Relax cron expression validation

Accept all expressions that croniter accepts.
If cron-descriptor throws an exception, don't show the
description to the user.

CHANGELOG.md

@@ -8,6 +8,7 @@ All notable changes to this project will be documented in this file.
 
 ## Bug Fixes
 - Fix downtime summary to handle months when the check didn't exist yet (#472)
+- Relax cron expression validation: accept all expressions that croniter accepts
 
 ## v1.19.0 - 2021-02-03
 

hc/front/tests/test_cron_preview.py

@@ -12,8 +12,14 @@ class CronPreviewTestCase(BaseTestCase):
         self.assertContains(r, "cron-preview-title", status_code=200)
         self.assertContains(r, "“Every minute”")
 
+    def test_it_accepts_sunday_7(self):
+        payload = {"schedule": "* * * * 7", "tz": "UTC"}
+        r = self.client.post("/checks/cron_preview/", payload)
+        self.assertContains(r, "Expected Ping Dates", status_code=200)
+        self.assertNotContains(r, "Invalid cron expression", status_code=200)
+
     def test_it_rejects_invalid_cron_expression(self):
-        samples = ["", "*", "100 100 100 100 100", "* * * * * *", "1,2 3,* * * *"]
+        samples = ["", "*", "100 100 100 100 100", "* * * * * *"]
 
         for schedule in samples:
             payload = {"schedule": schedule, "tz": "UTC"}

hc/front/views.py

@@ -477,12 +477,22 @@ def cron_preview(request):
         for i in range(0, 6):
             ctx["dates"].append(it.get_next(datetime))
 
-        ctx["desc"] = str(ExpressionDescriptor(schedule, use_24hour_time_format=True))
+            pass
     except UnknownTimeZoneError:
         ctx["bad_tz"] = True
     except:
         ctx["bad_schedule"] = True
 
+    if ctx["dates"]:
+        try:
+            descriptor = ExpressionDescriptor(schedule, use_24hour_time_format=True)
+            ctx["desc"] = descriptor.get_description()
+        except:
+            # We assume the schedule is valid if croniter accepts it.
+            # If cron-descriptor throws an exception, don't show the description
+            # to the user.
+            pass
+
     return render(request, "front/cron_preview.html", ctx)