Fix GC.stat on Native Image

* ManagementFactory.getMemoryPoolMXBeans() is empty on Native Image

Author: eregon

src/main/java/org/truffleruby/core/GCNodes.java

@@ -14,6 +14,7 @@
 import java.lang.management.MemoryPoolMXBean;
 import java.lang.management.MemoryUsage;
 import java.time.Duration;
+import java.util.Arrays;
 
 import com.oracle.truffle.api.dsl.Cached;
 import org.jcodings.specific.UTF8Encoding;
@@ -178,6 +179,8 @@ protected RubyArray stat(
 
             // Get memory usage values from relevant memory pools (2-3 / ~8 are relevant)
             memoryPools = new Object[memoryPoolNames.length];
+            // On Native Image, ManagementFactory.getMemoryPoolMXBeans() is empty
+            Arrays.fill(memoryPools, nil);
             for (int i = 0; i < memoryPoolNames.length; i++) {
                 String memoryPoolName = memoryPoolNames[i];
                 for (MemoryPoolMXBean bean : ManagementFactory.getMemoryPoolMXBeans()) {

src/main/ruby/truffleruby/core/gc.rb

@@ -102,27 +102,29 @@ def self.stat(option = nil)
       heap_free_slots: committed - used,
     }
 
-    (0...memory_pool_names.length).each do |i|
+    memory_pool_names.each_with_index do |memory_pool_name, i|
       # Populate memory pool specific stats
       info = memory_pool_info[i]
-      stat[memory_pool_names[i]] = {
-        committed: info[0],
-        init: info[1],
-        max: info[2],
-        used: info[3],
-        peak_committed: info[4],
-        peak_init: info[5],
-        peak_max: info[6],
-        peak_used: info[7],
-        last_committed: info[8],
-        last_init: info[9],
-        last_max: info[10],
-        last_used: info[11],
-      }
-
-      # Calculate stats across memory pools
-      stat[memory_pool_names[i]].each_pair do |key, value|
-        stat[key] += value
+      if info
+        stat[memory_pool_name] = data = {
+          committed: info[0],
+          init: info[1],
+          max: info[2],
+          used: info[3],
+          peak_committed: info[4],
+          peak_init: info[5],
+          peak_max: info[6],
+          peak_used: info[7],
+          last_committed: info[8],
+          last_init: info[9],
+          last_max: info[10],
+          last_used: info[11],
+        }
+
+        # Calculate stats across memory pools
+        data.each_pair do |key, value|
+          stat[key] += value
+        end
       end
     end