Ensure empty error tables in scripts don't crash Valkey (#2229)

When calling the command `EVAL error{} 0`, Valkey crashes with the
following stack trace. This patch ensures we never leave the
`err_info.msg` field null when we fail to extract a proper error
message.

---------

Signed-off-by: Fusl <[email protected]>
Signed-off-by: Binbin <[email protected]>
Co-authored-by: Binbin <[email protected]>

Author: 2 people

src/script_lua.c

@@ -1645,6 +1645,11 @@ void luaExtractErrorInformation(lua_State *lua, errorInfo *err_info) {
         err_info->ignore_err_stats_update = lua_toboolean(lua, -1);
     }
     lua_pop(lua, 1);
+
+    if (err_info->msg == NULL) {
+        /* Ensure we never return a NULL msg. */
+        err_info->msg = sdsnew("ERR unknown error");
+    }
 }
 
 void luaCallFunction(scriptRunCtx* run_ctx, lua_State *lua, robj** keys, size_t nkeys, robj** args, size_t nargs, int debug_enabled) {

tests/unit/scripting.tcl

@@ -2266,4 +2266,21 @@ start_server {tags {"scripting"}} {
             assert { [r memory usage foo] <= $expected_memory};
         }
     }
+
+    test {EVAL - explicit error() call handling} {
+        # error("simple string error")
+        assert_error {ERR user_script:1: simple string error script: *} {
+            r eval "error('simple string error')" 0
+        }
+
+        # error({"err": "ERR table error"})
+        assert_error {ERR table error script: *} {
+            r eval "error({err='ERR table error'})" 0
+        }
+
+        # error({})
+        assert_error {ERR unknown error script: *} {
+            r eval "error({})" 0
+        }
+    }
 }