geo-rep: Fix deadlock during worker start

Analysis:
Monitor process spawns monitor threads (one per brick).
Each monitor thread, forks worker and agent processes.
Each monitor thread, while intializing, updates the
monitor status file. It is synchronized using flock.
The race is that, some thread can fork worker while
other thread opened the status file resulting in
holding the reference of fd in worker process.

Cause:
flock gets unlocked either by specifically unlocking it
or by closing all duplicate fds referring to the file.
The code was relying on fd close, hence a reference
in worker/agent process by fork could cause the deadlock.

Fix:
1. flock is unlocked specifically.
2. Also made sure to update status file in approriate places so that
the reference is not leaked to worker/agent process.

With this fix, both the deadlock and possible fd
leaks is solved.

Backport of:
 > Patch: https://review.gluster.org/20704
 > BUG: bz#1614799
 > Change-Id: I0d1ce93072dab07d0dbcc7e779287368cd9f093d
 > Signed-off-by: Kotresh HR <khiremat@redhat.com>

fixes: bz#1630145
Change-Id: I0d1ce93072dab07d0dbcc7e779287368cd9f093d
Signed-off-by: Kotresh HR <khiremat@redhat.com>

2 files changed, 15 insertions, 4 deletions

diff --git a/geo-replication/syncdaemon/gsyncdstatus.py b/geo-replication/syncdaemon/gsyncdstatus.py
index e75f9dcd62c..e93e0519890 100644
--- a/geo-replication/syncdaemon/gsyncdstatus.py
+++ b/geo-replication/syncdaemon/gsyncdstatus.py
@@ -100,6 +100,7 @@ class LockedOpen(object):
         return f
 
     def __exit__(self, _exc_type, _exc_value, _traceback):
+        fcntl.flock(self.fileobj, fcntl.LOCK_UN)
         self.fileobj.close()
 
 
diff --git a/geo-replication/syncdaemon/monitor.py b/geo-replication/syncdaemon/monitor.py
index 40818427bfe..047efa75346 100644
--- a/geo-replication/syncdaemon/monitor.py
+++ b/geo-replication/syncdaemon/monitor.py
@@ -105,10 +105,6 @@ class Monitor(object):
                                                     master,
                                                     "%s::%s" % (slave_host,
                                                                 slave_vol))
-
-        set_monitor_status(gconf.get("state-file"), self.ST_STARTED)
-        self.status[w[0]['dir']].set_worker_status(self.ST_INIT)
-
         ret = 0
 
         def nwait(p, o=0):
@@ -153,6 +149,7 @@ class Monitor(object):
             # Spawn the worker and agent in lock to avoid fd leak
             self.lock.acquire()
 
+            self.status[w[0]['dir']].set_worker_status(self.ST_INIT)
             logging.info(lf('starting gsyncd worker',
                             brick=w[0]['dir'],
                             slave_node=remote_host))
@@ -349,6 +346,19 @@ class Monitor(object):
             t = Thread(target=wmon, args=[wx])
             t.start()
             ta.append(t)
+
+        # monitor status was being updated in each monitor thread. It
+        # should not be done as it can cause deadlock for a worker start.
+        # set_monitor_status uses flock to synchronize multple instances
+        # updating the file. Since each monitor thread forks worker and
+        # agent, these processes can hold the reference to fd of status
+        # file causing deadlock to workers which starts later as flock
+        # will not be release until all references to same fd is closed.
+        # It will also cause fd leaks.
+
+        self.lock.acquire()
+        set_monitor_status(gconf.get("state-file"), self.ST_STARTED)
+        self.lock.release()
         for t in ta:
             t.join()