librlist: add rlist_subtract()

Problem: It would be useful to have an rlist diff routine the modifies
the rlist argument, but this code is embedded in rlist_diff(), which
returns a new rlist.

Split rlist_subtract() out of rlist_diff() and make it public. Have
rlist_diff() call rlist_subtract() internally.

Author: grondo

src/common/librlist/rlist.c

@@ -533,17 +533,9 @@ static struct rnode *rlist_detach_rank (struct rlist *rl, uint32_t rank)
     return n;
 }
 
-struct rlist *rlist_diff (const struct rlist *rla, const struct rlist *rlb)
+int rlist_subtract (struct rlist *rl, const struct rlist *rlb)
 {
-    struct rnode *n;
-    struct rlist *rl = rlist_create ();
-
-    if (!rl || rlist_append (rl, rla) < 0) {
-        rlist_destroy (rl);
-        return NULL;
-    }
-
-    n = zlistx_first (rlb->nodes);
+    struct rnode *n = zlistx_first (rlb->nodes);
     while (n) {
         /*  Attempt to find and "detach" the rank which we're diffing.
          */
@@ -565,6 +557,18 @@ struct rlist *rlist_diff (const struct rlist *rla, const struct rlist *rlb)
         }
         n = zlistx_next (rlb->nodes);
     }
+    return 0;
+}
+
+struct rlist *rlist_diff (const struct rlist *rla, const struct rlist *rlb)
+{
+    struct rlist *rl = rlist_create ();
+    if (!rl
+        || rlist_append (rl, rla) < 0
+        || rlist_subtract (rl, rlb) < 0) {
+        rlist_destroy (rl);
+        return NULL;
+    }
     return rl;
 }
 

src/common/librlist/rlist.h

@@ -151,6 +151,11 @@ int rlist_append (struct rlist *rl, const struct rlist *rl2);
  */
 int rlist_add (struct rlist *rl, const struct rlist *rl2);
 
+/*  Subtract resources in `rl2` from `rl`. It is not an error if
+ *   resources in rl2 are not present in `rl`.
+ */
+int rlist_subtract (struct rlist *rl, const struct rlist *rl2);
+
 /*  Return the set difference of 'rlb' from 'rla'.
  */
 struct rlist *rlist_diff (const struct rlist *rla, const struct rlist *rlb);