init: Support loading awesome with shebang (#!) executables.

This commits re-use the modeline code from the previous commit
to support Lua scripts starting with #!. Previously, it was
non-trivial to add support since most *nix OS wont parse the
command line arguments correctly. Since we now have a state
machine good enough for 95%+ of the use case, it is easy to
support them.

awesome.c

@@ -589,8 +589,11 @@ main(int argc, char **argv)
     /* Text won't be printed correctly otherwise */
     setlocale(LC_CTYPE, "");
 
-    /* check args */
-    char *confpath = options_check_args(argc, argv, &default_init_flags, &searchpath);
+    char *confpath = options_detect_shebang(argc, argv);
+
+    /* if no shebang is detected, check the args. Shebang (#!) args are parsed later */
+    if (!confpath)
+        confpath = options_check_args(argc, argv, &default_init_flags, &searchpath);
 
     /* Get XDG basedir data */
     if(!xdgInitHandle(&xdg))

options.c

@@ -24,6 +24,7 @@
 
 #include <unistd.h>
 #include <stdio.h>
+#include <sys/stat.h>
 #include <getopt.h>
 
 #define KEY_VALUE_BUF_MAX 64
@@ -267,6 +268,56 @@ options_init_config(char *execpath, char *configpath, int *init_flags, string_ar
     return state == MODELINE_STATE_COMPLETE;
 }
 
+
+char *
+options_detect_shebang(int argc, char **argv)
+{
+    /* There is no cross-platform ways to check if it is *really* called by a
+     * shebang. There is a couple Linux specific hacks which work with the
+     * most common C libraries, but they wont work on *BSD.
+     *
+     * On some platforms, the argv is going to be parsed by the OS, in other
+     * they will be concatenated in one big string. There is some ambiguities
+     * caused by that. For example, `awesome -s foo` and and `#!/bin/awesome -s`
+     * are both technically valid if `foo` is a directory in the first and
+     * lua file (without extension) in the second. While `-s` with a file
+     * wont work, it is hard to know by looking at the string.
+     *
+     * The trick to avoid any ambiguity is to just read the file and see if
+     * the args match. `options_init_config` will be called later and the args
+     * will be parsed properly.
+     */
+
+    /* On WSL and some other *nix this isn't true, but it is true often enough */
+    if (argc > 3 || argc == 1)
+        return NULL;
+
+    /* Check if it is executable */
+    struct stat inf;
+    if (stat(argv[argc-1], &inf) || !(inf.st_mode & S_IXUSR))
+        return NULL;
+
+    FILE *fp = fopen(argv[argc-1], "r");
+
+    if (!fp)
+        return NULL;
+
+    char buf[3];
+
+    if (!fgets(buf, 2, fp)) {
+        fclose(fp);
+        return NULL;
+    }
+
+    fclose(fp);
+
+    if (!strcmp(buf, "#!"))
+        return NULL;
+
+    /* Ok, good enough, this is a shebang script, assume it called `awesome` */
+    return a_strdup(argv[argc-1]);
+}
+
 /** Print help and exit(2) with given exit_code.
  * \param exit_code The exit code.
  */

options.h

@@ -34,5 +34,6 @@ typedef enum {
     INIT_FLAG_FORCE_CMD_ARGS = 0x1 << 5,
 } awesome_init_config_t;
 
+char *options_detect_shebang(int argc, char **argv);
 bool options_init_config(char *execpath, char *configpath, int *init_flags, string_array_t *paths);
 char *options_check_args(int argc, char **argv, int *init_flags, string_array_t *paths);