Improve handling of nagbar processes

Other changes in nagbar_exited:
- Remove ERROR from ELOG as it shows up as "ERROR: ERROR:"
- Make sure to reset the PID even when the process did not exit
normally.
- Use ELOG when exitcode != 0
- Remove the not found error: if nagbar is not in $PATH, exec_i3_utility
prints an error as well.

For the record, I also implemented a more complicated approach with a
new watcher data structure:
orestisfl/i3@bd3aaf3
While writing the commit message, it occurred to me to compare
watcher->pid with *watcher->data, which fixes the problems mentioned in
that patch.

Fixes #4104

RELEASE-NOTES-next

@@ -42,3 +42,4 @@ working. Please reach out to us in that case!
   • build: correctly provide auxiliary functions when needed
   • build: fix issues with parallel build
   • set _NET_DESKTOP_VIEWPORT after randr changes
+  • fix a bug with i3-nagbar not starting after it has already started once

src/commands.c

@@ -1615,8 +1615,16 @@ void cmd_exit(I3_CMD) {
  */
 void cmd_reload(I3_CMD) {
     LOG("reloading\n");
+
     kill_nagbar(config_error_nagbar_pid, false);
     kill_nagbar(command_error_nagbar_pid, false);
+    /* start_nagbar() will refuse to start a new process if the passed pid is
+     * set. This will happen when our child watcher is triggered by libev when
+     * the loop re-starts. However, config errors might be detected before
+     * that since we will read the config right now with load_configuration.
+     * See #4104. */
+    config_error_nagbar_pid = command_error_nagbar_pid = -1;
+
     load_configuration(NULL, C_RELOAD);
     x_set_i3_atoms();
     /* Send an IPC event just in case the ws names have changed */

src/util.c

@@ -336,18 +336,20 @@ char *pango_escape_markup(char *input) {
 static void nagbar_exited(EV_P_ ev_child *watcher, int revents) {
     ev_child_stop(EV_A_ watcher);
 
+    int exitcode = WEXITSTATUS(watcher->rstatus);
     if (!WIFEXITED(watcher->rstatus)) {
-        ELOG("ERROR: i3-nagbar did not exit normally.\n");
-        return;
+        ELOG("i3-nagbar (%d) did not exit normally. This is not an error if the config was reloaded while a nagbar was active.\n", watcher->pid);
+    } else if (exitcode != 0) {
+        ELOG("i3-nagbar (%d) process exited with status %d\n", watcher->pid, exitcode);
+    } else {
+        DLOG("i3-nagbar (%d) process exited with status %d\n", watcher->pid, exitcode);
     }
 
-    int exitcode = WEXITSTATUS(watcher->rstatus);
-    DLOG("i3-nagbar process exited with status %d\n", exitcode);
-    if (exitcode == 2) {
-        ELOG("ERROR: i3-nagbar could not be found. Is it correctly installed on your system?\n");
+    pid_t *nagbar_pid = watcher->data;
+    if (*nagbar_pid == watcher->pid) {
+        /* Only reset if the watched nagbar is the active nagbar */
+        *nagbar_pid = -1;
     }
-
-    *((pid_t *)watcher->data) = -1;
 }
 
 /*