ytdl: implement user-agent and cookie overrides

For some sites, youtube-dl sends a special user-agent. If we don't send
the same user-agent, the server will reject mpv's connection attempt.
This was observed with trailers.apple.com. Fix it by forcing the
user-agent youtube-dl uses.

Some sites set cookies when doing a website access, and require the
client to provide these cookies when downloading the actual media. This
is needed at least by nicovideo.jp. Fix by adding youtube-dl's cookies
to our request headers.

Both of these require a very recent youtube-dl version (youtube-dl added
the necessary headers a few hours ago). The script still works with
older youtube-dl versions, though.

player/lua/ytdl_hook.lua

@@ -7,13 +7,37 @@ local ytdl = {
     vercheck = nil,
 }
 
-mp.add_hook("on_load", 10, function ()
+local function exec(args)
+    local ret = utils.subprocess({args = args})
+    return ret.status, ret.stdout
+end
 
-    local function exec(args)
-        local ret = utils.subprocess({args = args})
-        return ret.status, ret.stdout
+-- return if it was explicitly set on the command line
+local function option_was_set(name)
+    return mp.get_property_bool("option-info/" .. name .. "/set-from-commandline",
+                                false)
+end
+
+-- youtube-dl may set special http headers for some sites (user-agent, cookies)
+local function set_http_headers(http_headers)
+    if not http_headers then
+        return
     end
+    local headers = {}
+    local useragent = http_headers["User-Agent"]
+    if useragent and not option_was_set("user-agent") then
+        mp.set_property("file-local-options/user-agent", useragent)
+    end
+    local cookies = http_headers["Cookie"]
+    if cookies then
+        headers[#headers + 1] = "Cookie: " .. cookies
+    end
+    if #headers > 0 and not option_was_set("http-header-fields") then
+        mp.set_property_native("file-local-options/http-header-fields", headers)
+    end
+end
 
+mp.add_hook("on_load", 10, function ()
     local url = mp.get_property("stream-open-filename")
 
     if (url:find("http://") == 1) or (url:find("https://") == 1)
@@ -117,6 +141,10 @@ mp.add_hook("on_load", 10, function ()
 
                 msg.debug("EDL: " .. playlist)
 
+                -- can't change the http headers for each entry, so use the 1st
+                if json.entries[1] then
+                    set_http_headers(json.entries[1].http_headers)
+                end
 
                 mp.set_property("stream-open-filename", playlist)
                 if not (json.title == nil) then
@@ -168,6 +196,7 @@ mp.add_hook("on_load", 10, function ()
             elseif not (json.url == nil) then
                 -- normal video
                 streamurl = json.url
+                set_http_headers(json.http_headers)
             else
                 msg.error("No URL found in JSON data.")
                 return