Also, handle first user differently under the hood

cmd/model/users.go

@@ -1,6 +1,8 @@
 package model
 
 import (
+	"context"
+	"database/sql"
 	"fmt"
 	"log"
 
@@ -198,3 +200,71 @@ func (db *DB) UpdateUserAttributes(id int64, user, first, last, oldPass, newPass
 
 	return fmt.Errorf("error: %v unsuccessful retries for DB operation, aborting", TxMaxRetries)
 }
+
+func (db *DB) AddFirstUser(u *User, pass string) (int64, error) {
+	var numUsers int64
+	txOptions := &sql.TxOptions{Isolation: sql.LevelSerializable}
+	selectQuery := "SELECT COUNT(*) FROM users"
+	insertQuery := `
+    INSERT INTO users (username, password, first_name, last_name, role)
+    VALUES (?, ?, ?, ?, ?)
+    `
+
+	for i := 0; i < TxMaxRetries; i++ {
+		id, err := func() (int64, error) {
+			tx, err := db.BeginTx(context.Background(), txOptions)
+			if err != nil {
+				return 0, fmt.Errorf("error starting transaction: %v", err)
+			}
+
+			if err := tx.QueryRow(selectQuery).Scan(&numUsers); err != nil {
+				if rollbackErr := tx.Rollback(); rollbackErr != nil {
+					log.Fatalf("transaction error: %v, rollback error: %v", err, rollbackErr)
+				}
+				return 0, fmt.Errorf("error getting ID of %v: %v", u.UserName, err)
+			}
+			if numUsers != 0 {
+				if err = tx.Commit(); err != nil {
+					return 0, fmt.Errorf("error committing transaction: %v", err)
+				}
+				return 2, nil
+			}
+
+			hashedPass, err := bcrypt.GenerateFromPassword([]byte(pass), bcrypt.DefaultCost)
+			if err != nil {
+				if rollbackErr := tx.Rollback(); rollbackErr != nil {
+					log.Fatalf("transaction error: %v, rollback error: %v", err, rollbackErr)
+				}
+				return 0, fmt.Errorf("error creating password hash: %v", err)
+			}
+
+			result, err := tx.Exec(insertQuery, u.UserName, string(hashedPass), u.FirstName, u.LastName, u.Role)
+			if err != nil {
+				if rollbackErr := tx.Rollback(); rollbackErr != nil {
+					log.Fatalf("transaction error: %v, rollback error: %v", err, rollbackErr)
+				}
+				return 0, fmt.Errorf("error inserting new user %v into DB: %v", u.UserName, err)
+			}
+
+			id, err := result.LastInsertId()
+			if err != nil {
+				if rollbackErr := tx.Rollback(); rollbackErr != nil {
+					log.Fatalf("transaction error: %v, rollback error: %v", err, rollbackErr)
+				}
+				return 0, fmt.Errorf("error inserting user into DB: %v", err)
+			}
+
+			if err = tx.Commit(); err != nil {
+				return 0, fmt.Errorf("error committing transaction: %v", err)
+			}
+			return id, nil
+		}()
+		if err == nil {
+			return id, nil
+		}
+
+		log.Println(err)
+		wait(i)
+	}
+	return 0, fmt.Errorf("error: %v unsuccessful retries for DB operation, aborting", TxMaxRetries)
+}

cmd/view/users.go

@@ -236,12 +236,18 @@ func AddFirstUser(c *control.CliArgs, db *model.DB, s *control.CookieStore) http
 			return
 		}
 
-		htmlData.ID, err = db.AddUser(htmlData.User, pass)
+		htmlData.ID, err = db.AddFirstUser(htmlData.User, pass)
 		if err != nil {
 			log.Println(err)
 			http.Error(w, err.Error(), http.StatusInternalServerError)
 			return
 		}
+		if htmlData.ID > 1 {
+			errString := "error: there is already a first user"
+			log.Println(errString)
+			http.Error(w, errString, http.StatusInternalServerError)
+			return
+		}
 
 		if err := saveSession(w, r, s, htmlData.User); err != nil {
 			log.Println(err)

web/templates/first-user.html

@@ -1,5 +1,6 @@
 {{define "page-content"}}
-<h2>Neuer Benutzer</h2>
+<h2>Erster Benutzer (Administrator)</h2>
+
 <form>
     <div class="grid grid-cols-3 gap-4">
         <div>