fix: upload part tmp+rename (B4), transactional last-admin DeleteUser (B5), RowsAffected order (B13)

backend/internal/store/store.go

@@ -2,12 +2,23 @@ package store
 
 import (
 	"context"
+	"errors"
 	"time"
 
 	"github.com/jackc/pgx/v5"
+	"github.com/jackc/pgx/v5/pgconn"
 	"github.com/jackc/pgx/v5/pgxpool"
 )
 
+// ErrLastAdmin is returned by DeleteUser when attempting to delete the last admin.
+var ErrLastAdmin = errors.New("cannot delete the last admin")
+
+// IsUniqueViolation reports whether err is a PostgreSQL unique constraint violation (23505).
+func IsUniqueViolation(err error) bool {
+	var pgErr *pgconn.PgError
+	return errors.As(err, &pgErr) && pgErr.Code == "23505"
+}
+
 type Store struct{ P *pgxpool.Pool }
 
 func New(p *pgxpool.Pool) *Store { return &Store{P: p} }
@@ -103,9 +114,31 @@ func (s *Store) ListUsers(ctx context.Context) ([]User, error) {
 	return out, rows.Err()
 }
 
+// DeleteUser deletes a user with a transactional last-admin check (B5).
+// Returns ErrLastAdmin if the target is the last admin.
 func (s *Store) DeleteUser(ctx context.Context, id int64) error {
-	_, err := s.P.Exec(ctx, "DELETE FROM users WHERE id=$1", id)
-	return err
+	tx, err := s.P.Begin(ctx)
+	if err != nil {
+		return err
+	}
+	defer tx.Rollback(ctx)
+	var role string
+	if err := tx.QueryRow(ctx, "SELECT role FROM users WHERE id=$1 FOR UPDATE", id).Scan(&role); err != nil {
+		return err // includes pgx.ErrNoRows
+	}
+	if role == "admin" {
+		var n int
+		if err := tx.QueryRow(ctx, "SELECT count(*) FROM users WHERE role='admin'").Scan(&n); err != nil {
+			return err
+		}
+		if n <= 1 {
+			return ErrLastAdmin
+		}
+	}
+	if _, err := tx.Exec(ctx, "DELETE FROM users WHERE id=$1", id); err != nil {
+		return err
+	}
+	return tx.Commit(ctx)
 }
 
 func scanUser(row pgx.Row) (User, error) {
@@ -114,12 +147,7 @@ func scanUser(row pgx.Row) (User, error) {
 	return u, err
 }
 
-// CountAdmins 供 Task 5 的"最后一个 admin 不可删"保护
-func (s *Store) CountAdmins(ctx context.Context) (int, error) {
-	var n int
-	err := s.P.QueryRow(ctx, "SELECT count(*) FROM users WHERE role='admin'").Scan(&n)
-	return n, err
-}
+
 
 // ---------- libraries ----------
 
@@ -357,12 +385,18 @@ func (s *Store) ListBookmarks(ctx context.Context, userID, libID int64, bookPath
 
 func (s *Store) UpdateBookmarkNote(ctx context.Context, userID, id int64, note string) (bool, error) {
 	res, err := s.P.Exec(ctx, `UPDATE bookmarks SET note=$3 WHERE id=$1 AND user_id=$2`, id, userID, note)
-	return res.RowsAffected() > 0, err
+	if err != nil {
+		return false, err
+	}
+	return res.RowsAffected() > 0, nil
 }
 
 func (s *Store) DeleteBookmark(ctx context.Context, userID, id int64) (bool, error) {
 	res, err := s.P.Exec(ctx, `DELETE FROM bookmarks WHERE id=$1 AND user_id=$2`, id, userID)
-	return res.RowsAffected() > 0, err
+	if err != nil {
+		return false, err
+	}
+	return res.RowsAffected() > 0, nil
 }
 
 func (s *Store) GetProgress(ctx context.Context, userID, libID int64, bookPath string) (Progress, error) {