No per-command bootstrap mount leftovers

The d4c1269 commit broke this use-case:

    $ mock --chain ....
    $ mock --scrub=chroot
    $ mock --scrub=bootstrap

The problem is that --chain always creates a host-local temporary
repository (directory) for built results, which is also bind-mounted to
the bootstrap chroot (so the inner packager may use the built results).

The commit started modifying bootstrap's dnf.conf, thus it added a new
bind-mount that never got unmounted.

Complements: d4c1269
Relates: #1414
Relates: #1286

mock/py/mockbuild/buildroot.py

@@ -978,6 +978,8 @@ def finalize(self):
         buildroot (exclusive lock can be acquired) also kill orphan processes,
         unmount mounts and call postumount hooks.
         """
+        self.mounts.umount_temporary()
+
         if self.tmpdir:
             for d in self.tmpdir, self.make_chroot_path(self.tmpdir):
                 if os.path.exists(d):

mock/py/mockbuild/mounts.py

@@ -192,6 +192,9 @@ def __init__(self, rootObj):
         self.user_mounts = []  # mounts injected by user
         self.bootstrap_mounts = []
 
+        # managed mounts that disappear after each mock command
+        self.temporary_mounts = []
+
         # Instead of mounting a fresh procfs and sysfs, we bind mount /proc
         # and /sys. This avoids problems with kernel restrictions if running
         # within a user namespace, and is pretty much identical otherwise.
@@ -323,6 +326,15 @@ def mountall_managed(self):
             self.mountall_essential()
         for m in self.managed_mounts:
             m.mount()
+        for m in self.temporary_mounts:
+            m.mount()
+
+    def umount_temporary(self):
+        """
+        Umount temporary mounts whenever mock command quits (finalize()).
+        """
+        for m in reversed(self.temporary_mounts):
+            m.umount()
 
     @traceLog()
     def mountall_user(self):
@@ -339,7 +351,7 @@ def umountall(self, force=False, nowarn=False):
             # as long as in every loop at least one umount succeed.
             failed_old = failed_new
             failed_new = 0
-            for m in reversed(self.managed_mounts + self.user_mounts):
+            for m in reversed(self.temporary_mounts + self.managed_mounts + self.user_mounts):
                 if m.umount() is False:
                     failed_new += 1
         if self._essential_mounted:

mock/py/mockbuild/util.py

@@ -1009,8 +1009,8 @@ def _fix_cfg(cfg):
         return
 
     mountpoint = bootstrap.make_chroot_path(local_dir)
-    bootstrap.mounts.add(BindMountPoint(srcpath=local_dir,
-                                        bindpath=mountpoint))
+    bootstrap.mounts.temporary_mounts.append(
+        BindMountPoint(srcpath=local_dir, bindpath=mountpoint))
 
 
 def subscription_redhat_init(opts, uidManager):