cfg: Fix implementation of timeout in attempting to lock the config file.

Without removing SA_RESTART from the SIGALRM handler, the fcntl call will
never return, even after the signal handler is invoked and returns.

We haven't seen a problem in practice, at least not recently, but that's
probably just luck combined with not holding the configuration file lock
for very long.

lib/cfg.c

@@ -320,12 +320,19 @@ static int
 try_lock(int fd, bool block)
 {
     struct flock l;
+    int error;
+
     memset(&l, 0, sizeof l);
     l.l_type = F_WRLCK;
     l.l_whence = SEEK_SET;
     l.l_start = 0;
     l.l_len = 0;
-    return fcntl(fd, block ? F_SETLKW : F_SETLK, &l) == -1 ? errno : 0;
+
+    time_disable_restart();
+    error = fcntl(fd, block ? F_SETLKW : F_SETLK, &l) == -1 ? errno : 0;
+    time_enable_restart();
+    
+    return error;
 }
 
 /* Locks the configuration file against modification by other processes and

lib/timeval.c

@@ -44,6 +44,7 @@ static struct timeval now;
 static time_t deadline = TIME_MIN;
 
 static void setup_timer(void);
+static void setup_signal(int flags);
 static void sigalrm_handler(int);
 static void refresh_if_ticked(void);
 static time_t time_add(time_t, time_t);
@@ -56,7 +57,6 @@ static void log_poll_interval(long long int last_wakeup,
 void
 time_init(void)
 {
-    struct sigaction sa;
     if (inited) {
         return;
     }
@@ -67,17 +67,49 @@ time_init(void)
     gettimeofday(&now, NULL);
     tick = false;
 
-    /* Set up signal handler. */
+    setup_signal(SA_RESTART);
+    setup_timer();
+}
+
+static void
+setup_signal(int flags)
+{
+    struct sigaction sa;
+
     memset(&sa, 0, sizeof sa);
     sa.sa_handler = sigalrm_handler;
     sigemptyset(&sa.sa_mask);
-    sa.sa_flags = SA_RESTART;
+    sa.sa_flags = flags;
     if (sigaction(SIGALRM, &sa, NULL)) {
         ovs_fatal(errno, "sigaction(SIGALRM) failed");
     }
+}
 
-    /* Set up periodic signal. */
-    setup_timer();
+/* Remove SA_RESTART from the flags for SIGALRM, so that any system call that
+ * is interrupted by the periodic timer interrupt will return EINTR instead of
+ * continuing after the signal handler returns.
+ *
+ * time_disable_restart() and time_enable_restart() may be usefully wrapped
+ * around function calls that might otherwise block forever unless interrupted
+ * by a signal, e.g.:
+ *
+ *   time_disable_restart();
+ *   fcntl(fd, F_SETLKW, &lock);
+ *   time_enable_restart();
+ */
+void
+time_disable_restart(void)
+{
+    setup_signal(0);
+}
+
+/* Add SA_RESTART to the flags for SIGALRM, so that any system call that
+ * is interrupted by the periodic timer interrupt will continue after the
+ * signal handler returns instead of returning EINTR. */
+void
+time_enable_restart(void)
+{
+    setup_signal(SA_RESTART);
 }
 
 static void

lib/timeval.h

@@ -41,6 +41,8 @@ BUILD_ASSERT_DECL(TYPE_IS_SIGNED(time_t));
 #define TIME_UPDATE_INTERVAL 100
 
 void time_init(void);
+void time_disable_restart(void);
+void time_enable_restart(void);
 void time_postfork(void);
 void time_refresh(void);
 time_t time_now(void);