DynamicBuildrequires: Detect when no new packages were installed

Prior to this change, the DynamicBuildrequires loop never ended.

This commit fixes it in 2 ways:

 1. It now actually stops when max_loops (10) happened.
 2. It also stops when the package manager hasn't installed anything.

Both is consistent with the comment present in the previously broken code.

The detection of 2. is based on packages returned by `rpm -qa` before
and after the installation. If they haven't changed, the package
manager hasn't installed anything.

mock/py/mockbuild/backend.py

@@ -721,11 +721,12 @@ def get_command(mode):
         max_loops = int(self.config.get('dynamic_buildrequires_max_loops'))
         success = False
         if dynamic_buildrequires and self.config.get('dynamic_buildrequires'):
-            while not success or max_loops > 0:
+            while not success and max_loops > 0:
                 # run rpmbuild+installSrpmDeps until
                 # * it fails
                 # * installSrpmDeps does nothing
                 # * or we run out of dynamic_buildrequires_max_loops tries
+                packages_before = self.buildroot.all_chroot_packages()
                 try:
                     self.buildroot.doChroot(get_command(['-br']),
                                             shell=False, logger=self.buildroot.build_log, timeout=timeout,
@@ -743,6 +744,9 @@ def get_command(mode):
                     self.buildroot.root_log.info("Going to install missing buildrequires")
                     buildreqs = glob.glob(bd_out + '/SRPMS/*.buildreqs.nosrc.rpm')
                     self.installSrpmDeps(*buildreqs)
+                    packages_after = self.buildroot.all_chroot_packages()
+                    if packages_after == packages_before:
+                        success = True
                     for f_buildreqs in buildreqs:
                         os.remove(f_buildreqs)
                     if not sc:

mock/py/mockbuild/buildroot.py

@@ -207,6 +207,12 @@ def doChroot(self, command, nosync=False, *args, **kargs):
             self.uid_manager.restorePrivs()
         return result
 
+    def all_chroot_packages(self):
+        """package set, result of rpm -qa in the buildroot"""
+        out, _ = self.doChroot([self.config['rpm_command'], "-qa"],
+                               returnOutput=True, printOutput=False, shell=False)
+        return set(out.splitlines())
+
     @traceLog()
     def _copy_config(self, filename):
         etcdir = self.make_chroot_path('etc')