Implement shell builtins: cd, exec, umask (issue #15)

Add builtin dispatch in lcmd.c that checks __builtins table before
fork(), so cd/exec/umask operate on the shell process itself.

lbuiltin.c

@@ -0,0 +1,159 @@
+/*
+** $Id: lbuiltin.c $
+** Shell builtins (cd, exec, umask)
+** See Copyright Notice in lua.h
+*/
+
+#define lbuiltin_c
+#define LUA_LIB
+
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include <sys/stat.h>
+#include <unistd.h>
+
+#include "lua.h"
+#include "lauxlib.h"
+#include "lbuiltin.h"
+
+
+static void push_builtin_result (lua_State *L, int code,
+                                 const char *out, const char *err) {
+  lua_createtable(L, 0, 3);
+  lua_pushinteger(L, code);
+  lua_setfield(L, -2, "code");
+  lua_pushstring(L, out ? out : "");
+  lua_setfield(L, -2, "stdout");
+  lua_pushstring(L, err ? err : "");
+  lua_setfield(L, -2, "stderr");
+}
+
+
+/*
+** builtin_cd(cmd, [dir])
+** Change working directory. No arg → $HOME, "-" → $OLDPWD.
+** Updates $PWD and $OLDPWD.
+*/
+static int builtin_cd (lua_State *L) {
+  const char *dir;
+  char oldpwd[4096];
+  char newpwd[4096];
+
+  if (lua_gettop(L) < 2 || lua_isnil(L, 2)) {
+    /* no dir argument: use $HOME */
+    dir = getenv("HOME");
+    if (dir == NULL) {
+      push_builtin_result(L, 1, NULL, "cd: HOME not set");
+      return 1;
+    }
+  }
+  else {
+    dir = luaL_checkstring(L, 2);
+    if (strcmp(dir, "-") == 0) {
+      dir = getenv("OLDPWD");
+      if (dir == NULL) {
+        push_builtin_result(L, 1, NULL, "cd: OLDPWD not set");
+        return 1;
+      }
+    }
+  }
+
+  /* save current directory */
+  if (getcwd(oldpwd, sizeof(oldpwd)) == NULL)
+    oldpwd[0] = '\0';
+
+  if (chdir(dir) != 0) {
+    char errbuf[4200];
+    snprintf(errbuf, sizeof(errbuf), "cd: %s: %s", dir, strerror(errno));
+    push_builtin_result(L, 1, NULL, errbuf);
+    return 1;
+  }
+
+  /* get resolved path */
+  if (getcwd(newpwd, sizeof(newpwd)) == NULL)
+    newpwd[0] = '\0';
+
+  setenv("OLDPWD", oldpwd, 1);
+  setenv("PWD", newpwd, 1);
+
+  push_builtin_result(L, 0, NULL, NULL);
+  return 1;
+}
+
+
+/*
+** builtin_exec(cmd, program, [args...])
+** Replace the shell process via execvp(2).
+*/
+static int builtin_exec (lua_State *L) {
+  int nargs = lua_gettop(L);
+  int i;
+  char **argv;
+  char errbuf[4200];
+
+  if (nargs < 2) {
+    push_builtin_result(L, 1, NULL, "exec: command required");
+    return 1;
+  }
+
+  /* build argv from args 2..nargs (skip "exec" at arg 1) */
+  argv = (char **)malloc((size_t)(nargs) * sizeof(char *));
+  if (argv == NULL)
+    return luaL_error(L, "out of memory");
+
+  for (i = 2; i <= nargs; i++)
+    argv[i - 2] = (char *)luaL_checkstring(L, i);
+  argv[nargs - 1] = NULL;
+
+  execvp(argv[0], argv);
+
+  /* execvp only returns on failure */
+  snprintf(errbuf, sizeof(errbuf), "exec: %s: %s", argv[0], strerror(errno));
+  free(argv);
+  push_builtin_result(L, 1, NULL, errbuf);
+  return 1;
+}
+
+
+/*
+** builtin_umask(cmd, [mode])
+** No arg: query and print current umask. With arg: set umask from octal string.
+*/
+static int builtin_umask (lua_State *L) {
+  if (lua_gettop(L) < 2 || lua_isnil(L, 2)) {
+    /* query mode */
+    mode_t old = umask(0);
+    char buf[16];
+    umask(old);  /* restore */
+    snprintf(buf, sizeof(buf), "%04o\n", (unsigned)old);
+    push_builtin_result(L, 0, buf, NULL);
+    return 1;
+  }
+  else {
+    const char *modestr = luaL_checkstring(L, 2);
+    char *endp;
+    unsigned long val;
+    errno = 0;
+    val = strtoul(modestr, &endp, 8);
+    if (errno != 0 || *endp != '\0' || endp == modestr || val > 0777) {
+      push_builtin_result(L, 1, NULL, "umask: invalid mode");
+      return 1;
+    }
+    umask((mode_t)val);
+    push_builtin_result(L, 0, NULL, NULL);
+    return 1;
+  }
+}
+
+
+void luaopen_builtins (lua_State *L) {
+  lua_createtable(L, 0, 3);
+  lua_pushcfunction(L, builtin_cd);
+  lua_setfield(L, -2, "cd");
+  lua_pushcfunction(L, builtin_exec);
+  lua_setfield(L, -2, "exec");
+  lua_pushcfunction(L, builtin_umask);
+  lua_setfield(L, -2, "umask");
+  lua_setglobal(L, "__builtins");
+}