[PATCH] Feature: s6-style readiness notification with flag

If a file descriptor is specified by the -n flag then write a
newline to it when initialisation is complete.

https://skarnet.org/software/s6/notifywhenup.html
---
 src/main.c | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/src/main.c b/src/main.c
index dd7eb7c..4b5db01 100644
--- a/src/main.c
+++ b/src/main.c
@@ -106,6 +106,7 @@
 
 static bool background;
 static char *pidfile = NULL;
+static int notify_fd = 0;
 
 static void pidfile_update(ice_config_t *config, int always_try);
 
@@ -129,6 +130,7 @@ static void _print_usage(void)
     printf("\t-c <file>       Specify configuration file\n");
     printf("\t-v or --version Display version info\n");
     printf("\t-b              Run icecast in the background\n");
+    printf("\t-n FD           Write a newline to a file description when ready\n");
     printf("\n");
 }
 
@@ -329,6 +331,15 @@ static bool _parse_config_opts(int argc, char **argv, char *filename, size_t siz
             } else {
                 return false;
             }
+        } else if (strcmp(opt, "-n") == 0) {
+            if ((i + 1) < argc) {
+              if (sscanf(argv[++i], "%d", &notify_fd) != 1) {
+                fprintf(stderr, "FATAL: Invalid notify descriptor: %s\n", opt);
+                return false;
+              }
+            } else {
+                return false;
+            }
         } else {
             fprintf(stderr, "FATAL: Invalid option: %s\n", opt);
             return false;
@@ -794,6 +805,14 @@ int main(int argc, char **argv)
     event_stream_initialise();
 
     event_emit_global("icecast-start");
+    if (notify_fd) {
+      /* Notify a service supervisor that we are ready */
+      if (write(notify_fd, "\n", 1) != 1) {
+        ICECAST_LOG_WARN("Failed to write ready notification to file descriptor %d.", notify_fd);
+      } else {
+        close(notify_fd);
+      };
+    }
     _server_proc();
     event_emit_global("icecast-stop");
 
-- 
2.52.0

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