[JENKINS-26739] Backporting #1592 to rc.

(cherry picked from commit cf2b02dc)

Conflicts:
	changelog.html

changelog.html

@@ -66,6 +66,9 @@ Upcoming changes</a>
   <li class="major bug">
     Regression with environment variables in 1.600.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-27188">issue 27188</a>)
+  <li class="major bug">
+    Errors with concurrent matrix builds since 1.597.
+    (<a href="https://issues.jenkins-ci.org/browse/JENKINS-26739">issue 26739</a>)
   <li class=bug>
     Errors in Dashboard View plugin since 1.597.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-26690">issue 26690</a>)

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

@@ -186,7 +186,9 @@ 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());
+        if (!r.getClass().getName().equals("hudson.matrix.MatrixRun")) { // JENKINS-26739: grandfathered in
+            proposeNewNumber(r.getNumber());
+        }
         rootDir.mkdirs();
         return super._put(r);
     }