[bugfix] media.Processor{}.GetFile() returning 404s on first call, correctly loading on 2nd (#3129)

* refactor file handling a tiny bit

* whoops

* make processing media / emoji defers a bit clear to see that it's the "on finished processing" path

* some wording

* add some debug logging

* add mutex locks for processing remote media

* try removing freshness check

* fix derefMedia not being allocated

* fix log format string

* handle case of empty file paths (i.e. not stored)

* remove media / emoji once finished processing from dereferencer maps

* whoops, fix the cached / force checks

* move url parsing outside of 'process___Safely()' funcs to prevalidate url

* use emoji.ShortcodeDomain()

* update RefreshEmoji() to also match RefreshMedia() changes

---------

Co-authored-by: tobi <tobi.smethurst@protonmail.com>

internal/federation/dereferencing/dereferencer.go

@@ -85,12 +85,9 @@ type Dereferencer struct {
 	mediaManager        *media.Manager
 	visibility          *visibility.Filter
 
-	// in-progress dereferencing emoji. we already perform
-	// locks per-status and per-account so we don't need
-	// processing maps for other media which won't often
-	// end up being repeated. worst case we run into an
-	// db.ErrAlreadyExists error which then gets handled
-	// appropriately by enrich{Account,Status}Safely().
+	// in-progress dereferencing media / emoji
+	derefMedia    map[string]*media.ProcessingMedia
+	derefMediaMu  sync.Mutex
 	derefEmojis   map[string]*media.ProcessingEmoji
 	derefEmojisMu sync.Mutex
 
@@ -119,6 +116,7 @@ func NewDereferencer(
 		transportController: transportController,
 		mediaManager:        mediaManager,
 		visibility:          visFilter,
+		derefMedia:          make(map[string]*media.ProcessingMedia),
 		derefEmojis:         make(map[string]*media.ProcessingEmoji),
 		handshakes:          make(map[string][]*url.URL),
 	}