fix(backend): dbErr 503 degradation for connection-class DB errors

backend/internal/api/api.go

@@ -1,10 +1,17 @@
 package api
 
 import (
+	"errors"
+	"io"
+	"log"
+	"net"
 	"net/http"
 	"strings"
+	"syscall"
 
 	"github.com/gin-gonic/gin"
+	"github.com/jackc/pgx/v5/pgconn"
+	"github.com/jackc/puddle/v2"
 
 	"booklib/internal/auth"
 	"booklib/internal/config"
@@ -24,6 +31,21 @@ func err(c *gin.Context, status int, code, msg string) {
 	c.AbortWithStatusJSON(status, gin.H{"error": gin.H{"code": code, "message": msg}})
 }
 
+// dbErr 统一处理 store 层失败:记日志;连接类错误 503(Service Unavailable),其余 500
+// 注:brief 里的 pgxpool.ErrClosedPool 在 pgx v5 不存在,实际由 puddle 原样透出,用它替代;
+// PG 停机时池内连接先收到 SQLSTATE 57P01(administrator shutdown),故把 08xx/57Pxx 也归为 503
+func dbErr(c *gin.Context, e error) {
+	log.Printf("db: %v", e)
+	status, code := http.StatusInternalServerError, "internal"
+	var pgErr *pgconn.PgError
+	connClass := errors.As(e, &pgErr) && (strings.HasPrefix(pgErr.Code, "08") || strings.HasPrefix(pgErr.Code, "57P"))
+	if connClass || errors.Is(e, syscall.ECONNREFUSED) || errors.Is(e, io.ErrUnexpectedEOF) ||
+		errors.Is(e, net.ErrClosed) || errors.Is(e, puddle.ErrClosedPool) {
+		status, code = http.StatusServiceUnavailable, "unavailable"
+	}
+	err(c, status, code, "db error")
+}
+
 func (a *api) authMw() gin.HandlerFunc {
 	return func(c *gin.Context) {
 		h := c.GetHeader("Authorization")