[core] truncate pidfile on exit (fixes #2695)

If the server has changed its uid or is running in a chroot
it may be unable to remove the pid file when it exits.

However, if it holds on to an open handle to the pid file
that has write permission, it will be able to truncate the
pid file to 0 bytes in length.

Most monitoring software recognizes a 0-length pid file
as indicating there is no process running.

Therefore always attempt to truncate the pid file before
trying to remove it so that it's not left containing the
pid of a process that is no longer running.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

git-svn-id: svn://svn.lighttpd.net/lighttpd/branches/lighttpd-1.4.x@3112 152afb58-edef-0310-8abb-c4023f1b3aa9

NEWS

@@ -40,6 +40,7 @@ NEWS
   * [core] log remote address on request timeouts (fixes #652)
   * [autobuild] use AC_CANONICAL_HOST instead of AC_CANONICAL_TARGET (fixes #1866)
   * [core] fix request_start in keep-alive requests to mark time when received first byte (fixes #2412)
+  * [core] truncate pidfile on exit (fixes #2695)
 
 - 1.4.39 - 2016-01-02
   * [core] fix memset_s call (fixes #2698)

src/server.c

@@ -355,6 +355,34 @@ static void server_free(server *srv) {
 	free(srv);
 }
 
+static void remove_pid_file(server *srv, int *pid_fd) {
+	if (!buffer_string_is_empty(srv->srvconf.pid_file) && 0 <= *pid_fd) {
+		if (0 != ftruncate(*pid_fd, 0)) {
+			log_error_write(srv, __FILE__, __LINE__, "sbds",
+					"ftruncate failed for:",
+					srv->srvconf.pid_file,
+					errno,
+					strerror(errno));
+		}
+	}
+	if (0 <= *pid_fd) {
+		close(*pid_fd);
+		*pid_fd = -1;
+	}
+	if (!buffer_string_is_empty(srv->srvconf.pid_file) &&
+	    buffer_string_is_empty(srv->srvconf.changeroot)) {
+		if (0 != unlink(srv->srvconf.pid_file->ptr)) {
+			if (errno != EACCES && errno != EPERM) {
+				log_error_write(srv, __FILE__, __LINE__, "sbds",
+						"unlink failed for:",
+						srv->srvconf.pid_file,
+						errno,
+						strerror(errno));
+			}
+		}
+	}
+}
+
 static void show_version (void) {
 #ifdef USE_OPENSSL
 # define TEXT_SSL " (ssl)"
@@ -717,6 +745,7 @@ int main (int argc, char **argv) {
 				return -1;
 			}
 		}
+		fd_close_on_exec(pid_fd);
 	}
 
 	if (srv->event_handler == FDEVENT_HANDLER_SELECT) {
@@ -1014,8 +1043,6 @@ int main (int argc, char **argv) {
 			close(pid_fd);
 			return -1;
 		}
-		close(pid_fd);
-		pid_fd = -1;
 	}
 
 	/* Close stderr ASAP in the child process to make sure that nothing
@@ -1148,6 +1175,7 @@ int main (int argc, char **argv) {
 				kill(0, SIGTERM);
 			}
 
+			remove_pid_file(srv, &pid_fd);
 			log_error_close(srv);
 			network_close(srv);
 			connections_free(srv);
@@ -1155,6 +1183,15 @@ int main (int argc, char **argv) {
 			server_free(srv);
 			return 0;
 		}
+
+		/**
+		 * make sure workers do not muck with pid-file
+		 */
+		if (0 <= pid_fd) {
+			close(pid_fd);
+			pid_fd = -1;
+		}
+		buffer_reset(srv->srvconf.pid_file);
 	}
 #endif
 
@@ -1441,23 +1478,11 @@ int main (int argc, char **argv) {
 						srv_socket->fd = -1;
 
 						/* network_close() will cleanup after us */
-
-						if (!buffer_string_is_empty(srv->srvconf.pid_file) &&
-						    buffer_string_is_empty(srv->srvconf.changeroot)) {
-							if (0 != unlink(srv->srvconf.pid_file->ptr)) {
-								if (errno != EACCES && errno != EPERM) {
-									log_error_write(srv, __FILE__, __LINE__, "sbds",
-											"unlink failed for:",
-											srv->srvconf.pid_file,
-											errno,
-											strerror(errno));
-								}
-							}
-						}
 					}
 				}
 
 				if (graceful_shutdown) {
+					remove_pid_file(srv, &pid_fd);
 					log_error_write(srv, __FILE__, __LINE__, "s", "[note] graceful shutdown started");
 				} else if (srv->conns->used >= srv->max_conns) {
 					log_error_write(srv, __FILE__, __LINE__, "s", "[note] sockets disabled, connection limit reached");
@@ -1559,18 +1584,8 @@ int main (int argc, char **argv) {
 		srv->joblist->used = 0;
 	}
 
-	if (!buffer_string_is_empty(srv->srvconf.pid_file) &&
-	    buffer_string_is_empty(srv->srvconf.changeroot) &&
-	    0 == graceful_shutdown) {
-		if (0 != unlink(srv->srvconf.pid_file->ptr)) {
-			if (errno != EACCES && errno != EPERM) {
-				log_error_write(srv, __FILE__, __LINE__, "sbds",
-						"unlink failed for:",
-						srv->srvconf.pid_file,
-						errno,
-						strerror(errno));
-			}
-		}
+	if (0 == graceful_shutdown) {
+		remove_pid_file(srv, &pid_fd);
 	}
 
 #ifdef HAVE_SIGACTION