execute: avoid logging to closed fds

Several functions called from the "sd(EXEC)" process try to log messages
when all the file descriptors are already closed, including the logging
ones. The logging functions do not expect their fds to be closed and
they hit an assertion failure. The failure wants to be logged too,
so there is an infinite recursion, ended by a SIGSEGV.

When we close all fds, we must let log.c know about it.

src/execute.c

@@ -1016,6 +1016,7 @@ int exec_spawn(ExecCommand *command,
                 /* Close sockets very early to make sure we don't
                  * block init reexecution because it cannot bind its
                  * sockets */
+                log_forget_fds();
                 if (close_all_fds(socket_fd >= 0 ? &socket_fd : fds,
                                   socket_fd >= 0 ? 1 : n_fds) < 0) {
                         r = EXIT_FDS;

src/log.c

@@ -237,6 +237,10 @@ void log_close(void) {
         log_close_syslog();
 }
 
+void log_forget_fds(void) {
+        console_fd = kmsg_fd = syslog_fd = -1;
+}
+
 void log_set_max_level(int level) {
         assert((level & LOG_PRIMASK) == level);
 

src/log.h

@@ -57,6 +57,7 @@ int log_get_max_level(void);
 
 int log_open(void);
 void log_close(void);
+void log_forget_fds(void);
 
 void log_close_syslog(void);
 void log_close_kmsg(void);