feat(lua): add safe_function template that catches Lean and C++ exceptions and convert them into Lua errors

I'm using the approach described at:
http://stackoverflow.com/questions/4615890/how-to-handle-c-exceptions-when-calling-functions-from-lua

BTW, in some Lua versions, the C++ exceptions are correctly propagated.
I think we should not rely on features of particular implementations.
For example, LuaJIT does not propagate C++ exceptions.
Whenever an exception is thrown from C++ code invoked from LuaJit, LuaJit interrupts the execution and converts it to an error "C++ exception".
On the other hand, Lua 5.2 PUC-Rio interpreter (for Ubuntu) seem to propagate the C++ exceptions.
The template safe_function solves the issue. It will also produce a Lua error whenever the function being wrapped throws an exception. The error message is based on the "what()" method.

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>

src/bindings/lua/name.cpp

@@ -17,7 +17,11 @@ static int name_eq(lua_State * L);
 static int name_lt(lua_State * L);
 
 static const struct luaL_Reg name_m[] = {
-    {"__gc", name_gc}, {"__tostring", name_tostring}, {"__eq", name_eq}, {"__lt", name_lt}, {0, 0}
+    {"__gc",       name_gc}, // never throws
+    {"__tostring", safe_function<name_tostring>},
+    {"__eq",       safe_function<name_eq>},
+    {"__lt",       safe_function<name_lt>},
+    {0, 0}
 };
 
 static int mk_name(lua_State * L) {
@@ -84,7 +88,7 @@ void init_name(lua_State * L) {
     luaL_newmetatable(L, "name.mt");
     setfuncs(L, name_m, 0);
 
-    lua_pushcfunction(L, mk_name);
+    lua_pushcfunction(L, safe_function<mk_name>);
     lua_setglobal(L, "name");
 }
 }

src/bindings/lua/numerics.cpp

@@ -102,17 +102,23 @@ public:
         luaL_newmetatable(L, M);
         setfuncs(L, m, 0);
 
-        lua_pushcfunction(L, mk);
+        lua_pushcfunction(L, safe_function<mk>);
         lua_setglobal(L, N);
     }
 };
 
 template<typename T, char const * N, char const * M>
 const struct luaL_Reg num_bindings<T, N, M>::m[] = {
-    {"__gc", num_bindings<T, N, M>::gc}, {"__tostring", num_bindings<T, N, M>::tostring}, {"__eq", num_bindings<T, N, M>::eq},
+    {"__gc",       num_bindings<T, N, M>::gc}, // never throws
-    {"__lt", num_bindings<T, N, M>::lt}, {"__add", num_bindings<T, N, M>::add}, {"__add", num_bindings<T, N, M>::sub},
+    {"__tostring", safe_function<num_bindings<T, N, M>::tostring>},
-    {"__mul", num_bindings<T, N, M>::mul}, {"__div", num_bindings<T, N, M>::div}, {"__pow", num_bindings<T, N, M>::power},
+    {"__eq",       safe_function<num_bindings<T, N, M>::eq>},
-    {"__unm", num_bindings<T, N, M>::umn},
+    {"__lt",       safe_function<num_bindings<T, N, M>::lt>},
+    {"__add",      safe_function<num_bindings<T, N, M>::add>},
+    {"__add",      safe_function<num_bindings<T, N, M>::sub>},
+    {"__mul",      safe_function<num_bindings<T, N, M>::mul>},
+    {"__div",      safe_function<num_bindings<T, N, M>::div>},
+    {"__pow",      safe_function<num_bindings<T, N, M>::power>},
+    {"__unm",      safe_function<num_bindings<T, N, M>::umn>},
     {0, 0}
 };
 
@@ -121,6 +127,7 @@ constexpr char const mpz_metatable[] = "mpz.mt";
 void init_mpz(lua_State * L) {
     num_bindings<mpz, mpz_name, mpz_metatable>::init(L);
 }
+
 constexpr char const mpq_name[]      = "mpq";
 constexpr char const mpq_metatable[] = "mpq.mt";
 void init_mpq(lua_State * L) {

src/bindings/lua/util.cpp

@@ -6,6 +6,9 @@ Author: Leonardo de Moura
 */
 #ifdef LEAN_USE_LUA
 #include <lua.hpp>
+#include <exception>
+#include "util/exception.h"
+
 namespace lean {
 /**
    \brief luaL_setfuncs replacement. The function luaL_setfuncs is only available in Lua 5.2.
@@ -21,5 +24,24 @@ void setfuncs(lua_State * L, luaL_Reg const * l, int nup) {
     }
     lua_pop(L, nup);  // remove upvalues
 }
+
+int safe_function_wrapper(lua_State * L, lua_CFunction f){
+    static thread_local std::string _error_msg;
+    char const * error_msg;
+    try {
+        return f(L);
+    } catch (exception & e) {
+        _error_msg = e.what();
+        error_msg  = _error_msg.c_str();
+    } catch (std::bad_alloc &) {
+        error_msg = "out of memory";
+    } catch (std::exception & e) {
+        _error_msg = e.what();
+        error_msg  = _error_msg.c_str();
+    } catch(...) {
+        error_msg = "unknown error";
+    }
+    return luaL_error(L, error_msg);
+}
 }
 #endif

src/bindings/lua/util.h

@@ -8,5 +8,13 @@ Author: Leonardo de Moura
 #include <lua.hpp>
 namespace lean {
 void setfuncs(lua_State * L, luaL_Reg const * l, int nup);
+/**
+   \brief Wrapper for invoking function f, and catching Lean exceptions.
+*/
+int safe_function_wrapper(lua_State * L, lua_CFunction f);
+template<lua_CFunction F>
+int safe_function(lua_State * L) {
+    return safe_function_wrapper(L, F);
+}
 }
 #endif

src/shell/lua/leanlua.cpp

@@ -9,14 +9,13 @@ Author: Leonardo de Moura
 
 #ifdef LEAN_USE_LUA
 #include <lua.hpp>
-#include "util/exception.h"
 #include "bindings/lua/name.h"
 #include "bindings/lua/numerics.h"
 
 int main(int argc, char ** argv) {
     int status, result;
     lua_State *L;
-
+    int exitcode = 0;
     L = luaL_newstate();
     luaL_openlibs(L);
     lean::init_name(L);
@@ -27,21 +26,17 @@ int main(int argc, char ** argv) {
         status = luaL_loadfile(L, argv[i]);
         if (status) {
             std::cerr << "Couldn't load file: " << lua_tostring(L, -1) << "\n";
-            return 1;
+            exitcode = 1;
-        }
+        } else {
-        try {
             result = lua_pcall(L, 0, LUA_MULTRET, 0);
             if (result) {
                 std::cerr << "Failed to run script: " << lua_tostring(L, -1) << "\n";
-                return 1;
+                exitcode = 1;
             }
-        } catch (lean::exception & ex) {
-            std::cerr << "Lean exception when running: " << argv[i] << "\n";
-            std::cerr << ex.what() << "\n";
         }
     }
     lua_close(L);
-    return 0;
+    return exitcode;
 }
 #else
 int main() {