chore: close db properly in early exit paths in ConnectToPostgres (#18448)

There were some code paths where if we exited early from the function
the postgres connection would never get cleaned up.

This is the mechanism that cleans up the db - it requires the err
variable to be not nil:
https://github.com/coder/coder/blob/118bf981454188c4989e8b565dec67906616f885/cli/server.go#L2319-L2328

cli/server.go

@@ -2312,19 +2312,20 @@ func ConnectToPostgres(ctx context.Context, logger slog.Logger, driver string, d
 
 	var err error
 	var sqlDB *sql.DB
+	dbNeedsClosing := true
 	// Try to connect for 30 seconds.
 	ctx, cancel := context.WithTimeout(ctx, 30*time.Second)
 	defer cancel()
 
 	defer func() {
-		if err == nil {
+		if !dbNeedsClosing {
 			return
 		}
 		if sqlDB != nil {
 			_ = sqlDB.Close()
 			sqlDB = nil
+			logger.Debug(ctx, "closed db before returning from ConnectToPostgres")
 		}
-		logger.Error(ctx, "connect to postgres failed", slog.Error(err))
 	}()
 
 	var tries int
@@ -2361,12 +2362,7 @@ func ConnectToPostgres(ctx context.Context, logger slog.Logger, driver string, d
 	}
 	defer version.Close()
 	if !version.Next() {
-		// it's critical we assign to the err variable, otherwise the defer statement
-		// that runs db.Close() will not execute it
-		if err = version.Err(); err != nil {
-			return nil, xerrors.Errorf("no rows returned for version select: %w", err)
-		}
-		return nil, xerrors.Errorf("no rows returned for version select")
+		return nil, xerrors.Errorf("no rows returned for version select: %w", version.Err())
 	}
 	var versionNum int
 	err = version.Scan(&versionNum)
@@ -2408,6 +2404,7 @@ func ConnectToPostgres(ctx context.Context, logger slog.Logger, driver string, d
 	// of connection churn.
 	sqlDB.SetMaxIdleConns(3)
 
+	dbNeedsClosing = false
 	return sqlDB, nil
 }