libflux: make flux_msg_fprint() output clearer

Problem: With zmsg_t being removed from the internals of the
flux_msg_t data structure (commit XXX), flux_msg_fprint() is no
longer beholden to the output format from zframe_fprint().  A more
clear format for debugging can be output.

Solution: Update flux_msg_fprint() to output all fields in the
message using clear key=value output and prefixes indicating the
"section" they belong to in the message.

src/common/libflux/message.c

@@ -1525,8 +1525,6 @@ void flux_msg_fprint (FILE *f, const flux_msg_t *msg)
 {
     int hops;
     const char *prefix;
-    uint8_t proto[PROTO_SIZE];
-    int i;
 
     fprintf (f, "--------------------------------------\n");
     if (!msg) {
@@ -1538,36 +1536,67 @@ void flux_msg_fprint (FILE *f, const flux_msg_t *msg)
      */
     hops = flux_msg_get_route_count (msg); /* -1 if no route stack */
     if (hops >= 0) {
-        int len = flux_msg_get_route_size (msg);
         char *rte = flux_msg_get_route_string (msg);
         assert (rte != NULL);
-        fprintf (f, "%s[%3.3d] |%s|\n", prefix, len, rte);
+        fprintf (f, "%s[routes] |%s|\n", prefix, rte);
         free (rte);
     };
     /* Topic (keepalive has none)
      */
     if (msg->topic)
-        fprintf (f, "%s[%3.3zu] %s\n", prefix, strlen (msg->topic), msg->topic);
+        fprintf (f, "%s[topic] %s\n", prefix, msg->topic);
     /* Payload
      */
     if (flux_msg_has_payload (msg)) {
         const char *s;
         const void *buf;
         int size;
         if (flux_msg_get_string (msg, &s) == 0)
-            fprintf (f, "%s[%3.3zu] %s\n", prefix, strlen (s), s);
+            fprintf (f, "%s[payload] size=%zu %s\n", prefix, strlen (s), s);
         else if (flux_msg_get_payload (msg, &buf, &size) == 0)
-            fprintf (f, "%s[%3.3d] ...\n", prefix, size);
+            fprintf (f, "%s[payload] size=%d ...\n", prefix, size);
         else
             fprintf (f, "malformed payload\n");
     }
-    /* Proto block
+    /* Proto info
      */
-    msg_proto_setup (msg, proto, PROTO_SIZE);
-    fprintf (f, "%s[%03d] ", prefix, PROTO_SIZE);
-    for (i = 0; i < PROTO_SIZE; i++)
-        fprintf (f, "%02X", proto[i]);
-    fprintf (f, "\n");
+    fprintf (f, "%s[proto header] type=%s flags=0x%X userid=%u rolemask=0x%X\n",
+             prefix,
+             flux_msg_typestr (msg->type),
+             msg->flags,
+             msg->userid,
+             msg->rolemask);
+    switch (msg->type) {
+        case FLUX_MSGTYPE_REQUEST:
+            fprintf (f, "%s[proto aux] nodeid=%u matchtag=%u\n",
+                     prefix,
+                     msg->nodeid,
+                     msg->matchtag);
+            break;
+        case FLUX_MSGTYPE_RESPONSE:
+            fprintf (f, "%s[proto aux] errnum=%u matchtag=%u\n",
+                     prefix,
+                     msg->errnum,
+                     msg->matchtag);
+            break;
+        case FLUX_MSGTYPE_EVENT:
+            fprintf (f, "%s[proto aux] sequence=%u\n",
+                     prefix,
+                     msg->sequence);
+            break;
+        case FLUX_MSGTYPE_KEEPALIVE:
+            fprintf (f, "%s[proto aux] errnum=%u status=%u\n",
+                     prefix,
+                     msg->errnum,
+                     msg->status);
+            break;
+        default:
+            fprintf (f, "%s[proto aux] aux1=0x%X aux2=0x%X\n",
+                     prefix,
+                     msg->aux1,
+                     msg->aux2);
+            break;
+    }
 }
 
 static zmsg_t *msg_to_zmsg (const flux_msg_t *msg)

src/common/libflux/test/message.c

@@ -921,7 +921,7 @@ void check_print (void)
 {
     flux_msg_t *msg;
     char buf[] = "xxxxxxxx";
-    FILE *f = fopen ("/dev/null", "w");
+    FILE *f = stderr;
     if (!f)
         BAIL_OUT ("cannot open /dev/null for writing");