mon/Elector: do a trivial write on every election cycle

Currently we already do a small write when the *first* election in
a round happens (to update the election epoch).  If the backend
happens to fail while we are already in the midst of elections,
however, we may continue to call elections without verifying we
are still writeable.

Signed-off-by: Sage Weil <sage@redhat.com>
(cherry picked from commit ef909ccbdc303cce8a39edef255325127832ff16)

diff --git a/src/mon/Elector.cc b/src/mon/Elector.cc
index 6f54cb4bbc70079037d0db1bbc32efcd9ba28cf9..79d66de6189597288881c825fe5fb6cd1702a6e6 100644 (file)
--- a/src/mon/Elector.cc
+++ b/src/mon/Elector.cc
@@ -78,8 +78,15 @@ void Elector::start()
   init();
   
   // start by trying to elect me
-  if (epoch % 2 == 0) 
+  if (epoch % 2 == 0) {
     bump_epoch(epoch+1);  // odd == election cycle
+  } else {
+    // do a trivial db write just to ensure it is writeable.
+    MonitorDBStore::TransactionRef t(new MonitorDBStore::Transaction);
+    t->put(Monitor::MONITOR_NAME, "election_writeable_test", rand());
+    int r = mon->store->apply_transaction(t);
+    assert(r >= 0);
+  }
   start_stamp = ceph_clock_now(g_ceph_context);
   electing_me = true;
   acked_me[mon->rank] = CEPH_FEATURES_ALL;