Merge pull request #1142 from mmd-osm/patch/issue_1136

Provide a stack trace on a lua error with flex backend

Author: pnorman

src/lua-utils.cpp

@@ -135,3 +135,34 @@ bool luaX_get_table_bool(lua_State *lua_state, char const *key, int table_index,
     throw std::runtime_error{
         "{} must contain a '{}' boolean field"_format(error_msg, key)};
 }
+
+static int pcall_error_traceback_handler(lua_State *lua_state)
+{
+    assert(lua_state);
+
+    char const *msg = lua_tostring(lua_state, 1);
+    if (msg == nullptr) { // is error object not a string?
+        if (luaL_callmeta(lua_state, 1,
+                          "__tostring") && // does it have a metamethod
+            lua_type(lua_state, -1) == LUA_TSTRING) { // that produces a string?
+            return 1;                                 // that is the message
+        } else {
+            msg = lua_pushfstring(lua_state, "(error object is a %s value)",
+                                  luaL_typename(lua_state, 1));
+        }
+    }
+    luaL_traceback(lua_state, lua_state, msg, 1); // append a standard traceback
+    return 1;                                     // return the traceback
+}
+
+// wrapper function for lua_pcall to include a stack traceback
+int luaX_pcall(lua_State *lua_state, int narg, int nres)
+{
+    int const base = lua_gettop(lua_state) - narg; // function index
+    lua_pushcfunction(lua_state,
+                      pcall_error_traceback_handler); // push message handler
+    lua_insert(lua_state, base); // put it under function and args
+    int const status = lua_pcall(lua_state, narg, nres, base);
+    lua_remove(lua_state, base); // remove message handler from the stack
+    return status;
+}

src/lua-utils.hpp

@@ -49,4 +49,6 @@ char const *luaX_get_table_string(lua_State *lua_state, char const *key,
 bool luaX_get_table_bool(lua_State *lua_state, char const *key, int table_index,
                          char const *error_msg, bool default_value);
 
+int luaX_pcall(lua_State *lua_state, int narg, int nres);
+
 #endif // OSM2PGSQL_FLEX_LUA_HPP

src/output-flex.cpp

@@ -996,7 +996,7 @@ void output_flex_t::call_process_function(int index,
         get_options()->extra_attributes); // the single argument
 
     luaX_set_context(lua_state(), this);
-    if (lua_pcall(lua_state(), 1, 0, 0)) {
+    if (luaX_pcall(lua_state(), 1, 0)) {
         throw std::runtime_error{"Failed to execute lua processing function:"
                                  " {}"_format(lua_tostring(lua_state(), -1))};
     }