fix(AggregationCursor): make next() error if schema pre('aggregate') middleware throws error

lib/cursor/aggregationCursor.js

@@ -8,6 +8,7 @@ const MongooseError = require('../error/mongooseError');
 const Readable = require('stream').Readable;
 const eachAsync = require('../helpers/cursor/eachAsync');
 const immediate = require('../helpers/immediate');
+const kareem = require('kareem');
 const util = require('util');
 
 /**
@@ -62,7 +63,11 @@ util.inherits(AggregationCursor, Readable);
 
 function _init(model, c, agg) {
   if (!model.collection.buffer) {
-    model.hooks.execPre('aggregate', agg, function() {
+    model.hooks.execPre('aggregate', agg, function(err) {
+      if (err != null) {
+        _handlePreHookError(c, err);
+        return;
+      }
       if (typeof agg.options?.cursor?.transform === 'function') {
         c._transforms.push(agg.options.cursor.transform);
       }
@@ -72,7 +77,12 @@ function _init(model, c, agg) {
     });
   } else {
     model.collection.emitter.once('queue', function() {
-      model.hooks.execPre('aggregate', agg, function() {
+      model.hooks.execPre('aggregate', agg, function(err) {
+        if (err != null) {
+          _handlePreHookError(c, err);
+          return;
+        }
+
         if (typeof agg.options?.cursor?.transform === 'function') {
           c._transforms.push(agg.options.cursor.transform);
         }
@@ -84,6 +94,38 @@ function _init(model, c, agg) {
   }
 }
 
+/**
+* Handles error emitted from pre middleware. In particular, checks for `skipWrappedFunction`, which allows skipping
+* the actual aggregation and overwriting the function's return value. Because aggregation cursors don't return a value,
+* we need to make sure the user doesn't accidentally set a value in skipWrappedFunction.
+*
+* @param {QueryCursor} queryCursor
+* @param {Error} err
+* @returns
+*/
+
+function _handlePreHookError(queryCursor, err) {
+  if (err instanceof kareem.skipWrappedFunction) {
+    const resultValue = err.args[0];
+    if (resultValue != null && (!Array.isArray(resultValue) || resultValue.length)) {
+      const err = new MongooseError(
+        'Cannot `skipMiddlewareFunction()` with a value when using ' +
+        '`.aggregate().cursor()`, value must be nullish or empty array, got "' +
+        util.inspect(resultValue) +
+        '".'
+      );
+      queryCursor._markError(err);
+      queryCursor.listeners('error').length > 0 && queryCursor.emit('error', err);
+      return;
+    }
+    queryCursor.emit('cursor', null);
+    return;
+  }
+  queryCursor._markError(err);
+  queryCursor.listeners('error').length > 0 && queryCursor.emit('error', err);
+}
+
+
 /**
  * Necessary to satisfy the Readable API
  * @method _read
@@ -424,6 +466,7 @@ function _next(ctx, cb) {
       err => callback(err)
     );
   } else {
+    ctx.once('error', cb);
     ctx.once('cursor', function() {
       _next(ctx, cb);
     });

test/aggregate.test.js

@@ -1300,4 +1300,34 @@ describe('aggregate: ', function() {
       });
     }, /Aggregate `near\(\)` argument has invalid coordinates, got ""/);
   });
+
+  it('cursor() errors out if schema pre aggregate hook throws an error (gh-15279)', async function() {
+    const schema = new Schema({ name: String });
+
+    schema.pre('aggregate', function(next) {
+      if (!this.options.allowed) {
+        throw new Error('Unauthorized aggregate operation: only allowed operations are permitted');
+      }
+      next();
+    });
+
+    const Test = db.model('Test', schema);
+
+    await Test.create({ name: 'test1' });
+
+    await assert.rejects(
+      async() => {
+        await Test.aggregate([{ $limit: 1 }], { allowed: false }).exec();
+      },
+      err => err.message === 'Unauthorized aggregate operation: only allowed operations are permitted'
+    );
+
+    const cursor = Test.aggregate([{ $limit: 1 }], { allowed: false }).cursor();
+    await assert.rejects(
+      async() => {
+        await cursor.next();
+      },
+      err => err.message === 'Unauthorized aggregate operation: only allowed operations are permitted'
+    );
+  });
 });