SERVER-4243 Allow reading on secondaries+fsyncLock

Allow reading on secondaries with fsyncLock mode enabled.
Previously oplog application would cause blocking.

jstests/fsync_lock_read_secondaries.js

@@ -0,0 +1,61 @@
+/* @file : jstests/fsync_lock_read_secondaries.js
+ * 
+ * SERVER 4243 : If there is a pending write due to an fsync lock, all reads are blocked
+ *
+ * This test validates part of SERVER-4243 ticket. Allow reading on secondaries with fsyncLock
+ * mode enabled. Previously oplog application would cause blocking.
+ * The corresponding commit :
+ * https://github.com/mongodb/mongo/commit/73aa870d129bd7d51de946b91c16cc056aaacbc7
+ */
+
+/*
+ * 1) Create a replica set.
+ * 2) Add some documents to master.
+ * 3) Wait until the secondary nodes are in state "SECONDARY".
+ * 4) Set slaveOk on secondary.
+ * 5) Take the fsync lock on a secondary. This will stop replication.
+ * 6) Insert some more documents to master.
+ * 7) Expect to be able to read from the secondary; the count of documents should
+ *    be equal to the number of documents added in step 2.
+ * 8) Release the fsync lock. This will resume replication.
+ * 9) Soon, the secondary should be applying the oplog again, which we should
+ *    witness as an increase in the count of documents stored on the secondary.
+ */
+
+// Load utility methods for replica set tests
+load("jstests/replsets/rslib.js");
+
+var replTest = new ReplSetTest({name: 'testSet', nodes: 2, oplogSize: 5});
+// Start each mongod in the replica set. Returns a list of nodes
+var nodes = replTest.startSet();
+// This will wait for initiation
+replTest.initiate();
+var master = replTest.getMaster();
+var docNum = 100;
+for(var i=0; i<docNum; i++) {
+    master.getDB("foo").bar.save({a: i});
+}
+waitForAllMembers(master.getDB("foo"));
+
+// Calling getMaster also makes available the liveNodes structure, which looks like this:
+// liveNodes = {master: masterNode, slaves: [slave1, slave2] }
+var slaves = replTest.liveNodes.slaves;
+slaves[0].setSlaveOk();
+
+assert.commandWorked(slaves[0].getDB("admin").runCommand({fsync:1, lock: 1}));
+var docNum = 1000;
+for (var i=0; i<docNum; i++) {
+    master.getDB("foo").bar.save({a: i});
+}
+// Issue a read query on the secondary while holding the fsync lock.
+// This is what we are testing. Previously this would block. After the fix 
+// this should work just fine.
+var slave0count = slaves[0].getDB("foo").bar.count();
+assert.eq(slave0count, 100, "Doc count in fsync lock wrong. Expected (=100), found " + slave0count);
+assert(slaves[0].getDB("admin").$cmd.sys.unlock.findOne().ok);
+
+// The secondary should have equal or more documents than what it had before.
+assert.soon(function() {
+        return slaves[0].getDB("foo").bar.count() > 100
+    }, "count of documents stored on the secondary did not increase");
+replTest.stopSet();