SERVER-49690 Retry "CappedPositionLost" to find oldest oplog entry

(cherry picked from commit b58968562034c206cd041c083d1ac3cc5e749ec1)
(cherry picked from commit a53e991dafb3860662acd3e698ac394273ce3357)

3 files changed, 26 insertions, 8 deletions

diff --git a/jstests/replsets/initial_sync_oplog_rollover.js b/jstests/replsets/initial_sync_oplog_rollover.js
index 0d8e60e5501..b0f7b10ad9f 100644
--- a/jstests/replsets/initial_sync_oplog_rollover.js
+++ b/jstests/replsets/initial_sync_oplog_rollover.js
@@ -10,6 +10,8 @@
 (function() {
 "use strict";
 
+load("jstests/replsets/rslib.js");
+
 var name = 'initial_sync_oplog_rollover';
 var replSet = new ReplSetTest({
     name: name,
@@ -31,10 +33,6 @@ var primary = replSet.getPrimary();
 var coll = primary.getDB('test').foo;
 assert.writeOK(coll.insert({a: 1}));
 
-function getFirstOplogEntry(conn) {
-    return conn.getDB('local').oplog.rs.find().sort({$natural: 1}).limit(1)[0];
-}
-
 var firstOplogEntry = getFirstOplogEntry(primary);
 
 // Add a secondary node but make it hang before copying databases.
diff --git a/jstests/replsets/rslib.js b/jstests/replsets/rslib.js
index 25bfee53cfb..3b385e70e29 100644
--- a/jstests/replsets/rslib.js
+++ b/jstests/replsets/rslib.js
@@ -13,6 +13,7 @@ var waitForState;
 var reInitiateWithoutThrowingOnAbortedMember;
 var awaitRSClientHosts;
 var getLastOpTime;
+var getFirstOplogEntry;
 var setLogVerbosity;
 var stopReplicationAndEnforceNewPrimaryToCatchUp;
 var setFailPoint;
@@ -444,6 +445,29 @@ getLastOpTime = function(conn) {
 };
 
 /**
+ * Returns the oldest oplog entry.
+ */
+getFirstOplogEntry = function(conn) {
+    let firstEntry;
+    // The query plan may yield between the cursor establishment and iterating to retrieve the first
+    // result. During this yield it's possible for the oplog to "roll over" or shrink. This is rare,
+    // but if these both happen the cursor will be unable to resume after yielding and return a
+    // "CappedPositionLost" error. This can be safely retried.
+    assert.soon(() => {
+        try {
+            firstEntry = conn.getDB('local').oplog.rs.find().sort({$natural: 1}).limit(1)[0];
+            return true;
+        } catch (e) {
+            if (e.code == ErrorCodes.CappedPositionLost) {
+                return false;
+            }
+            throw e;
+        }
+    });
+    return firstEntry;
+};
+
+/**
  * Set log verbosity on all given nodes.
  * e.g. setLogVerbosity(replTest.nodes, { "replication": {"verbosity": 3} });
  */
diff --git a/jstests/replsets/too_stale_secondary.js b/jstests/replsets/too_stale_secondary.js
index bc382274250..5bde4a3f320 100644
--- a/jstests/replsets/too_stale_secondary.js
+++ b/jstests/replsets/too_stale_secondary.js
@@ -33,10 +33,6 @@
 
 load('jstests/replsets/rslib.js');
 
-function getFirstOplogEntry(conn) {
-    return conn.getDB('local').oplog.rs.find().sort({$natural: 1}).limit(1)[0];
-}
-
 /**
  * Overflows the oplog of a given node.
  *