sqlite storage: Track total blob data size manually

Works around possible full table scan at startup, and possible lack of caching around cast(data as blob).

storage/sqlite/sqlite-storage.go

@@ -23,7 +23,13 @@ import (
 type conn = *sqlite.Conn
 
 func initConn(conn conn, wal bool) error {
-	err := sqlitex.ExecTransient(conn, `pragma synchronous=off`, nil)
+	// Recursive triggers are required because we need to trim the blob_meta size after trimming to
+	// capacity. Hopefully we don't hit the recursion limit, and if we do, there's an error thrown.
+	err := sqlitex.ExecTransient(conn, "pragma recursive_triggers=on", nil)
+	if err != nil {
+		return err
+	}
+	err = sqlitex.ExecTransient(conn, `pragma synchronous=off`, nil)
 	if err != nil {
 		return err
 	}
@@ -42,6 +48,9 @@ func initConn(conn conn, wal bool) error {
 
 func initSchema(conn conn) error {
 	return sqlitex.ExecScript(conn, `
+-- We have to opt into this before creating any tables, or before a vacuum to enable it. It means we
+-- can trim the database file size with partial vacuums without having to do a full vacuum, which 
+-- locks everything.
 pragma auto_vacuum=incremental;
 
 create table if not exists blob (
@@ -51,6 +60,16 @@ create table if not exists blob(
 	primary key (name)
 );
 
+create table if not exists blob_meta (
+	key text primary key,
+	value
+);
+
+-- While sqlite *seems* to be faster to get sum(length(data)) instead of 
+-- sum(length(cast(data as blob))), it may still require a large table scan at start-up or with a 
+-- cold-cache. With this we can be assured that it doesn't.
+insert or ignore into blob_meta values ('size', 0);
+
 create table if not exists setting (
 	name primary key on conflict replace,
 	value
@@ -63,27 +82,47 @@ with recursive excess(
 	blob_rowid,
 	data_length
 ) as (
-	select * from (select (select sum(length(cast(data as blob))) from blob) as usage_with, last_used, rowid, length(cast(data as blob)) from blob order by last_used, rowid limit 1)
+	select * 
+	from (
+		select 
+			(select value from blob_meta where key='size') as usage_with,
+			last_used,
+			rowid,
+			length(cast(data as blob))
+		from blob order by last_used, rowid limit 1
+	)
 	where usage_with >= (select value from setting where name='capacity')
 	union all
-	select usage_with-data_length, blob.last_used, blob.rowid, length(cast(data as blob)) from excess join blob
+	select 
+		usage_with-data_length,
+		blob.last_used,
+		blob.rowid,
+		length(cast(data as blob))
+	from excess join blob
 	on blob.rowid=(select rowid from blob where (last_used, rowid) > (excess.last_used, blob_rowid))
 	where usage_with >= (select value from setting where name='capacity')
-) select * from excess;
+)
+select * from excess;
 
-create trigger if not exists trim_blobs_to_capacity_after_update
+create trigger if not exists after_insert_blob
-after update of data on blob
+after insert on blob
-when length(new.data)>length(old.data) and (select sum(length(cast(data as blob))) from blob)>(select value from setting where name='capacity')
 begin
+	update blob_meta set value=value+length(cast(new.data as blob)) where key='size';
 	delete from blob where rowid in (select blob_rowid from deletable_blob);
 end;
 
-create trigger if not exists trim_blobs_to_capacity_after_insert 
+create trigger if not exists after_update_blob
-after insert on blob
+after update of data on blob
-when (select sum(length(cast(data as blob))) from blob)>(select value from setting where name='capacity')
 begin
+	update blob_meta set value=value+length(cast(new.data as blob))-length(cast(old.data as blob)) where key='size';
 	delete from blob where rowid in (select blob_rowid from deletable_blob);
 end;
+
+create trigger if not exists after_delete_blob
+after delete on blob
+begin
+	update blob_meta set value=value-length(cast(old.data as blob)) where key='size';
+end;
 `)
 }