environ: implement __pairs metamethod

Implements: https://todo.sr.ht/~jeremy/callisto/11

lenviron.c

@@ -10,11 +10,15 @@
 
 #include <errno.h>
 #include <stdlib.h>
+#include <string.h>
 
 #include <lua/lauxlib.h>
 #include <lua/lua.h>
 
 #include "callisto.h"
+#include "util.h"
+
+extern char **environ;
 
 /***
  * Table enabling easy access to environment
@@ -94,11 +98,55 @@ environ__newindex(lua_State *L)
 	return 0;
 }
 
+int
+pairs_iter(lua_State *L)
+{
+	char *line, *name;
+	size_t *envi, len;
+	int i;
+
+	len = 0;
+	envi = (size_t *)lua_touserdata(L, 1);
+	line = environ[*envi];
+
+	if (line == NULL) {
+		lua_pushnil(L);
+		return 1;
+	}
+
+	name = calloc(strlen(line), sizeof(char));
+
+	/* extract the name portion from the environ line */
+	for (i = 0; line[i] != '='; i++) {
+		if (line[i] == '\0')
+			return lfailm(L, "process environment is corrupted");
+
+		name[len] = line[i];
+		len++;
+	}
+
+	(*envi)++;
+	lua_pushlstring(L, name, len);
+	return 1;
+}
+
+int
+environ__pairs(lua_State *L)
+{
+	size_t *p;
+
+	lua_pushcfunction(L, pairs_iter);
+	p = lua_newuserdatauv(L, sizeof(size_t), 0);
+	*p = 0;
+	return 2;
+}
+
 /* clang-format off */
 
 static const luaL_Reg mt[] = {
 	{"__index",    environ__index},
 	{"__newindex", environ__newindex},
+	{"__pairs",    environ__pairs},
 	{NULL, NULL}
 };
 

test.lua

@@ -41,6 +41,17 @@ local tests = {
 			environ[var] = val
 			assert(environ[var] == "hello")
 			return 'environ["' .. var .. '"] = "' .. val .. '"'
+		end,
+		pairs = function ()
+			local ev = {}
+			local var = "VAR"
+
+			environ[var] = "1"
+			for env in pairs(environ) do
+				ev[env] = environ[env]
+			end
+			assert(ev[var] == "1")
+			return "pairs(environ)"
 		end
 	},
 
@@ -251,6 +262,7 @@ do
 	-- environ
 	test(environ.getvar)
 	test(environ.setvar)
+	test(environ.pairs)
 
 	-- fs
 	test(fs.copy)