implement execution timeouts for executors

Previously, it was possible for an executor to hang forever.  To mitigate
this, we now implement process execution timeouts for executors, by looping
with waitpid(..., WNOHANG) and sleeping.

This could be implemented more efficiently with process descriptors, see
pidfd_open(2), but that interface is Linux-specific and is only available
on Linux 5.3 or newer.

cmd/multicall-exec-options.c

@@ -21,10 +21,13 @@
 #include <getopt.h>
 #include "cmd/multicall.h"
 
+#define DEFAULT_TIMEOUT		300
+
 struct lif_execute_opts exec_opts = {
 	.interfaces_file = INTERFACES_FILE,
 	.executor_path = EXECUTOR_PATH,
-	.state_file = STATE_FILE
+	.state_file = STATE_FILE,
+	.timeout = DEFAULT_TIMEOUT,
 };
 
 static void
@@ -74,6 +77,14 @@ set_force(const char *opt_arg)
 	exec_opts.force = true;
 }
 
+static void
+set_timeout(const char *opt_arg)
+{
+	exec_opts.timeout = atoi(opt_arg);
+	if (exec_opts.timeout < 0)
+		exec_opts.timeout = DEFAULT_TIMEOUT;
+}
+
 static struct if_option exec_options[] = {
 	{'f', "force", NULL, "force (de)configuration", false, set_force},
 	{'i', "interfaces", "interfaces FILE", "use FILE for interface definitions", true, set_interfaces_file},
@@ -82,6 +93,7 @@ static struct if_option exec_options[] = {
 	{'v', "verbose", NULL, "show what commands are being run", false, set_verbose},
 	{'E', "executor-path", "executor-path PATH", "use PATH for executor directory", true, set_executor_path},
 	{'S', "state-file", "state-file FILE", "use FILE for state", true, set_state_file},
+	{'T', "timeout", "timeout TIMEOUT", "wait TIMEOUT seconds for executors to complete", true, set_timeout},
 };
 
 struct if_option_group exec_option_group = {

libifupdown/execute.c

@@ -30,6 +30,31 @@
 
 #define SHELL	"/bin/sh"
 
+/* TODO: Add support for Linux process descriptors once it is okay to require
+ * Linux 5.3 or newer.
+ */
+static inline bool
+lif_process_monitor(const char *cmdbuf, pid_t child, int timeout_sec)
+{
+	int status;
+	int ticks = 0;
+
+	while (ticks < timeout_sec)
+	{
+		if (waitpid(child, &status, WNOHANG) == child)
+			return WIFEXITED(status) && WEXITSTATUS(status) == 0;
+
+		sleep(1);
+		ticks++;
+	}
+
+	fprintf(stderr, "execution of '%s': timeout after %d seconds\n", cmdbuf, timeout_sec);
+	kill(child, SIGKILL);
+	waitpid(child, &status, 0);
+
+	return false;
+}
+
 bool
 lif_execute_fmt(const struct lif_execute_opts *opts, char *const envp[], const char *fmt, ...)
 {
@@ -55,10 +80,7 @@ lif_execute_fmt(const struct lif_execute_opts *opts, char *const envp[], const c
 		return false;
 	}
 
-	int status;
+	return lif_process_monitor(cmdbuf, child, opts->timeout);
-	waitpid(child, &status, 0);
-
-	return WIFEXITED(status) && WEXITSTATUS(status) == 0;
 }
 
 bool
@@ -118,11 +140,8 @@ lif_execute_fmt_with_result(const struct lif_execute_opts *opts, char *buf, size
 		return false;
 	}
 
-	int status;
 no_result:
-	waitpid(child, &status, 0);
+	return lif_process_monitor(cmdbuf, child, opts->timeout);
-
-	return WIFEXITED(status) && WEXITSTATUS(status) == 0;
 }
 
 bool

libifupdown/execute.h

@@ -27,6 +27,7 @@ struct lif_execute_opts {
 	const char *executor_path;
 	const char *interfaces_file;
 	const char *state_file;
+	int timeout;
 };
 
 extern bool lif_execute_fmt(const struct lif_execute_opts *opts, char *const envp[], const char *fmt, ...);

libifupdown/lifecycle.c

@@ -94,7 +94,8 @@ query_dependents_from_executors(const struct lif_execute_opts *opts, char *const
 		struct lif_execute_opts exec_opts = {
 			.verbose = opts->verbose,
 			.executor_path = opts->executor_path,
-			.interfaces_file = opts->interfaces_file
+			.interfaces_file = opts->interfaces_file,
+			.timeout = opts->timeout,
 		};
 
 		if (strcmp(entry->key, "use"))