8345684: OperatingSystemMXBean.getSystemCpuLoad() throws NPE

Backport-of: d7ef3ac0b7e677c4901c69ad4361b5de8408e8d3

Author: GoeLin

src/jdk.management/unix/classes/com/sun/management/internal/OperatingSystemImpl.java

@@ -282,8 +282,13 @@ public double getProcessCpuLoad() {
     }
 
     private boolean isCpuSetSameAsHostCpuSet() {
-        if (containerMetrics != null && containerMetrics.getCpuSetCpus() != null) {
-            return containerMetrics.getCpuSetCpus().length == getHostOnlineCpuCount0();
+        if (containerMetrics != null) {
+            // The return value may change (including from non-null to null) and
+            // the call may involve I/O, so keep the result in a local variable.
+            int[] cpuSetCpus = containerMetrics.getCpuSetCpus();
+            if (cpuSetCpus != null) {
+                return cpuSetCpus.length == getHostOnlineCpuCount0();
+            }
         }
         return false;
     }