Critical - P2 ISSUE SUMMARY
The OplogReplay query flag is an internal optimization for queries on the oplog (and other capped collections) using the ts (timestamp) field. Version 2.6.0 introduced a bug whereby a query using the OplogReplay flag with additional predicates not on the ts field could return incorrect results.
USER IMPACT
Internally, replication issues queries with predicates only on the ts field, and thus is unaffected by the bug. Third-party tools or other user implementations to query or tail the oplog can break, if they use the query flag and have additional query predicates.
WORKAROUNDS
A query without the OplogReplay query flag would return correct results but may be less efficient as it would not employ the OplogReplay optimization.
RESOLUTION
The fix restores the behavior to ignore predicates on other fields when walking the collection backwards to find the earliest oplog entry that matches the query. This brings the behavior back in line with that of version 2.4.x. Additionally, validation was added to ensure that the predicate on ts is either $gt or $gte, as the OplogReplay optimization does not work with other operators.
AFFECTED VERSIONS
Version 2.6.0 was affected by this bug.
PATCHES
The patch is included in the 2.6.1 production release.
Original description
Querying the oplog on a 2.6.0 secondary with the oplog replay flag and another predicate in addition to "ts" gives incorrect results vs. a 2.4.8 primary.
The query returns correct results if "ts" is the only predicate in the find.
query: db.oplog.rs.find({ts : {$gte : Timestamp(1397347250,1)}, "o2._id" : ObjectId("52a41544e4b04cb2bcc59814")}).addOption(8).limit(1)
I do not have a 2.4 secondary to test this on now, but I've issued similar queries on 2.4 secondaries in the past and they returned correct results.
- is related to
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SERVER-13562 Queries that use tailable cursors do not stream results if skip() is applied
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- Closed
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- related to
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SERVER-29843 Make oplog queries with point equality on ts field use getOplogStartHack
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- Closed
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