ERROR cursor does not exist after first loop in PL/pgSQL

I need to load a large number of csv files in to a PostgreSQL database. I have a table source_files which contains the file paths and a flag which indicates whether a file has already been loaded for all the csv files I need to load.

I have written the following code which loads the first file correctly but then throws the error:

ERROR: cursor "curs" does not exist

Why am I getting this error and how can I fix it?

DO $$
DECLARE
    file_record record;
    curs CURSOR
        FOR SELECT id, file_path
            FROM source_files
            WHERE added_to_db=FALSE
            ORDER BY id;
BEGIN
    OPEN curs;
    LOOP
        -- Get next entry in source file which contains name of csv to load
        FETCH curs INTO file_record;
        exit WHEN NOT found;
        BEGIN
            -- As we need to add a column to the data after loading csv but before inserting
            -- into final table we use a temporary table mytemp
            DROP TABLE mytemp;

            CREATE TABLE mytemp
            (
                dataA numeric,
                dataB numeric
            );

            -- Load csv file   
            EXECUTE FORMAT('COPY mytemp
                            FROM ''%s''
                            DELIMITER '',''
                            CSV HEADER;', file_record.file_path);

            -- Add Column specifying what source file the data is from
            ALTER TABLE mytemp
                ADD COLUMN source_id int;

            UPDATE mytemp 
                SET source_id=file_record.id;

            -- Add the data to the destination table
            INSERT INTO data_table(
                dataA,
                dataB,
                source_id
            )
            SELECT 
                mytemp.dataA,
                mytemp.dataB
                mytemp.source_id
            FROM 
                mytemp

            -- Set a flag to indicate that the current file in source_files has been loaded
            UPDATE source_files
                SET added_to_db=TRUE WHERE CURRENT OF curs;

            COMMIT;
        END;

    END LOOP;
    CLOSE curs;

END $$;

Answer

There big problem with your code is the COMMIT. You can use COMMIT in a DO statement, but the cursor is closed as soon as the transaction ends. In SQL you can create a cursor WITH HOLD that remains valid after the transaction has ended, but that is not available in PL/pgSQL.

I suggest removing the COMMIT.

Another error in your code is your use of the format function, which exposes you to SQL injection. Instead of

FORMAT('COPY mytemp
        FROM ''%s''
        DELIMITER '',''
        CSV HEADER;', file_record.file_path);

use

FORMAT('COPY mytemp
        FROM %L
        DELIMITER '',''
        CSV HEADER;', file_record.file_path);

You can make the code much simpler by using an implicit cursor in the loop:

FOR file_record IN
   SELECT id, file_path
   FROM source_files
   WHERE added_to_db=FALSE
   ORDER BY id
LOOP
   ...
END LOOP;

That saves you declaring the cursor and the EXIT WHEN. The OPEN and CLOSE statements are unnecessary anyway.