Add -W keepopenfds option. (#12211)

When this option is specified the agent will not close any inherited open
FDs. This option is useful when starting the agent under profiling tools that
maintain their own file descriptors, eg. to save profiling data.

daemon/main.c

@@ -690,6 +690,7 @@ int main(int argc, char **argv) {
     int i;
     int config_loaded = 0;
     int dont_fork = 0;
+    bool close_open_fds = true;
     size_t default_stacksize;
     char *user = NULL;
 
@@ -1038,7 +1039,13 @@ int main(int argc, char **argv) {
                             print_build_info_json();
                             return 0;
                         }
-                        else {
+                        else if(strcmp(optarg, "keepopenfds") == 0) {
+                            // Internal dev option to skip closing inherited
+                            // open FDs. Useful, when we want to run the agent
+                            // under profiling tools that open/maintain their
+                            // own FDs.
+                            close_open_fds = false;
+                        } else {
                             fprintf(stderr, "Unknown -W parameter '%s'\n", optarg);
                             return help(1);
                         }
@@ -1053,12 +1060,12 @@ int main(int argc, char **argv) {
     }
 
 #ifdef _SC_OPEN_MAX
-    // close all open file descriptors, except the standard ones
-    // the caller may have left open files (lxc-attach has this issue)
-    {
-        int fd;
-        for(fd = (int) (sysconf(_SC_OPEN_MAX) - 1); fd > 2; fd--)
-            if(fd_is_valid(fd)) close(fd);
+    if (close_open_fds == true) {
+        // close all open file descriptors, except the standard ones
+        // the caller may have left open files (lxc-attach has this issue)
+        for(int fd = (int) (sysconf(_SC_OPEN_MAX) - 1); fd > 2; fd--)
+            if(fd_is_valid(fd))
+                close(fd);
     }
 #endif