shell: fetch enclosing instance security.owner attribute

Problem: The job shell doesn't capture the owner of the enclosing
instance, so there is no way to determine if the job is running in
single vs multiuser mode.

Capture the local broker security.owner attribute.

Author: grondo

src/shell/internal.h

@@ -26,6 +26,7 @@
 struct flux_shell {
     flux_jobid_t jobid;
     int broker_rank;
+    uid_t broker_owner;
     char hostname [MAXHOSTNAMELEN + 1];
     int protocol_fd[2];
 

src/shell/shell.c

@@ -253,6 +253,25 @@ static int reconnect (flux_t *h, void *arg)
     return 0;
 }
 
+static uid_t get_instance_owner (flux_t *h)
+{
+    const char *s;
+    char *endptr;
+    int id;
+
+    if (!(s = flux_attr_get (h, "security.owner"))) {
+        shell_log_errno ("error fetching security.owner attribute");
+        return (uid_t) 0;
+    }
+    errno = 0;
+    id = strtoul (s, &endptr, 10);
+    if (errno != 0 || *endptr != '\0') {
+        shell_log_error ("error parsing security.owner=%s", s);
+        return (uid_t) 0;
+    }
+    return (uid_t) id;
+}
+
 static void shell_connect_flux (flux_shell_t *shell)
 {
     uint32_t rank;
@@ -274,6 +293,8 @@ static void shell_connect_flux (flux_shell_t *shell)
         shell_log_errno ("error fetching broker rank");
     shell->broker_rank = rank;
 
+    shell->broker_owner = get_instance_owner (shell->h);
+
     if (plugstack_call (shell->plugstack, "shell.connect", NULL) < 0)
         shell_log_errno ("shell.connect");
 }