Store media usn per-file

Since anki.media.MediaManager does not store per-file usn (as it's
not really needed for anything in the client), this requires us to
drop it and implement a custom media manager.

ankisyncd/media.py

@@ -0,0 +1,56 @@
+# Based on anki.media.MediaManager, © Ankitects Pty Ltd and contributors
+# License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
+# Original source: https://raw.githubusercontent.com/dae/anki/62481ddc1aa78430cb8114cbf00a7739824318a8/anki/media.py
+
+import re
+import os
+import os.path
+
+import anki.db
+
+
+class ServerMediaManager:
+    def __init__(self, col):
+        self._dir = re.sub(r"(?i)\.(anki2)$", ".media", col.path)
+        self.connect()
+
+    def connect(self):
+        path = self.dir() + ".server.db"
+        # TODO: migrate old db
+        create = not os.path.exists(path)
+        self.db = anki.db.DB(path)
+        if create:
+            self.db.executescript(
+                """CREATE TABLE media (
+                       fname TEXT NOT NULL PRIMARY KEY,
+                       usn INT NOT NULL,
+                       csum TEXT -- null if deleted
+                   );
+                CREATE INDEX idx_media_usn ON media (usn);"""
+            )
+
+    def close(self):
+        self.db.close()
+
+    def dir(self):
+        return self._dir
+
+    def lastUsn(self):
+        return self.db.scalar("SELECT usn FROM media ORDER BY usn DESC LIMIT 1") or 0
+
+    def mediaCount(self):
+        return self.db.scalar("SELECT count() FROM media WHERE csum IS NOT NULL")
+
+    # used only in unit tests
+    def syncInfo(self, fname):
+        return self.db.first("SELECT csum, 0 FROM media WHERE fname=?", fname)
+
+    def syncDelete(self, fname):
+        fpath = os.path.join(self.dir(), fname)
+        if os.path.exists(fpath):
+            os.remove(fpath)
+        self.db.execute(
+            "UPDATE media SET csum = NULL, usn = ? WHERE fname = ?",
+            self.lastUsn() + 1,
+            fname,
+        )