Defend against out-of-order builds even if nextBuildNumber is wrong.

core/src/main/java/hudson/model/RunMap.java

@@ -172,6 +172,7 @@ public final class RunMap<R extends Run<?,R>> extends AbstractLazyLoadRunMap<R>
         if (rootDir.isDirectory()) {
             throw new IllegalStateException(rootDir + " already existed; will not overwite with " + r);
         }
+        proposeNewNumber(r.getNumber());
         rootDir.mkdirs();
         return super._put(r);
     }

core/src/main/java/jenkins/model/lazy/AbstractLazyLoadRunMap.java

@@ -356,6 +356,12 @@ public abstract class AbstractLazyLoadRunMap<R> extends AbstractMap<Integer,R> i
         return load(n, null);
     }
 
+    protected final synchronized void proposeNewNumber(int number) throws IllegalStateException {
+        if (numberOnDisk.isInRange(numberOnDisk.ceil(number))) {
+            throw new IllegalStateException("cannot create a build with number " + number + " since that (or higher) is already in use among " + numberOnDisk);
+        }
+    }
+
     public R put(R value) {
         return _put(value);
     }

core/src/test/java/jenkins/model/lazy/SortedIntListTest.java

 package jenkins.model.lazy;
 
+import static org.junit.Assert.*;
 import org.junit.Test;
 
 /**
@@ -12,6 +13,25 @@ public class SortedIntListTest {
         l.add(0);
         l.add(5);
         l.add(10);
-        System.out.println(l.lower(Integer.MAX_VALUE));
+        assertEquals(2, l.lower(Integer.MAX_VALUE));
     }
+
+    @Test public void ceil() {
+        SortedIntList l = new SortedIntList(5);
+        l.add(1);
+        l.add(3);
+        l.add(5);
+        assertEquals(0, l.ceil(0));
+        assertEquals(0, l.ceil(1));
+        assertEquals(1, l.ceil(2));
+        assertEquals(1, l.ceil(3));
+        assertEquals(2, l.ceil(4));
+        assertEquals(2, l.ceil(5));
+        assertEquals(3, l.ceil(6));
+        assertTrue(l.isInRange(0));
+        assertTrue(l.isInRange(1));
+        assertTrue(l.isInRange(2));
+        assertFalse(l.isInRange(3));
+    }
+
 }