Avoiding deadlock when jenkins updates the build iterator while acces…

…sing the build to count the number of builds

src/main/java/org/jenkinsci/plugins/prometheus/collectors/jobs/NbBuildsGauge.java

@@ -5,10 +5,12 @@
 import io.prometheus.client.SimpleCollector;
 import org.jenkinsci.plugins.prometheus.collectors.CollectorType;
 import org.jenkinsci.plugins.prometheus.collectors.builds.BuildsMetricCollector;
-
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 
 public class NbBuildsGauge extends BuildsMetricCollector<Job<?, ?>, Gauge> {
 
+    private final ReentrantReadWriteLock lock = new ReentrantReadWriteLock();
+
     protected NbBuildsGauge(String[] labelNames, String namespace, String subsystem) {
         super(labelNames, namespace, subsystem);
     }
@@ -30,7 +32,12 @@ protected SimpleCollector.Builder<?, Gauge> getCollectorBuilder() {
 
     @Override
     public void calculateMetric(Job<?, ?> jenkinsObject, String[] labelValues) {
-        int nbBuilds = jenkinsObject.getBuildsAsMap().size();
-        this.collector.labels(labelValues).set(nbBuilds);
+        lock.readLock().lock();
+        try  {
+            int nbBuilds = jenkinsObject.getBuildsAsMap().size();
+            this.collector.labels(labelValues).set(nbBuilds);
+        } finally {
+            lock.readLock().unlock();
+        }
     }
 }