Fix checkUserExists functions

[biofa98]

This PR fixes an issue that occurs when removing an existing user from a group.
The error occurs during the terraform apply phase, the error that gets thrown is the following:

│ Error: error reading info about user: %!s(<nil>)
│
│   with module.groups["group_name"].redshift_group.group,
│   on ../../modules/group/main.tf line 1, in resource "redshift_group" "group":
│    1: resource "redshift_group" "group" {

Currently, the query used to check for the existence of a user is: SELECT 1 from pg_user_info WHERE usename=$1

The issue is that the usename is not enclosed in single quotes '', when manually executed this query causes redshift to return the following error:

db=# SELECT 1 from pg_user_info WHERE usename=existinguser;
ERROR:  column "existinguser" does not exist in pg_user_info

The same query executed with the usename enclosed in quotes returns the correct result:

db=# SELECT 1 from pg_user_info WHERE usename='existinguser';
?column?
----------
        1
(1 row)

[winglot]

Hi, thanks for your report.

Currently, the query used to check for the existence of a user is: SELECT 1 from pg_user_info WHERE usename=$1

The issue is that the usename is not enclosed in single quotes '', when manually executed this query causes redshift to return the following error:

db=# SELECT 1 from pg_user_info WHERE usename=existinguser;
ERROR:  column "existinguser" does not exist in pg_user_info

The same query executed with the usename enclosed in quotes returns the correct result:

db=# SELECT 1 from pg_user_info WHERE usename='existinguser';
?column?
----------
        1
(1 row)

This is all true about the quoting. Except this is not how the query is executed in the Go code. This is a prepared statement with one text parameter ($1). If you wish to simulate the same behavior, you would have to write it like this:

terraform_provider_tester=# PREPARE userExists(text) AS
terraform_provider_tester-# SELECT 1 from pg_user_info WHERE usename=$1;
PREPARE
terraform_provider_tester=# EXECUTE userExists('root');
 ?column?
----------
        1
(1 row)

terraform_provider_tester=# EXECUTE userExists('nonexisting');
 ?column?
----------
(0 rows)

which as you can see, works fine.

But not everything is lost. Thanks to your report I skimmed through the code and noticed the issue you are having is hiding just a few lines below your change in the checkIfUserExists function. There is this switch statement that makes absolutely no sense:

	switch err {
	case sql.ErrNoRows:
		return false, nil
	case nil:
		return false, fmt.Errorf("error reading info about user: %s", err)
	}

what the author (most likely me) meant to do here is:

	switch {
	case err == sql.ErrNoRows:
		return false, nil
	case err != nil:
		return false, fmt.Errorf("error reading info about user: %s", err)
	}

which, when fixed, makes removing existing users from groups possible.

I will close your PR and prepare a new one with a fixed switch statement and additional tests.