Fix selected_images index

1. For a fresh database make sure the index is created correctly
2. In the database migration it seems the unique index creation might fail if non-unique
   data are in the table, to avoid this conflict we drop & recreate both the table and index,
   the only downside being the selection is lost after database upgrade.

src/common/database.c

@@ -1,6 +1,6 @@
 /*
     This file is part of darktable,
-    Copyright (C) 2011-2023 darktable developers.
+    Copyright (C) 2011-2024 darktable developers.
 
     darktable is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -48,7 +48,7 @@
 
 // whenever _create_*_schema() gets changed you HAVE to bump this version and add an update path to
 // _upgrade_*_schema_step()!
-#define CURRENT_DATABASE_VERSION_LIBRARY 51
+#define CURRENT_DATABASE_VERSION_LIBRARY 52
 #define CURRENT_DATABASE_VERSION_DATA    10
 
 // #define USE_NESTED_TRANSACTIONS
@@ -2683,30 +2683,30 @@ static int _upgrade_library_schema_step(dt_database_t *db, int version)
     TRY_EXEC("CREATE TABLE whitebalance"
              " (id INTEGER PRIMARY KEY AUTOINCREMENT,"
              "  name VARCHAR)",
-              "[init] can't create table whitebalance\n"); 
+              "[init] can't create table whitebalance\n");
     TRY_EXEC("CREATE UNIQUE INDEX whitebalance_name ON whitebalance (name)",
-             "[init] can't create index `whitebalance_name' on table `whitebalance'\n"); 
+             "[init] can't create index `whitebalance_name' on table `whitebalance'\n");
 
     TRY_EXEC("CREATE TABLE flash"
              " (id INTEGER PRIMARY KEY AUTOINCREMENT,"
              "  name VARCHAR)",
-              "[init] can't create table flash\n"); 
+              "[init] can't create table flash\n");
     TRY_EXEC("CREATE UNIQUE INDEX flash_name ON flash (name)",
-             "[init] can't create index `flash_name' on table `flash'\n"); 
+             "[init] can't create index `flash_name' on table `flash'\n");
 
     TRY_EXEC("CREATE TABLE exposure_program"
              " (id INTEGER PRIMARY KEY AUTOINCREMENT,"
              "  name VARCHAR)",
-              "[init] can't create table exposure_program\n"); 
+              "[init] can't create table exposure_program\n");
     TRY_EXEC("CREATE UNIQUE INDEX exposure_program_name ON exposure_program (name)",
-             "[init] can't create index `exposure_program_name' on table `exposure_program'\n"); 
+             "[init] can't create index `exposure_program_name' on table `exposure_program'\n");
 
     TRY_EXEC("CREATE TABLE metering_mode"
              " (id INTEGER PRIMARY KEY AUTOINCREMENT,"
              "  name VARCHAR)",
-              "[init] can't create table metering_mode\n"); 
+              "[init] can't create table metering_mode\n");
     TRY_EXEC("CREATE UNIQUE INDEX metering_mode_name ON metering_mode (name)",
-             "[init] can't create index `metering_mode_name' on table `metering_mode'\n"); 
+             "[init] can't create index `metering_mode_name' on table `metering_mode'\n");
 
     TRY_EXEC("CREATE TABLE images_new"
              " (id INTEGER PRIMARY KEY AUTOINCREMENT, group_id INTEGER, film_id INTEGER,"
@@ -2736,7 +2736,7 @@ static int _upgrade_library_schema_step(dt_database_t *db, int version)
              "FOREIGN KEY(exposure_program_id) REFERENCES exposure_program(id) ON DELETE CASCADE ON UPDATE CASCADE, "
              "FOREIGN KEY(metering_mode_id) REFERENCES metering_mode(id) ON DELETE CASCADE ON UPDATE CASCADE)",
              "[init] can't create new table images");
-    
+
     TRY_EXEC("INSERT INTO images_new"
              " SELECT id, group_id, film_id, width, height, filename,"
              "        maker_id, model_id, lens_id, camera_id,"
@@ -2802,6 +2802,29 @@ static int _upgrade_library_schema_step(dt_database_t *db, int version)
 
     new_version = 51;
   }
+  else if(version == 51)
+  {
+    sqlite3_exec(db->handle, "BEGIN TRANSACTION", NULL, NULL, NULL);
+
+    // As the selected_images table might have non-unique data the UNIQUE INDEX could fail,
+    // we avoid this by recreating both the table & index.
+    // minor downside: selection is lost while updating database sheme.
+    TRY_EXEC("DROP INDEX selected_images_ni",
+             "[init] can't drop index selected_images_ni\n");
+
+    TRY_EXEC("DROP TABLE selected_images",
+             "[init] can't drop selected_images\n");
+
+    TRY_EXEC("CREATE TABLE selected_images (num INTEGER PRIMARY KEY AUTOINCREMENT, imgid INTEGER)",
+             "[init] can't create selected_images\n");
+
+    TRY_EXEC("CREATE UNIQUE INDEX selected_images_ni"
+             " ON selected_images (imgid)",
+             "[init] can't create index selected_images_ni\n");
+
+    sqlite3_exec(db->handle, "COMMIT", NULL, NULL, NULL);
+    new_version = 52;
+  }
   else
     new_version = version; // should be the fallback so that calling code sees that we are in an infinite loop
 
@@ -3246,7 +3269,7 @@ static void _create_library_schema(dt_database_t *db)
 
   sqlite3_exec(db->handle,
                "CREATE UNIQUE INDEX main.selected_images_ni"
-               " ON main.selected_images (num, imgid)",
+               " ON selected_images (num, imgid)",
                NULL, NULL, NULL);
   ////////////////////////////// history
   sqlite3_exec(