
+      s.prepare(new_source)
+      next_source := new_source
     end
   end