flux-config: add get subcommand

Problem: flux_get_conf() only works in the broker or its loadable
modules; there is no way to access the broker's config from other
locations, for example from rc files.

Add flux-config get subcommand.
Run without arguments, the entire config object is dumped
Run wtih a 'path' argument, a subset of the object, or an individual key
may be dumped.

Output is in compact JSON form.

Fixes flux-framework#4161

src/cmd/builtin/config.c

@@ -8,8 +8,16 @@
  * SPDX-License-Identifier: LGPL-3.0
 \************************************************************/
 
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
+#include <jansson.h>
+
+#include "src/common/libutil/jpath.h"
+
 #include "builtin.h"
 
+
 static int internal_config_reload (optparse_t *p, int ac, char *av[])
 {
     flux_t *h;
@@ -30,6 +38,43 @@ static int internal_config_reload (optparse_t *p, int ac, char *av[])
     return (0);
 }
 
+static int config_get (optparse_t *p, int ac, char *av[])
+{
+    int optindex = optparse_option_index (p);
+    flux_t *h;
+    flux_future_t *f = NULL;
+    json_t *o;
+    const char *path = NULL;
+    char *val;
+
+    if (optindex < ac)
+        path = av[optindex++];
+    if (ac - optindex > 0) {
+        optparse_print_usage (p);
+        exit (1);
+    }
+    if (!(h = builtin_get_flux_handle (p)))
+        log_err_exit ("flux_open");
+    if (!(f = flux_rpc (h, "config.get", NULL, FLUX_NODEID_ANY, 0))
+        || flux_rpc_get_unpack (f, "o", &o) < 0)
+        log_msg_exit ("Error fetching config object: %s",
+                      future_strerror (f, errno));
+    if (path) {
+        if (!(o = jpath_get (o, path))) {
+            if (errno == ENOENT)
+                log_msg_exit ("Unknown config key '%s'", path);
+            log_err_exit ("%s", path);
+        }
+    }
+    if (!(val = json_dumps (o, JSON_COMPACT | JSON_ENCODE_ANY)))
+        log_msg_exit ("error encoding json object");
+    printf ("%s\n", val);
+    free (val);
+    flux_future_destroy (f);
+    flux_close (h);
+    return (0);
+}
+
 int cmd_config (optparse_t *p, int ac, char *av[])
 {
     log_init ("flux-config");
@@ -42,11 +87,18 @@ int cmd_config (optparse_t *p, int ac, char *av[])
 static struct optparse_subcommand config_subcmds[] = {
     { "reload",
       "[OPTIONS]",
-      "Reload configuration from files",
+      "Reload broker configuration from files",
       internal_config_reload,
       0,
       NULL,
     },
+    { "get",
+      "[OPTIONS]",
+      "Query broker configuration",
+      config_get,
+      0,
+      NULL,
+    },
     OPTPARSE_SUBCMD_END
 };