libjsc: add jsc_query_rdesc_efficiently()

Problem: JSON encode/decode of JSC rdesc objects proved
to be a significant bottleneck affecting scheduling
performance for high throughput workloads.

Add jsc_query_rdesc_efficiently() to allow nnodes, ncores,
ntasks, and walltime to be queries directly.

This was solved in a different way in current flux-sched
master, but this is needed as a temporary workaround until
older forks of flux-sched used with the Splash application
catch up with master per @trws.

Fixes #1455

src/common/libjsc/jstatctl.c

@@ -622,6 +622,29 @@ static int query_rdesc (flux_t *h, int64_t j, json_object **jcb)
     return 0;
 }
 
+int jsc_query_rdesc_efficiently (flux_t *h, int64_t jobid,
+                                 int64_t *nnodes, int64_t *ntasks,
+                                 int64_t *ncores, int64_t *walltime)
+{
+    if (nnodes) {
+        if (extract_raw_nnodes (h, jobid, nnodes) < 0)
+            return -1;
+    }
+    if (ntasks) {
+        if (extract_raw_ntasks (h, jobid, ntasks) < 0)
+            return -1;
+    }
+    if (ncores) {
+        if (extract_raw_ncores (h, jobid, ncores) < 0)
+            return -1;
+    }
+    if (walltime) {
+        if (extract_raw_walltime (h, jobid, walltime) < 0)
+            return -1;
+    }
+    return 0;
+}
+
 static int query_rdl (flux_t *h, int64_t j, json_object **jcb)
 {
     char *rdlstr = NULL;

src/common/libjsc/jstatctl.h

@@ -122,6 +122,15 @@ int jsc_update_jcb (flux_t *h, int64_t jobid, const char *key, const char *jcb);
 const char *jsc_job_num2state (job_state_t s);
 int jsc_job_state2num (const char *s);
 
+/**
+ * Accessor for nnodes, ntasks, ncores, walltime that circumvents
+ * JSON encode/decode.  N.B. this is a workaround for performance issu
+ * encountered when scheduling high throughput workloads.
+ */
+int jsc_query_rdesc_efficiently (flux_t *h, int64_t jobid,
+                                 int64_t *nnodes, int64_t *ntasks,
+                                 int64_t *ncores, int64_t *walltime);
+
 #ifdef __cplusplus
 }
 #endif