resource: add subsystem for core reservation

Problem: There is no way to reserve cores so they are not used for
scheduling.

Add a 'reserve' subsystem to the core resource module that takes a
reserved set of cores via the 'cores[@Ranks]' spec. Nothing is done
with the reserve subsystem at this point.

Author: grondo

src/modules/resource/Makefile.am

@@ -40,7 +40,9 @@ libresource_la_SOURCES = \
 	drainset.h \
 	drainset.c \
 	upgrade.h \
-	upgrade.c
+	upgrade.c \
+	reserve.c \
+	reserve.h
 
 TESTS = \
 	test_rutil.t \

src/modules/resource/reserve.c

@@ -0,0 +1,81 @@
+/************************************************************\
+ * Copyright 2025 Lawrence Livermore National Security, LLC
+ * (c.f. AUTHORS, NOTICE.LLNS, COPYING)
+ *
+ * This file is part of the Flux resource manager framework.
+ * For details, see https://github.com/flux-framework.
+ *
+ * SPDX-License-Identifier: LGPL-3.0
+\************************************************************/
+
+/* reserve.c - get static set of resources to reserve for the OS
+ */
+
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
+#include <flux/core.h>
+#include <jansson.h>
+
+#include "src/common/librlist/rlist.h"
+
+#include "resource.h"
+#include "inventory.h"
+
+struct reserve {
+    struct resource_ctx *ctx;
+    struct rlist *rl;
+};
+
+const struct rlist *reserve_get (struct reserve *reserve)
+{
+    return reserve->rl;
+}
+
+void reserve_destroy (struct reserve *reserve)
+{
+    if (reserve) {
+        int saved_errno = errno;
+        rlist_destroy (reserve->rl);
+        free (reserve);
+        errno = saved_errno;
+    }
+}
+
+struct reserve *reserve_create (struct resource_ctx *ctx,
+                                const char *spec)
+{
+    struct reserve *reserve;
+    struct rlist *rl = NULL;
+
+    if (!(reserve = calloc (1, sizeof (*reserve))))
+        return NULL;
+    reserve->ctx = ctx;
+    if (spec) {
+        flux_error_t error;
+        if (!(rl = rlist_from_json (inventory_get (ctx->inventory), NULL))) {
+            flux_log (ctx->h,
+                      LOG_ERR,
+                      "reserve: failed to get resources from inventory");
+            goto error;
+        }
+        if (!(reserve->rl = rlist_copy_core_spec (rl, spec, &error))) {
+            flux_log (ctx->h,
+                      LOG_ERR,
+                      "error decoding reserve spec %s: %s",
+                      spec,
+                      error.text);
+            goto error;
+        }
+        rlist_destroy (rl);
+    }
+    return reserve;
+error:
+    rlist_destroy (rl);
+    reserve_destroy (reserve);
+    return NULL;
+}
+
+/*
+ * vi:tabstop=4 shiftwidth=4 expandtab
+ */

src/modules/resource/reserve.h

@@ -0,0 +1,24 @@
+/************************************************************\
+ * Copyright 2025 Lawrence Livermore National Security, LLC
+ * (c.f. AUTHORS, NOTICE.LLNS, COPYING)
+ *
+ * This file is part of the Flux resource manager framework.
+ * For details, see https://github.com/flux-framework.
+ *
+ * SPDX-License-Identifier: LGPL-3.0
+\************************************************************/
+
+#ifndef _FLUX_RESOURCE_RESERVE_H
+#define _FLUX_RESOURCE_RESERVE_H
+
+struct reserve *reserve_create (struct resource_ctx *ctx, const char *spec);
+void reserve_destroy (struct reserve *exclude);
+
+const struct rlist *reserve_get (struct reserve *reserve);
+
+#endif /* !_FLUX_RESOURCE_RESERVE_H */
+
+/*
+ * vi:tabstop=4 shiftwidth=4 expandtab
+ */
+