commonio: Use do_lock_file again

This avoids regressions introduced with do_fcntl_lock.

Signed-off-by: Samanta Navarro <ferivoz@riseup.net>

lib/commonio.c

@@ -34,6 +34,8 @@
 
 /* local function prototypes */
 static int lrename (const char *, const char *);
+static int check_link_count (const char *file, bool log);
+static int do_lock_file (const char *file, const char *lock, bool log);
 static /*@null@*/ /*@dependent@*/FILE *fopen_set_perms (
 	const char *name,
 	const char *mode,
@@ -91,6 +93,150 @@ int lrename (const char *old, const char *new)
 	return res;
 }
 
+static int check_link_count (const char *file, bool log)
+{
+	struct stat sb;
+
+	if (stat (file, &sb) != 0) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: %s file stat error: %s\n",
+			                shadow_progname, file, strerror (errno));
+		}
+		return 0;
+	}
+
+	if (sb.st_nlink != 2) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: %s: lock file already used (nlink: %u)\n",
+			                shadow_progname, file, sb.st_nlink);
+		}
+		return 0;
+	}
+
+	return 1;
+}
+
+
+static int do_lock_file (const char *file, const char *lock, bool log)
+{
+	int fd;
+	pid_t pid;
+	ssize_t len;
+	int retval;
+	char buf[32];
+
+	fd = open (file, O_CREAT | O_TRUNC | O_WRONLY, 0600);
+	if (-1 == fd) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: %s: %s\n",
+			                shadow_progname, file, strerror (errno));
+		}
+		return 0;
+	}
+
+	pid = getpid ();
+	snprintf (buf, sizeof buf, "%lu", (unsigned long) pid);
+	len = (ssize_t) strlen (buf) + 1;
+	if (write (fd, buf, (size_t) len) != len) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: %s file write error: %s\n",
+			                shadow_progname, file, strerror (errno));
+		}
+		(void) close (fd);
+		unlink (file);
+		return 0;
+	}
+	if (fdatasync (fd) == -1) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: %s file sync error: %s\n",
+			                shadow_progname, file, strerror (errno));
+		}
+		(void) close (fd);
+		unlink (file);
+		return 0;
+	}
+	close (fd);
+
+	if (link (file, lock) == 0) {
+		retval = check_link_count (file, log);
+		unlink (file);
+		return retval;
+	}
+
+	fd = open (lock, O_RDWR);
+	if (-1 == fd) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: %s: %s\n",
+			                shadow_progname, lock, strerror (errno));
+		}
+		unlink (file);
+		errno = EINVAL;
+		return 0;
+	}
+	len = read (fd, buf, sizeof (buf) - 1);
+	close (fd);
+	if (len <= 0) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: existing lock file %s without a PID\n",
+			                shadow_progname, lock);
+		}
+		unlink (file);
+		errno = EINVAL;
+		return 0;
+	}
+	buf[len] = '\0';
+	if (get_pid (buf, &pid) == 0) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: existing lock file %s with an invalid PID '%s'\n",
+			                shadow_progname, lock, buf);
+		}
+		unlink (file);
+		errno = EINVAL;
+		return 0;
+	}
+	if (kill (pid, 0) == 0) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: lock %s already used by PID %lu\n",
+			                shadow_progname, lock, (unsigned long) pid);
+		}
+		unlink (file);
+		errno = EEXIST;
+		return 0;
+	}
+	if (unlink (lock) != 0) {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: cannot get lock %s: %s\n",
+			                shadow_progname, lock, strerror (errno));
+		}
+		unlink (file);
+		return 0;
+	}
+
+	retval = 0;
+	if (link (file, lock) == 0) {
+		retval = check_link_count (file, log);
+	} else {
+		if (log) {
+			(void) fprintf (shadow_logfd,
+			                "%s: cannot get lock %s: %s\n",
+			                shadow_progname, lock, strerror (errno));
+		}
+	}
+
+	unlink (file);
+	return retval;
+}
+
 
 static /*@null@*/ /*@dependent@*/FILE *fopen_set_perms (
 	const char *name,
@@ -205,43 +351,38 @@ bool commonio_present (const struct commonio_db *db)
 }
 
 
-int do_fcntl_lock (const char *file, bool log, short type)
-{
-	int fd;
-	struct flock lck = {
-		.l_type = type,
-		.l_whence = SEEK_SET,
-		.l_start = 0,
-		.l_len = 0,
-	};
-
-	fd = open (file, O_WRONLY | O_CREAT, 0600);
-	if (-1 == fd) {
-		if (log) {
-			(void) fprintf (shadow_logfd, "%s: %s: %s\n",
-			                shadow_progname, file, strerror (errno));
-		}
-		return 0;
-	}
-
-	fcntl (fd, F_SETLKW, &lck);
-	return(1);
-}
-
 int commonio_lock_nowait (struct commonio_db *db, bool log)
 {
+	char* file = NULL;
+	char* lock = NULL;
+	size_t lock_file_len;
+	size_t file_len;
 	int err = 0;
 
 	if (db->locked) {
 		return 1;
 	}
-
-	if (do_fcntl_lock (db->filename, log, F_WRLCK | F_RDLCK) != 0) {
+	file_len = strlen(db->filename) + 11;/* %lu max size */
+	lock_file_len = strlen(db->filename) + 6; /* sizeof ".lock" */
+	file = MALLOCARRAY(file_len, char);
+	if (file == NULL) {
+		goto cleanup_ENOMEM;
+	}
+	lock = MALLOCARRAY(lock_file_len, char);
+	if (lock == NULL) {
+		goto cleanup_ENOMEM;
+	}
+	snprintf (file, file_len, "%s.%lu",
+	          db->filename, (unsigned long) getpid ());
+	snprintf (lock, lock_file_len, "%s.lock", db->filename);
+	if (do_lock_file (file, lock, log) != 0) {
 		db->locked = true;
 		lock_count++;
 		err = 1;
 	}
-
+cleanup_ENOMEM:
+	free(file);
+	free(lock);
 	return err;
 }
 
@@ -332,6 +473,8 @@ static void dec_lock_count (void)
 
 int commonio_unlock (struct commonio_db *db)
 {
+	char lock[1024];
+
 	if (db->isopen) {
 		db->readonly = true;
 		if (commonio_close (db) == 0) {
@@ -342,9 +485,13 @@ int commonio_unlock (struct commonio_db *db)
 		}
 	}
 	if (db->locked) {
+		/*
+		 * Unlock in reverse order: remove the lock file,
+		 * then call ulckpwdf() (if used) on last unlock.
+		 */
 		db->locked = false;
-
-		do_fcntl_lock (db->filename, false, F_UNLCK);
+		snprintf (lock, sizeof lock, "%s.lock", db->filename);
+		unlink (lock);
 		dec_lock_count ();
 		return 1;
 	}