SERVER-22537 Fix DBDirectClient's OperationContext not getting updated.

This fixes an issue where the DBDirectClient used by the $lookup stage
would have the previous OperationContext and trigger an invalid memory
access when sending a getMore request as it unwinds the results.

Calling PipelineProxyStage::doDetachFromOperationContext() now causes
the DBDirectClient's OperationContext to be set to nullptr, and
PipelineProxyStage::doReattachToOperationContext() causes it to be set
to the current OperationContext.

(cherry picked from commit e1928f36d21a4193802d7fbdcb8fcd6df58f7aa7)

1 files changed, 55 insertions, 0 deletions

diff --git a/jstests/aggregation/bugs/lookup_unwind_getmore.js b/jstests/aggregation/bugs/lookup_unwind_getmore.js
new file mode 100644
index 00000000000..e15e4155136
--- /dev/null
+++ b/jstests/aggregation/bugs/lookup_unwind_getmore.js
@@ -0,0 +1,55 @@
+/**
+ * Tests that the server correctly handles when the OperationContext of the DBDirectClient used by
+ * the $lookup stage changes as it unwinds the results.
+ *
+ * This test was designed to reproduce SERVER-22537.
+ */
+(function() {
+    'use strict';
+
+    // We use a batch size of 1 to ensure that the mongo shell issues a getMore when unwinding the
+    // results from the 'dest' collection for the same document in the 'source' collection under a
+    // different OperationContext.
+    const batchSize = 1;
+
+    db.source.drop();
+    db.dest.drop();
+
+    assert.writeOK(db.source.insert({local: 1}));
+
+    // We insert documents in the 'dest' collection such that their combined size is greater than
+    // 16MB in order to ensure that the DBDirectClient used by the $lookup stage issues a getMore
+    // under a different OperationContext.
+    const numMatches = 3;
+    const largeStr = new Array(6 * 1024 * 1024 + 1).join('x');
+
+    for (var i = 0; i < numMatches; ++i) {
+        assert.writeOK(db.dest.insert({foreign: 1, largeStr: largeStr}));
+    }
+
+    var res = db.runCommand({
+        aggregate: 'source',
+        pipeline: [
+            {
+                $lookup: {
+                    from: 'dest',
+                    localField: 'local',
+                    foreignField: 'foreign',
+                    as: 'matches',
+                }
+            },
+            {
+                $unwind: {
+                    path: '$matches',
+                },
+            },
+        ],
+        cursor: {
+            batchSize: batchSize,
+        },
+    });
+    assert.commandWorked(res);
+
+    var cursor = new DBCommandCursor(db.getMongo(), res, batchSize);
+    assert.eq(numMatches, cursor.itcount());
+})();