Avoid infinite loop with invalid YEARLY recurrence rule (#621)

Produce iterator with no occurrences for impossible YEARLY recurrence rule.

Author: leftmostcat

lib/ical/recur_iterator.js

@@ -179,7 +179,19 @@ class RecurIterator {
     this.initialized = options.initialized || false;
 
     if (!this.initialized) {
-      this.init();
+      try {
+        this.init();
+      } catch (e) {
+        if (e instanceof InvalidRecurrenceRuleError) {
+          // Init may error if there are no possible recurrence instances from
+          // the rule, but we don't want to bubble this error up. Instead, we
+          // create an empty iterator.
+          this.completed = true;
+        } else {
+          // Propagate other errors to consumers.
+          throw e;
+        }
+      }
     }
   }
 
@@ -251,14 +263,28 @@ class RecurIterator {
     }
 
     if (this.rule.freq == "YEARLY") {
-      for (;;) {
+      // Some yearly recurrence rules may be specific enough to not actually
+      // occur on a yearly basis, e.g. the 29th day of February or the fifth
+      // Monday of a given month. The standard isn't clear on the intended
+      // behavior in these cases, but `libical` at least will iterate until it
+      // finds a matching year.
+      // CAREFUL: Some rules may specify an occurrence that can never happen,
+      // e.g. the first Monday of April so long as it falls on the 15th
+      // through the 21st. Detecting these is non-trivial, so ensure that we
+      // stop iterating at some point.
+      const untilYear = this.rule.until ? this.rule.until.year : 20000;
+      while (this.last.year <= untilYear) {
         this.expand_year_days(this.last.year);
         if (this.days.length > 0) {
           break;
         }
         this.increment_year(this.rule.interval);
       }
 
+      if (this.days.length == 0) {
+        throw new InvalidRecurrenceRuleError();
+      }
+
       this._nextByYearDay();
     }
 
@@ -348,11 +374,10 @@ class RecurIterator {
 
     if ((this.rule.count && this.occurrence_number >= this.rule.count) ||
         (this.rule.until && this.last.compare(this.rule.until) > 0)) {
-
-      //XXX: right now this is just a flag and has no impact
-      //     we can simplify the above case to check for completed later.
       this.completed = true;
+    }
 
+    if (this.completed) {
       return null;
     }
 
@@ -362,7 +387,6 @@ class RecurIterator {
       return this.last;
     }
 
-
     let valid;
     do {
       valid = 1;
@@ -1391,4 +1415,18 @@ class RecurIterator {
     return result;
   }
 }
+
+/**
+ * An error indicating that a recurrence rule is invalid and produces no
+ * occurrences.
+ *
+ * @extends {Error}
+ * @class
+ */
+class InvalidRecurrenceRuleError extends Error {
+  constructor() {
+    super("Recurrence rule has no valid occurrences");
+  }
+}
+
 export default RecurIterator;

test/recur_iterator_test.js

@@ -143,14 +143,22 @@ suite('recur_iterator', function() {
 
       let start = ICAL.Time.fromString(options.dtStart);
       let recur = ICAL.Recur.fromString(ruleString);
+
       if (options.throws) {
         assert.throws(function() {
           recur.iterator(start);
         });
         return;
       }
+
       let iterator = recur.iterator(start);
 
+      if (options.noInstance) {
+        assert.equal(iterator.next(), null);
+        assert.ok(iterator.completed);
+        return;
+      }
+
       let inc = 0;
       let dates = [];
       let next, max;
@@ -1081,6 +1089,13 @@ suite('recur_iterator', function() {
         ]
       });
 
+      // Invalid recurrence rule. The first Monday can never fall later than the
+      // 7th.
+      testRRULE('FREQ=YEARLY;BYMONTHDAY=15,16,17,18,19,20,21;BYDAY=1MO', {
+        dtStart: '2015-01-01T08:00:00',
+        noInstance: true,
+      });
+
       // Tycho brahe days - yearly, byYearDay with negative offsets
       testRRULE('FREQ=YEARLY;BYYEARDAY=1,2,4,6,11,12,20,42,48,49,-306,-303,' +
                 '-293,-292,-266,-259,-258,-239,-228,-209,-168,-164,-134,-133,' +