WIP

changedetectionio/blueprint/ui/preview.py

@@ -57,7 +57,6 @@ def construct_blueprint(datastore: ChangeDetectionStore):
 
         # Fallback: if processor doesn't have preview module, use default text preview
         content = []
-        versions = []
         timestamp = None
 
         extra_stylesheets = [url_for('static_content', group='styles', filename='diff.css')]
@@ -70,20 +69,22 @@ def construct_blueprint(datastore: ChangeDetectionStore):
         ignored_line_numbers = []
         blocked_line_numbers = []
 
-        if datastore.data['watching'][uuid].history_n == 0 and (watch.get_error_text() or watch.get_error_snapshot()):
+        # Gate on whether there is a snapshot to show, NOT on whether an error was recorded: a
+        # watch that has never completed a check and has no error text either (freshly added, or
+        # paused before its first check) has nothing to index into, and asking for the newest of
+        # no versions used to be a 500 on a page reachable from the watch list.
+        versions = list(watch.history.keys())
+        if not versions:
             flash(gettext("Preview unavailable - No fetch/check completed or triggers not reached"), "error")
         else:
             # So prepare the latest preview or not
             preferred_version = request.values.get('version') if request.method == 'POST' else request.args.get('version')
 
-
-            versions = list(watch.history.keys())
             timestamp = versions[-1]
             if preferred_version and preferred_version in versions:
                 timestamp = preferred_version
 
             try:
-                versions = list(watch.history.keys())
                 content = watch.get_history_snapshot(timestamp=timestamp)
 
                 triggered_line_numbers = html_tools.strip_ignore_text(content=content,

changedetectionio/tests/test_preview_endpoints.py

@@ -66,3 +66,21 @@ def test_fetch_pdf(client, live_server, measure_memory_usage, datastore_path):
     assert changed_md5.encode('utf-8') in res.data
 
     assert b'here is a change' in res.data
+
+
+def test_preview_of_a_watch_with_no_history(client, live_server, measure_memory_usage, datastore_path):
+    """/preview must not 500 on a watch that has never completed a check.
+
+    The page is linked from the watch list, so it is reachable for a freshly added or paused
+    watch. The old guard only skipped the history lookup when history_n == 0 AND an error had
+    been recorded, so a clean never-checked watch asked for the newest of no versions and the
+    request died with IndexError.
+    """
+    datastore = client.application.config.get('DATASTORE')
+    uuid = datastore.add_watch(url="https://example.com", extras={'paused': True})
+    assert datastore.data['watching'][uuid].history_n == 0
+    assert not datastore.data['watching'][uuid].get_error_text()
+
+    res = client.get(url_for("ui.ui_preview.preview_page", uuid=uuid), follow_redirects=True)
+    assert res.status_code == 200
+    assert b'Preview unavailable' in res.data