Merge pull request #162 from siffiejoe/exception-meta

Support table errors in LuaSockets LTN13 C implementation.

doc/socket.html

@@ -220,13 +220,6 @@ Returns an equivalent function that instead of throwing exceptions,
 returns <tt><b>nil</b></tt> followed by an error message. 
 </p>
 
-<p class=note>
-Note: Beware that if your function performs some illegal operation that
-raises an error, the protected function will catch the error and return it
-as a string. This is because the <a href=#try><tt>try</tt></a> function
-uses errors as the mechanism to throw exceptions.  
-</p>
-
 <!-- select +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ -->
 
 <p class=name id=select> 
@@ -424,8 +417,7 @@ socket.<b>try(</b>ret<sub>1</sub> [, ret<sub>2</sub> ... ret<sub>N</sub>]<b>)</b
 
 <p class=description>
 Throws an exception in case of error. The exception can only be caught 
-by the <a href=#protect><tt>protect</tt></a> function. It does not explode
-into an error message.
+by the <a href=#protect><tt>protect</tt></a> function.
 </p>
 
 <p class=parameters>
@@ -436,7 +428,10 @@ nested with <tt>try</tt>.
 
 <p class=return> 
 The function returns <tt>ret</tt><sub>1</sub> to <tt>ret</tt><sub>N</sub> if
-<tt>ret</tt><sub>1</sub> is not <tt><b>nil</b></tt>. Otherwise, it calls <tt>error</tt> passing <tt>ret</tt><sub>2</sub>.
+<tt>ret</tt><sub>1</sub> is not <tt><b>nil</b></tt> or <tt><b>false</b></tt>.
+Otherwise, it calls <tt>error</tt> passing <tt>ret</tt><sub>2</sub> wrapped
+in a table with metatable used by <a href=#protect><tt>protect</tt></a> to
+distinguish exceptions from runtime errors.
 </p>
 
 <pre class=example>

src/except.c

@@ -12,7 +12,7 @@
 
 #if LUA_VERSION_NUM < 502
 #define lua_pcallk(L, na, nr, err, ctx, cont) \
-    ((void)ctx,(void)cont,lua_pcall(L, na, nr, err))
+    (((void)ctx),((void)cont),lua_pcall(L, na, nr, err))
 #endif
 
 #if LUA_VERSION_NUM < 503
@@ -39,12 +39,11 @@ static luaL_Reg func[] = {
 * Try factory
 \*-------------------------------------------------------------------------*/
 static void wrap(lua_State *L) {
-    lua_newtable(L);
-    lua_pushnumber(L, 1);
-    lua_pushvalue(L, -3);
-    lua_settable(L, -3);
-    lua_insert(L, -2);
-    lua_pop(L, 1);
+    lua_createtable(L, 1, 0);
+    lua_pushvalue(L, -2);
+    lua_rawseti(L, -2, 1);
+    lua_pushvalue(L, lua_upvalueindex(2));
+    lua_setmetatable(L, -2);
 }
 
 static int finalize(lua_State *L) {
@@ -58,15 +57,16 @@ static int finalize(lua_State *L) {
     } else return lua_gettop(L);
 }
 
-static int do_nothing(lua_State *L) { 
+static int do_nothing(lua_State *L) {
     (void) L;
-    return 0; 
+    return 0;
 }
 
 static int global_newtry(lua_State *L) {
     lua_settop(L, 1);
     if (lua_isnil(L, 1)) lua_pushcfunction(L, do_nothing);
-    lua_pushcclosure(L, finalize, 1);
+    lua_pushvalue(L, lua_upvalueindex(1));
+    lua_pushcclosure(L, finalize, 2);
     return 1;
 }
 
@@ -74,13 +74,16 @@ static int global_newtry(lua_State *L) {
 * Protect factory
 \*-------------------------------------------------------------------------*/
 static int unwrap(lua_State *L) {
-    if (lua_istable(L, -1)) {
-        lua_pushnumber(L, 1);
-        lua_gettable(L, -2);
-        lua_pushnil(L);
-        lua_insert(L, -2);
-        return 1;
-    } else return 0;
+    if (lua_istable(L, -1) && lua_getmetatable(L, -1)) {
+        int r = lua_rawequal(L, -1, lua_upvalueindex(2));
+        lua_pop(L, 1);
+        if (r) {
+            lua_pushnil(L);
+            lua_rawgeti(L, -2, 1);
+            return 1;
+        }
+    }
+    return 0;
 }
 
 static int protected_finish(lua_State *L, int status, lua_KContext ctx) {
@@ -110,7 +113,9 @@ static int protected_(lua_State *L) {
 }
 
 static int global_protect(lua_State *L) {
-    lua_pushcclosure(L, protected_, 1);
+    lua_settop(L, 1);
+    lua_pushvalue(L, lua_upvalueindex(1));
+    lua_pushcclosure(L, protected_, 2);
     return 1;
 }
 
@@ -118,6 +123,9 @@ static int global_protect(lua_State *L) {
 * Init module
 \*-------------------------------------------------------------------------*/
 int except_open(lua_State *L) {
-    luaL_setfuncs(L, func, 0);
+    lua_newtable(L); /* metatable for wrapped exceptions */
+    lua_pushboolean(L, 0);
+    lua_setfield(L, -2, "__metatable");
+    luaL_setfuncs(L, func, 1);
     return 0;
 }

src/except.h

@@ -9,21 +9,26 @@
 * error checking was taking a substantial amount of the coding. These
 * function greatly simplify the task of checking errors.
 *
-* The main idea is that functions should return nil as its first return
-* value when it finds an error, and return an error message (or value)
+* The main idea is that functions should return nil as their first return
+* values when they find an error, and return an error message (or value)
 * following nil. In case of success, as long as the first value is not nil,
 * the other values don't matter.
 *
 * The idea is to nest function calls with the "try" function. This function
-* checks the first value, and calls "error" on the second if the first is
-* nil. Otherwise, it returns all values it received. 
+* checks the first value, and, if it's falsy, wraps the second value in a
+* table with metatable and calls "error" on it. Otherwise, it returns all
+* values it received. Basically, it works like the Lua "assert" function,
+* but it creates errors targeted specifically at "protect".
 *
-* The protect function returns a new function that behaves exactly like the
-* function it receives, but the new function doesn't throw exceptions: it
-* returns nil followed by the error message instead.
+* The "newtry" function is a factory for "try" functions that call a
+* finalizer in protected mode before calling "error".
 *
-* With these two function, it's easy to write functions that throw
-* exceptions on error, but that don't interrupt the user script. 
+* The "protect" function returns a new function that behaves exactly like
+* the function it receives, but the new function catches exceptions thrown
+* by "try" functions and returns nil followed by the error message instead.
+*
+* With these three functions, it's easy to write functions that throw
+* exceptions on error, but that don't interrupt the user script.
 \*=========================================================================*/
 
 #include "lua.h"

test/excepttest.lua

@@ -1,6 +1,31 @@
 local socket = require("socket")
-try = socket.newtry(function()
-    print("finalized!!!")
+
+local finalizer_called
+
+local func = socket.protect(function(err, ...)
+    local try = socket.newtry(function()
+        finalizer_called = true
+        error("ignored")
+    end)
+
+    if err then
+        return error(err, 0)
+    else
+        return try(...)
+    end
 end)
-try = socket.protect(try)
-print(try(nil, "it works"))
+
+local ret1, ret2, ret3 = func(false, 1, 2, 3)
+assert(not finalizer_called, "unexpected finalizer call")
+assert(ret1 == 1 and ret2 == 2 and ret3 == 3, "incorrect return values")
+
+ret1, ret2, ret3 = func(false, false, "error message")
+assert(finalizer_called, "finalizer not called")
+assert(ret1 == nil and ret2 == "error message" and ret3 == nil, "incorrect return values")
+
+local err = {key = "value"}
+ret1, ret2 = pcall(func, err)
+assert(not ret1, "error not rethrown")
+assert(ret2 == err, "incorrect error rethrown")
+
+print("OK")