job-list: improve journal error handling

Problem: ENODATA is not treated specially (as per RFC 6) when
the journal RPC gets an error response.

Detect that case, log it, and stop the reactor without error.
Also, don't decode the eventlog initally, as it is not used yet.
Let any (unlikely) protocol errors be handled when the message is
processed.

src/modules/job-list/job_state.c

@@ -1297,6 +1297,10 @@ static int journal_process_events (struct job_state_ctx *jsctx,
                         "id", &id,
                         "events", &events) < 0)
         return -1;
+    if (!json_is_array (events)) {
+        errno = EPROTO;
+        return -1;
+    }
     json_array_foreach (events, index, value) {
         if (journal_process_event (jsctx, id, value) < 0)
             return -1;
@@ -1310,20 +1314,15 @@ static void job_events_journal_continuation (flux_future_t *f, void *arg)
     struct job_state_ctx *jsctx = arg;
     const flux_msg_t *msg;
     flux_jobid_t id;
-    json_t *events;
-
-    if (flux_future_get (f, (const void **)&msg) < 0
-        || flux_msg_unpack (msg,
-                            "{s:I s:o}",
-                            "id", &id,
-                            "events", &events) < 0) {
-        flux_log_error (jsctx->h, "error unpacking journal response");
-        goto error;
-    }
 
-    if (!json_is_array (events)) {
-        flux_log (jsctx->h, LOG_ERR, "%s: events EPROTO", __FUNCTION__);
-        errno = EPROTO;
+    if (flux_rpc_get_unpack (f, "{s:I}", "id", &id) < 0
+        || flux_future_get (f, (const void **)&msg) < 0) {
+        if (errno == ENODATA) {
+            flux_log (jsctx->h, LOG_INFO, "journal: EOF (exiting)");
+            flux_reactor_stop (flux_get_reactor (jsctx->h));
+            return;
+        }
+        flux_log (jsctx->h, LOG_ERR, "journal: %s", future_strerror (f, errno));
         goto error;
     }