When upgrading smos-sync-client, you may at times run into an error like this:

Database migration: manual intervention required.
The unsafe actions are prefixed by '***' below:

    CREATE TEMP TABLE "client_file_backup"("id" INTEGER PRIMARY KEY,"uuid" uuid NOT NULL,"path" VARCHAR NOT NULL,"sha256" BLOB NOT NULL,"time" INTEGER NOT NULL,CONSTRAINT "unique_path" UNIQUE ("path"),CONSTRAINT "unique_u_u_i_d" UNIQUE ("uuid"));
    INSERT INTO "client_file_backup"("id","uuid","path","sha256","time") SELECT "id","uuid","path","sha256","time" FROM "client_file";
*** DROP TABLE "client_file";
    CREATE TABLE "client_file"("id" INTEGER PRIMARY KEY,"uuid" uuid NOT NULL,"path" VARCHAR NOT NULL,"sha256" BLOB NOT NULL,"time" INTEGER NOT NULL,CONSTRAINT "unique_path" UNIQUE ("path"),CONSTRAINT "unique_u_u_i_d" UNIQUE ("uuid"));
    INSERT INTO "client_file" SELECT "id","uuid","path","sha256","time" FROM "client_file_backup";
    DROP TABLE "client_file_backup";

CallStack (from HasCallStack):
  error, called at ./Database/Persist/Sql/Migration.hs:101:14 in persistent-2.10.0-Jjk7XQn5Cwh2OYEvyhlR0s:Database.Persist.Sql.Migration

There are two ways to solve this problem, but first you should backup your smos files well.

Deleting the metadata database

You'll need to remove both the metadata database and the server uuid file for this to work. Otherwise all your files will be erased on the next sync.

rm ~/.smos/server-uuid.json 
rm ~/.smos/sync-metadata.sqlite3

Then sync again:

smos-sync-client sync

Performing the database migration

Go into an sqlite3 shell with the database:

sqlite3 ~/.smos/sync-metadata.sqlite3

Then run the commands listed in the error message one by one. You'll need to remove the *** in front of the dangerous commands to do so.