SERVER-71387 Add missing typecheck for range-based window

2 files changed, 43 insertions, 2 deletions

diff --git a/jstests/aggregation/sources/setWindowFields/range_wrong_type.js b/jstests/aggregation/sources/setWindowFields/range_wrong_type.js
new file mode 100644
index 00000000000..26001c40888
--- /dev/null
+++ b/jstests/aggregation/sources/setWindowFields/range_wrong_type.js
@@ -0,0 +1,36 @@
+/**
+ * Test that a window of the form [+N, unbounded] does not trigger a tassert
+ * on mixed-type input.
+ *
+ * Originally intended to reproduce SERVER-71387.
+ */
+(function() {
+"use strict";
+
+const coll = db.set_window_fields_range_wrong_type;
+coll.drop();
+assert.commandWorked(coll.insert([
+    // Numbers sort before strings, so we'll scan {a: 2} first.
+    {a: 2},
+    {a: 'xyz'},
+]));
+
+const err = assert.throws(() => {
+    return coll
+        .aggregate({
+            $setWindowFields: {
+                sortBy: {a: 1},
+                output: {
+                    // The lower bound +3 excludes the current document {a: 2}.
+                    // The only remaining document is {a: 'xyz'}.
+                    // We wrongly consider {a: 'xyz'} to be 'within' the lower bound.
+                    // Then, when we search for the upper bound, we are surprised
+                    // to be starting from 'xyz' which is the wrong type.
+                    b: {$max: 5, window: {range: [+3, 'unbounded']}},
+                },
+            }
+        })
+        .toArray();
+});
+assert.eq(err.code, 5429414, err);
+})();
diff --git a/src/mongo/db/pipeline/window_function/partition_iterator.cpp b/src/mongo/db/pipeline/window_function/partition_iterator.cpp
index ea59f5c6e09..872b4eb0b8d 100644
--- a/src/mongo/db/pipeline/window_function/partition_iterator.cpp
+++ b/src/mongo/db/pipeline/window_function/partition_iterator.cpp
@@ -301,8 +301,13 @@ optional<std::pair<int, int>> PartitionIterator::getEndpointsRangeBased(
                 for (int i = start; (doc = (*this)[i]); ++i) {
                     Value v = (*_sortExpr)->evaluate(*doc, &_expCtx->variables);
                     if (!lessThan(v, threshold)) {
-                        // This is the first doc we've scanned that crossed the threshold.
-                        return i;
+                        // This is the first doc we've scanned that crossed the threshold,
+                        // so it's the first doc in the window (as long as it's the expected type).
+                        if (hasExpectedType(v)) {
+                            return i;
+                        } else {
+                            return boost::none;
+                        }
                     }
                 }
                 // We scanned every document in the partition, and none crossed the