/* Copyright (C) 2010 Nick Bowler
 *
 * License WTFPL2: Do What The Fuck You Want To Public License, version 2.
 * This is free software: you are free to do what the fuck you want to.
 * There is NO WARRANTY, to the extent permitted by law.
 *
 * Putting . in PATH is generally not a good idea, for a variety of reasons.
 * However, some programs expect it to be there, and do not function correctly
 * without it.  This program allows one to invoke specific programs in the
 * current working directory as though . was in the PATH without it actually
 * being there.
 *
 * To work, place a copy of (or symlink to) this program somewhere that is
 * actually in the PATH.  The name of the command determines what program
 * gets invoked.  Basic checks are done to avoid accidental infinite
 * recursion.
 *
 * This is probably not portable to non-GNU/Linux systems.
 */

#define _GNU_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <limits.h>
#include <errno.h>

#include <unistd.h>
#include <fcntl.h>

/* Common shell return codes */
#define SHELL_CMD_NOT_FOUND 127
#define SHELL_CMD_NOT_EXEC  126

/*
 * A wrapper around readlink which dynamically allocates a buffer large
 * enough to contain the entire filename.  Also add a null terminator
 * so that the filename can be used as a C string.
 */
char *readlink_alloc(const char *path)
{
	ssize_t bufsize = 64, rc;
	char *buf = NULL, *tmp;

	while (1) {
		if (bufsize >= SSIZE_MAX/2) {
			fprintf(stderr, "cmdwrap: %s: %s\n",
			                path, strerror(ENAMETOOLONG));
			break;
		}

		tmp = realloc(buf, bufsize * 2);
		if (!tmp) {
			fprintf(stderr, "cmdwrap: %s\n", strerror(errno));
			break;
		}

		bufsize *= 2;
		buf = tmp;

		rc = readlink(path, buf, bufsize);
		if (rc == -1) {
			fprintf(stderr, "cmdwrap: %s: %s\n",
			                path, strerror(errno));
			break;
		} else if (rc < bufsize) {
			buf[rc] = 0;
			return buf;
		}
	}

	free(buf);
	return NULL;
}

int main(int argc, char **argv)
{
	char *curexe, *newexe, fdname[64];
	int fd;

	if (argc < 1) {
		fprintf(stderr, "cmdwrap: command name must be present in the argument list.\n");
		return SHELL_CMD_NOT_FOUND;
	}

	fd = open(argv[0], O_RDONLY);
	if (fd == -1) {
		fprintf(stderr, "cmdwrap: %s: %s\n", argv[0], strerror(errno));
		return SHELL_CMD_NOT_FOUND;
	}

	/* Try to avoid accidental infinite recursion. */
	sprintf(fdname, "/proc/self/fd/%d", fd);
	newexe = readlink_alloc(fdname);
	if (!newexe)
		return SHELL_CMD_NOT_FOUND;

	curexe = readlink_alloc("/proc/self/exe");
	if (!curexe)
		return SHELL_CMD_NOT_FOUND;

	if (strcmp(newexe, curexe) == 0) {
		fprintf(stderr, "cmdwrap: infinite recursion detected\n");
		return SHELL_CMD_NOT_EXEC;
	}

	/*
	 * Some programs expect argv[0] to point to their actual executable
	 * (potentially after a path lookup).  But if we leave argv[0] as it
	 * is, they'll tend to find this program instead of the right one.
	 *
	 * Fix that up, too.
	 */
	argv[0] = newexe;
	free(curexe);

	fexecve(fd, argv, environ);
	fprintf(stderr, "cmdwrap: %s: %s\n", argv[0], strerror(errno));
	return SHELL_CMD_NOT_EXEC;
}