channel: Add selfHasContentAccess, using canSubscribeGroup et al

Fixes #814.
Fixes part of #1786.

The general implementation of group-based permissions (#814)
was added in the commits leading up to this one.  This commit
demonstrates implementing a given such permission.

Then we also use the new permission in one place where we already
want to use it, toward #1786.  (There's another part of #1786 which
calls for a variation: determining whether the user would be able
to re-subscribe to a channel if they unsubscribe.  We'll leave that
for a follow-up.)  We'll also be adding a more prominent bit of UI
that wants this permission soon: the list of channels, #188.

To exercise this method on real data, I added some debugging prints
that printed a list of the channels where this returned true, and a
list of the channels where it returned false.  I also manually tested
the change for #1786, by visiting an unsubscribed private channel
(via the link in a #-mention), long-pressing the app bar to get the
channel action sheet, and then (as another user, on web) adding and
removing permission for the test user to subscribe themself.

Author: gnprice

lib/model/channel.dart

@@ -5,6 +5,7 @@ import 'package:flutter/foundation.dart';
 import '../api/model/events.dart';
 import '../api/model/initial_snapshot.dart';
 import '../api/model/model.dart';
+import 'realm.dart';
 import 'store.dart';
 import 'user.dart';
 
@@ -139,6 +140,44 @@ mixin ChannelStore on UserStore {
     }
   }
 
+  bool selfHasContentAccess(ZulipStream channel) {
+    // Compare web's stream_data.has_content_access.
+    if (channel.isWebPublic) return true;
+    if (channel is Subscription) return true;
+    // Here web calls has_metadata_access... but that always returns true,
+    // as its comment says.
+    if (selfUser.role == UserRole.guest) return false;
+    if (!channel.inviteOnly) return true;
+    return _selfHasContentAccessViaGroupPermissions(channel);
+  }
+
+  bool _selfHasContentAccessViaGroupPermissions(ZulipStream channel) {
+    // Compare web's stream_data.has_content_access_via_group_permissions.
+    // TODO(#814) try to clean up this logic; perhaps record more explicitly
+    //   what default/fallback value to use for a given group-based permission
+    //   on older servers.
+
+    if (channel.canAddSubscribersGroup != null
+        && selfHasPermissionForGroupSetting(channel.canAddSubscribersGroup!,
+             GroupSettingType.stream, 'can_add_subscribers_group')) {
+      // The behavior before this permission was introduced was equivalent to
+      // the "nobody" group.
+      // TODO(server-10): simplify
+      return true;
+    }
+
+    if (channel.canSubscribeGroup != null
+        && selfHasPermissionForGroupSetting(channel.canSubscribeGroup!,
+             GroupSettingType.stream, 'can_subscribe_group')) {
+      // The behavior before this permission was introduced was equivalent to
+      // the "nobody" group.
+      // TODO(server-10): simplify
+      return true;
+    }
+
+    return false;
+  }
+
   bool hasPostingPermission({
     required ZulipStream inChannel,
     required User user,

lib/widgets/action_sheet.dart

@@ -445,10 +445,11 @@ void showChannelActionSheet(BuildContext context, {
     && messageListPageNarrow.streamId == channelId;
 
   final unreadCount = store.unreads.countInChannelNarrow(channelId);
-  final isSubscribed = store.subscriptions[channelId] != null;
+  final channel = store.streams[channelId];
+  final isSubscribed = channel is Subscription;
   final buttonSections = [
-    if (!isSubscribed)
-      // TODO(#1786) check group-based can-subscribe permission
+    if (!isSubscribed
+        && channel != null && store.selfHasContentAccess(channel))
       [SubscribeButton(pageContext: pageContext, channelId: channelId)],
     [
       if (unreadCount > 0)