Test cache during init (#17852)

modules/cache/cache.go

 	"fmt"
 	"strconv"
 
+	"code.gitea.io/gitea/modules/log"
 	"code.gitea.io/gitea/modules/setting"
 
 	mc "gitea.com/go-chi/cache"
 		if conn, err = newCache(setting.CacheService.Cache); err != nil {
 			return err
 		}
+		const testKey = "__gitea_cache_test"
+		const testVal = "test-value"
+		if err = conn.Put(testKey, testVal, 10); err != nil {
+			return err
+		}
+		val := conn.Get(testKey)
+		if valStr, ok := val.(string); !ok || valStr != testVal {
+			// If the cache is full, the Get may not read the expected value stored by Put.
+			// Since we have checked that Put can success, so we just show a warning here, do not return an error to panic.
+			log.Warn("cache (adapter:%s, config:%s) doesn't seem to work correctly, set test value '%v' but get '%v'",
+				setting.CacheService.Cache.Adapter, setting.CacheService.Cache.Conn,
+				testVal, val,
+			)
+		}
 	}
 
 	return err