Fatal if more than one burst buffer plugin is configured

Without this commit, multiple burst buffer plugins could be configured
and would run, but because of both plugins manipulating shared state
(such as job state) this does not work properly and can exhibit
unexpected behavior.

Bug 12125

NEWS

@@ -172,6 +172,7 @@ documents those changes that are of interest to users and administrators.
     option has long been obsolete.
  -- cons_tres - change definition of the "least loaded node" (LLN) to the
     node with the greatest ratio of available cpus to total cpus.
+ -- Fatal if more than one burst buffer plugin is configured.
 * Changes in Slurm 21.08.7
 ==========================

RELEASE_NOTES

@@ -74,6 +74,7 @@ CONFIGURATION FILE CHANGES (see man appropriate man page for details)
     jobs within the same qos.
  -- Fatal error if CgroupReleaseAgentDir is configured in cgroup.conf. The
     option has long been obsolete.
+ -- Fatal if more than one burst buffer plugin is configured.
 
 COMMAND CHANGES (see man pages for details)
 ===========================================

src/slurmctld/burst_buffer.c

@@ -166,6 +166,17 @@ extern int bb_g_init(void)
 	}
 	init_run = true;
 
+	/*
+	 * Although the burst buffer plugin interface was designed to support
+	 * multiple burst buffer plugins, this currently does not work. For
+	 * now, do not allow multiple burst buffer plugins to be configured.
+	 */
+	if (g_context_cnt > 1) {
+		error("%d burst buffer plugins configured; can not run with more than one burst buffer plugin",
+		      g_context_cnt);
+		rc = SLURM_ERROR;
+	}
+
 fini:
 	slurm_mutex_unlock(&g_context_lock);