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This bug has varying severity depending on the version. The query is
db.coll.count({predicate})
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with secondary reads.
On 4.4, the result is incorrect, and includes orphan documents. A SHARDING_FILTER is missing from the explain plan.
On 5.0 and 6.0, orphan documents are filtered out and the count is correct, but the SHARDING_FILTER stage is not reported in the explain plan. This makes me think the explain is incorrect, and the stage is actually included.
Repro script (based on shard_filtering.js):
(function() {
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"use strict";
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load("jstests/libs/analyze_plan.js");
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// Deliberately inserts orphans outside of migration.
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TestData.skipCheckOrphans = true;
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const st = new ShardingTest({shards: 2, rs: {nodes: 2}});
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const collName = "test.shardfilter";
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const mongosDb = st.s.getDB("test");
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const mongosColl = st.s.getCollection(collName);
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assert.commandWorked(st.s.adminCommand({enableSharding: "test"}));
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st.ensurePrimaryShard("test", st.shard1.name);
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assert.commandWorked(
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st.s.adminCommand({shardCollection: collName, key: {a: 1, "b.c": 1, "d.e.f": 1}}));
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// Put a chunk with no data onto shard0 in order to make sure that both shards get targeted.
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assert.commandWorked(st.s.adminCommand({split: collName, middle: {a: 20, "b.c": 0, "d.e.f": 0}}));
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assert.commandWorked(st.s.adminCommand({split: collName, middle: {a: 30, "b.c": 0, "d.e.f": 0}}));
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assert.commandWorked(st.s.adminCommand(
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{moveChunk: collName, find: {a: 25, "b.c": 0, "d.e.f": 0}, to: st.shard0.shardName}));
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// Shard the collection and insert some docs.
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const docs = [
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{_id: 0, a: 1, b: {c: 1}, d: {e: {f: 1}}, g: 100, z: "z"},
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{_id: 1, a: 1, b: {c: 2}, d: {e: {f: 2}}, g: 100.9, z: "z"},
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{_id: 2, a: 1, b: {c: 3}, d: {e: {f: 3}}, g: "a", z: "z"},
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{_id: 3, a: 1, b: {c: 3}, d: {e: {f: 3}}, g: [1, 2, 3], z: "z"},
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{_id: 4, a: "a", b: {c: "b"}, d: {e: {f: "c"}}, g: null, z: "z"},
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{_id: 5, a: 1.0, b: {c: "b"}, d: {e: {f: Infinity}}, g: NaN, z: "z"},
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];
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assert.commandWorked(mongosColl.insert(docs));
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assert.eq(mongosColl.find().itcount(), 6);
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// Insert some documents with valid partial shard keys to both shards. The versions of these
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// documents on shard0 are orphans, since all of the data is owned by shard1.
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const docsWithMissingAndNullKeys = [
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{_id: 6, a: "missingParts", z: "z"},
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{_id: 7, a: null, b: {c: 1}, d: {e: {f: 1}}, z: "z"},
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{_id: 8, a: "null", b: {c: null}, d: {e: {f: 1}}, z: "z"},
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{_id: 9, a: "deepNull", b: {c: 1}, d: {e: {f: null}}, z: "z"},
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];
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assert.commandWorked(st.shard0.getCollection(collName).insert(docsWithMissingAndNullKeys));
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assert.commandWorked(st.shard1.getCollection(collName).insert(docsWithMissingAndNullKeys));
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// Insert orphan docs without missing or null shard keys onto shard0 and test that they get filtered
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// out.
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const orphanDocs = [
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{_id: 10, a: 100, b: {c: 10}, d: {e: {f: 999}}, g: "a", z: "z"},
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{_id: 11, a: 101, b: {c: 11}, d: {e: {f: 1000}}, g: "b", z: "z"}
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];
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assert.commandWorked(st.shard0.getCollection(collName).insert(orphanDocs));
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assert.eq(mongosColl.find().itcount(), 10);
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// With primary read pref, count with predicate filters out orphans
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assert.eq(mongosColl.count({z: "z"}), 10);
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// The explain plan includes a sharding filter
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jsTestLog(mongosColl.explain().count({z: "z"}));
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mongosDb.shardfilter.getMongo().setReadPref("secondary");
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// With secondary read pref, count with predicate still filters out orphans
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assert.eq(mongosColl.count({z: "z"}), 10);
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// The following explain doesn't include a sharding filter
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jsTestLog(mongosColl.explain().count({z: "z"}));
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st.stop();
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})();
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