SERVER-44619 truncate message for error code 16746 if too long

2 files changed, 20 insertions, 2 deletions

diff --git a/jstests/core/indexu.js b/jstests/core/indexu.js
index f8b6af7f15d..0e68429eaf7 100644
--- a/jstests/core/indexu.js
+++ b/jstests/core/indexu.js
@@ -6,7 +6,7 @@
 t = db.jstests_indexu;
 t.drop();
 
-var dupDoc = {a: [{'0': 1}]};  // There are two 'a.0' fields in this doc.
+var dupDoc = {_id: 0, a: [{'0': 1}]};  // There are two 'a.0' fields in this doc.
 var dupDoc2 = {a: [{'1': 1}, 'c']};
 var noDupDoc = {a: [{'1': 1}]};
 
@@ -14,6 +14,11 @@ var noDupDoc = {a: [{'1': 1}]};
 assert.commandWorked(t.save(dupDoc));
 assert.commandFailed(t.ensureIndex({'a.0': 1}));
 
+// Test that we can fail gracefully when dupDoc has a large array padded with nulls.
+// Index is based on max padding constant in mongo/db/update/path_support.h
+assert.commandWorked(t.update({_id: 0}, {$set: {'a.1500001': 1}}));
+assert.commandFailedWithCode(t.ensureIndex({'a.0': 1}), 16746);
+
 t.remove({});
 assert.commandWorked(t.ensureIndex({'a.0': 1}));
 assert.writeError(t.save(dupDoc));
diff --git a/src/mongo/db/index/btree_key_generator.cpp b/src/mongo/db/index/btree_key_generator.cpp
index c0daddfc3a5..b8dcb6f1024 100644
--- a/src/mongo/db/index/btree_key_generator.cpp
+++ b/src/mongo/db/index/btree_key_generator.cpp
@@ -94,11 +94,24 @@ BSONElement BtreeKeyGenerator::_extractNextElement(const BSONObj& obj,
 
     // An index component field name cannot exist in both a document
     // array and one of that array's children.
+    auto arrayObjAsString = [](const BSONObj& arrayObj) {
+        auto msg = arrayObj.toString();
+        const auto kMaxLength = 1024U;
+        if (msg.length() < kMaxLength) {
+            return msg;
+        }
+        str::stream ss;
+        ss << msg.substr(0, kMaxLength / 3);
+        ss << " .......... ";
+        ss << msg.substr(msg.size() - (kMaxLength / 3));
+        return std::string(ss);
+    };
     uassert(
         16746,
         str::stream() << "Ambiguous field name found in array (do not use numeric field names in "
                          "embedded elements in an array), field: '"
-                      << arrField.fieldName() << "' for array: " << positionalInfo.arrayObj,
+                      << arrField.fieldName()
+                      << "' for array: " << arrayObjAsString(positionalInfo.arrayObj),
         !haveObjField || !positionalInfo.hasPositionallyIndexedElt());
 
     *arrayNestedArray = false;