D-Bus: fix syncevo-dbus-server<->syncevo-dbus-helper communication when using GIO D-Bus

D-Bus objects in a process exist independently from a specific D-Bus
connection. They are only identified by their path. When syncevo-dbus-server
forked multiple syncevo-dbus-helper instances, it indirectly (in ForkExec)
created multiple callback objects for the childs "watch" functionality (a
pending method call that the parent never replies to). These method calls were
associated with the a random (oldest?) session instead of the current one.

This causes problems once an older session terminates and the callback object
destructs, because then the wrong children get a failure response, which they
treat as "connection lost = parent has terminated". Found while trying to fix
shutdown race conditions.

The solution is to generate a unique counter for each child and communicate
that counter to the child via a new env variable.

1 files changed, 8 insertions, 0 deletions

diff --git a/src/syncevo/ForkExec.h b/src/syncevo/ForkExec.h
index 5de1848c..e460e59d 100644
--- a/src/syncevo/ForkExec.h
+++ b/src/syncevo/ForkExec.h
@@ -100,8 +100,16 @@ class ForkExec : private boost::noncopyable {
     typedef boost::signals2::signal<void (SyncMLStatus, const std::string &)> OnFailure;
     OnFailure m_onFailure;
 
+    /**
+     * A unique string for the ForkExecParent/Child pair which can be used
+     * as D-Bus path component.
+     */
+    std::string getInstance() const { return m_instance; }
+
  protected:
     ForkExec();
+
+    std::string m_instance;
 };
 
 /**