Merge branch 'master' into zen-match-fix

18 files changed, 606 insertions, 14 deletions

diff --git a/bot/__main__.py b/bot/__main__.py
index 3df477a6d..8c3ae02e3 100644
--- a/bot/__main__.py
+++ b/bot/__main__.py
@@ -63,6 +63,7 @@ bot.load_extension("bot.cogs.tags")
 bot.load_extension("bot.cogs.token_remover")
 bot.load_extension("bot.cogs.utils")
 bot.load_extension("bot.cogs.watchchannels")
+bot.load_extension("bot.cogs.webhook_remover")
 bot.load_extension("bot.cogs.wolfram")
 
 # Apply `message_edited_at` patch if discord.py did not yet release a bug fix.
diff --git a/bot/cogs/alias.py b/bot/cogs/alias.py
index 9001e18f0..55c7efe65 100644
--- a/bot/cogs/alias.py
+++ b/bot/cogs/alias.py
@@ -29,7 +29,7 @@ class Alias (Cog):
             return log.info(f'Did not find command "{cmd_name}" to invoke.')
         elif not await cmd.can_run(ctx):
             return log.info(
-                f'{str(ctx.author)} tried to run the command "{cmd_name}"'
+                f'{str(ctx.author)} tried to run the command "{cmd_name}" but lacks permission.'
             )
 
         await ctx.invoke(cmd, *args, **kwargs)
diff --git a/bot/cogs/bot.py b/bot/cogs/bot.py
index f17135877..e897b30ff 100644
--- a/bot/cogs/bot.py
+++ b/bot/cogs/bot.py
@@ -67,7 +67,6 @@ class BotCog(Cog, name="Bot"):
             icon_url=URLs.bot_avatar
         )
 
-        log.info(f"{ctx.author} called !about. Returning information about the bot.")
         await ctx.send(embed=embed)
 
     @command(name='echo', aliases=('print',))
diff --git a/bot/cogs/error_handler.py b/bot/cogs/error_handler.py
index 261769efc..6a622d2ce 100644
--- a/bot/cogs/error_handler.py
+++ b/bot/cogs/error_handler.py
@@ -31,7 +31,9 @@ class ErrorHandler(Cog):
         Error handling emits a single error message in the invoking context `ctx` and a log message,
         prioritised as follows:
 
-        1. If the name fails to match a command but matches a tag, the tag is invoked
+        1. If the name fails to match a command:
+            * If it matches shh+ or unshh+, the channel is silenced or unsilenced respectively.
+              Otherwise if it matches a tag, the tag is invoked
             * If CommandNotFound is raised when invoking the tag (determined by the presence of the
               `invoked_from_error_handler` attribute), this error is treated as being unexpected
               and therefore sends an error message
@@ -48,9 +50,11 @@ class ErrorHandler(Cog):
             log.trace(f"Command {command} had its error already handled locally; ignoring.")
             return
 
-        # Try to look for a tag with the command's name if the command isn't found.
         if isinstance(e, errors.CommandNotFound) and not hasattr(ctx, "invoked_from_error_handler"):
+            if await self.try_silence(ctx):
+                return
             if ctx.channel.id != Channels.verification:
+                # Try to look for a tag with the command's name
                 await self.try_get_tag(ctx)
                 return  # Exit early to avoid logging.
         elif isinstance(e, errors.UserInputError):
@@ -89,6 +93,33 @@ class ErrorHandler(Cog):
         else:
             return self.bot.get_command("help")
 
+    async def try_silence(self, ctx: Context) -> bool:
+        """
+        Attempt to invoke the silence or unsilence command if invoke with matches a pattern.
+
+        Respecting the checks if:
+        * invoked with `shh+` silence channel for amount of h's*2 with max of 15.
+        * invoked with `unshh+` unsilence channel
+        Return bool depending on success of command.
+        """
+        command = ctx.invoked_with.lower()
+        silence_command = self.bot.get_command("silence")
+        ctx.invoked_from_error_handler = True
+        try:
+            if not await silence_command.can_run(ctx):
+                log.debug("Cancelling attempt to invoke silence/unsilence due to failed checks.")
+                return False
+        except errors.CommandError:
+            log.debug("Cancelling attempt to invoke silence/unsilence due to failed checks.")
+            return False
+        if command.startswith("shh"):
+            await ctx.invoke(silence_command, duration=min(command.count("h")*2, 15))
+            return True
+        elif command.startswith("unshh"):
+            await ctx.invoke(self.bot.get_command("unsilence"))
+            return True
+        return False
+
     async def try_get_tag(self, ctx: Context) -> None:
         """
         Attempt to display a tag by interpreting the command name as a tag name.
diff --git a/bot/cogs/moderation/__init__.py b/bot/cogs/moderation/__init__.py
index 5243cb92d..6880ca1bd 100644
--- a/bot/cogs/moderation/__init__.py
+++ b/bot/cogs/moderation/__init__.py
@@ -2,12 +2,14 @@ from bot.bot import Bot
 from .infractions import Infractions
 from .management import ModManagement
 from .modlog import ModLog
+from .silence import Silence
 from .superstarify import Superstarify
 
 
 def setup(bot: Bot) -> None:
-    """Load the Infractions, ModManagement, ModLog, and Superstarify cogs."""
+    """Load the Infractions, ModManagement, ModLog, Silence, and Superstarify cogs."""
     bot.add_cog(Infractions(bot))
     bot.add_cog(ModLog(bot))
     bot.add_cog(ModManagement(bot))
+    bot.add_cog(Silence(bot))
     bot.add_cog(Superstarify(bot))
diff --git a/bot/cogs/moderation/scheduler.py b/bot/cogs/moderation/scheduler.py
index f0b6b2c48..917697be9 100644
--- a/bot/cogs/moderation/scheduler.py
+++ b/bot/cogs/moderation/scheduler.py
@@ -222,7 +222,7 @@ class InfractionScheduler(Scheduler):
         # If multiple active infractions were found, mark them as inactive in the database
         # and cancel their expiration tasks.
         if len(response) > 1:
-            log.warning(
+            log.info(
                 f"Found more than one active {infr_type} infraction for user {user.id}; "
                 "deactivating the extra active infractions too."
             )
diff --git a/bot/cogs/moderation/silence.py b/bot/cogs/moderation/silence.py
new file mode 100644
index 000000000..1ef3967a9
--- /dev/null
+++ b/bot/cogs/moderation/silence.py
@@ -0,0 +1,159 @@
+import asyncio
+import logging
+from contextlib import suppress
+from typing import Optional
+
+from discord import TextChannel
+from discord.ext import commands, tasks
+from discord.ext.commands import Context
+
+from bot.bot import Bot
+from bot.constants import Channels, Emojis, Guild, MODERATION_ROLES, Roles
+from bot.converters import HushDurationConverter
+from bot.utils.checks import with_role_check
+
+log = logging.getLogger(__name__)
+
+
+class SilenceNotifier(tasks.Loop):
+    """Loop notifier for posting notices to `alert_channel` containing added channels."""
+
+    def __init__(self, alert_channel: TextChannel):
+        super().__init__(self._notifier, seconds=1, minutes=0, hours=0, count=None, reconnect=True, loop=None)
+        self._silenced_channels = {}
+        self._alert_channel = alert_channel
+
+    def add_channel(self, channel: TextChannel) -> None:
+        """Add channel to `_silenced_channels` and start loop if not launched."""
+        if not self._silenced_channels:
+            self.start()
+            log.info("Starting notifier loop.")
+        self._silenced_channels[channel] = self._current_loop
+
+    def remove_channel(self, channel: TextChannel) -> None:
+        """Remove channel from `_silenced_channels` and stop loop if no channels remain."""
+        with suppress(KeyError):
+            del self._silenced_channels[channel]
+            if not self._silenced_channels:
+                self.stop()
+                log.info("Stopping notifier loop.")
+
+    async def _notifier(self) -> None:
+        """Post notice of `_silenced_channels` with their silenced duration to `_alert_channel` periodically."""
+        # Wait for 15 minutes between notices with pause at start of loop.
+        if self._current_loop and not self._current_loop/60 % 15:
+            log.debug(
+                f"Sending notice with channels: "
+                f"{', '.join(f'#{channel} ({channel.id})' for channel in self._silenced_channels)}."
+            )
+            channels_text = ', '.join(
+                f"{channel.mention} for {(self._current_loop-start)//60} min"
+                for channel, start in self._silenced_channels.items()
+            )
+            await self._alert_channel.send(f"<@&{Roles.moderators}> currently silenced channels: {channels_text}")
+
+
+class Silence(commands.Cog):
+    """Commands for stopping channel messages for `verified` role in a channel."""
+
+    def __init__(self, bot: Bot):
+        self.bot = bot
+        self.muted_channels = set()
+        self._get_instance_vars_task = self.bot.loop.create_task(self._get_instance_vars())
+        self._get_instance_vars_event = asyncio.Event()
+
+    async def _get_instance_vars(self) -> None:
+        """Get instance variables after they're available to get from the guild."""
+        await self.bot.wait_until_guild_available()
+        guild = self.bot.get_guild(Guild.id)
+        self._verified_role = guild.get_role(Roles.verified)
+        self._mod_alerts_channel = self.bot.get_channel(Channels.mod_alerts)
+        self._mod_log_channel = self.bot.get_channel(Channels.mod_log)
+        self.notifier = SilenceNotifier(self._mod_log_channel)
+        self._get_instance_vars_event.set()
+
+    @commands.command(aliases=("hush",))
+    async def silence(self, ctx: Context, duration: HushDurationConverter = 10) -> None:
+        """
+        Silence the current channel for `duration` minutes or `forever`.
+
+        Duration is capped at 15 minutes, passing forever makes the silence indefinite.
+        Indefinitely silenced channels get added to a notifier which posts notices every 15 minutes from the start.
+        """
+        await self._get_instance_vars_event.wait()
+        log.debug(f"{ctx.author} is silencing channel #{ctx.channel}.")
+        if not await self._silence(ctx.channel, persistent=(duration is None), duration=duration):
+            await ctx.send(f"{Emojis.cross_mark} current channel is already silenced.")
+            return
+        if duration is None:
+            await ctx.send(f"{Emojis.check_mark} silenced current channel indefinitely.")
+            return
+
+        await ctx.send(f"{Emojis.check_mark} silenced current channel for {duration} minute(s).")
+        await asyncio.sleep(duration*60)
+        log.info(f"Unsilencing channel after set delay.")
+        await ctx.invoke(self.unsilence)
+
+    @commands.command(aliases=("unhush",))
+    async def unsilence(self, ctx: Context) -> None:
+        """
+        Unsilence the current channel.
+
+        If the channel was silenced indefinitely, notifications for the channel will stop.
+        """
+        await self._get_instance_vars_event.wait()
+        log.debug(f"Unsilencing channel #{ctx.channel} from {ctx.author}'s command.")
+        if await self._unsilence(ctx.channel):
+            await ctx.send(f"{Emojis.check_mark} unsilenced current channel.")
+
+    async def _silence(self, channel: TextChannel, persistent: bool, duration: Optional[int]) -> bool:
+        """
+        Silence `channel` for `self._verified_role`.
+
+        If `persistent` is `True` add `channel` to notifier.
+        `duration` is only used for logging; if None is passed `persistent` should be True to not log None.
+        Return `True` if channel permissions were changed, `False` otherwise.
+        """
+        current_overwrite = channel.overwrites_for(self._verified_role)
+        if current_overwrite.send_messages is False:
+            log.info(f"Tried to silence channel #{channel} ({channel.id}) but the channel was already silenced.")
+            return False
+        await channel.set_permissions(self._verified_role, **dict(current_overwrite, send_messages=False))
+        self.muted_channels.add(channel)
+        if persistent:
+            log.info(f"Silenced #{channel} ({channel.id}) indefinitely.")
+            self.notifier.add_channel(channel)
+            return True
+
+        log.info(f"Silenced #{channel} ({channel.id}) for {duration} minute(s).")
+        return True
+
+    async def _unsilence(self, channel: TextChannel) -> bool:
+        """
+        Unsilence `channel`.
+
+        Check if `channel` is silenced through a `PermissionOverwrite`,
+        if it is unsilence it and remove it from the notifier.
+        Return `True` if channel permissions were changed, `False` otherwise.
+        """
+        current_overwrite = channel.overwrites_for(self._verified_role)
+        if current_overwrite.send_messages is False:
+            await channel.set_permissions(self._verified_role, **dict(current_overwrite, send_messages=None))
+            log.info(f"Unsilenced channel #{channel} ({channel.id}).")
+            self.notifier.remove_channel(channel)
+            self.muted_channels.discard(channel)
+            return True
+        log.info(f"Tried to unsilence channel #{channel} ({channel.id}) but the channel was not silenced.")
+        return False
+
+    def cog_unload(self) -> None:
+        """Send alert with silenced channels on unload."""
+        if self.muted_channels:
+            channels_string = ''.join(channel.mention for channel in self.muted_channels)
+            message = f"<@&{Roles.moderators}> channels left silenced on cog unload: {channels_string}"
+            asyncio.create_task(self._mod_alerts_channel.send(message))
+
+    # This cannot be static (must have a __func__ attribute).
+    def cog_check(self, ctx: Context) -> bool:
+        """Only allow moderators to invoke the commands in this cog."""
+        return with_role_check(ctx, *MODERATION_ROLES)
diff --git a/bot/cogs/moderation/superstarify.py b/bot/cogs/moderation/superstarify.py
index 893cb7f13..ca3dc4202 100644
--- a/bot/cogs/moderation/superstarify.py
+++ b/bot/cogs/moderation/superstarify.py
@@ -59,7 +59,7 @@ class Superstarify(InfractionScheduler, Cog):
             return  # Nick change was triggered by this event. Ignore.
 
         log.info(
-            f"{after.display_name} is currently in superstar-prison. "
+            f"{after.display_name} ({after.id}) tried to escape superstar prison. "
             f"Changing the nick back to {before.display_name}."
         )
         await after.edit(
@@ -80,7 +80,7 @@ class Superstarify(InfractionScheduler, Cog):
         )
 
         if not notified:
-            log.warning("Failed to DM user about why they cannot change their nickname.")
+            log.info("Failed to DM user about why they cannot change their nickname.")
 
     @Cog.listener()
     async def on_member_join(self, member: Member) -> None:
diff --git a/bot/cogs/moderation/utils.py b/bot/cogs/moderation/utils.py
index 5052b9048..3598f3b1f 100644
--- a/bot/cogs/moderation/utils.py
+++ b/bot/cogs/moderation/utils.py
@@ -38,7 +38,7 @@ async def post_user(ctx: Context, user: UserSnowflake) -> t.Optional[dict]:
     log.trace(f"Attempting to add user {user.id} to the database.")
 
     if not isinstance(user, (discord.Member, discord.User)):
-        log.warning("The user being added to the DB is not a Member or User object.")
+        log.debug("The user being added to the DB is not a Member or User object.")
 
     payload = {
         'avatar_hash': getattr(user, 'avatar', 0),
diff --git a/bot/cogs/snekbox.py b/bot/cogs/snekbox.py
index 454836921..315383b12 100644
--- a/bot/cogs/snekbox.py
+++ b/bot/cogs/snekbox.py
@@ -301,7 +301,7 @@ class Snekbox(Cog):
             code = await self.continue_eval(ctx, response)
             if not code:
                 break
-            log.info(f"Re-evaluating message {ctx.message.id}")
+            log.info(f"Re-evaluating code from message {ctx.message.id}:\n{code}")
 
 
 def predicate_eval_message_edit(ctx: Context, old_msg: Message, new_msg: Message) -> bool:
diff --git a/bot/cogs/sync/syncers.py b/bot/cogs/sync/syncers.py
index c7ce54d65..003bf3727 100644
--- a/bot/cogs/sync/syncers.py
+++ b/bot/cogs/sync/syncers.py
@@ -131,7 +131,7 @@ class Syncer(abc.ABC):
             await message.edit(content=f':ok_hand: {mention}{self.name} sync will proceed.')
             return True
         else:
-            log.warning(f"The {self.name} syncer was aborted or timed out!")
+            log.info(f"The {self.name} syncer was aborted or timed out!")
             await message.edit(
                 content=f':warning: {mention}{self.name} sync aborted or timed out!'
             )
diff --git a/bot/cogs/utils.py b/bot/cogs/utils.py
index 0619296ad..3ed471bbf 100644
--- a/bot/cogs/utils.py
+++ b/bot/cogs/utils.py
@@ -40,6 +40,8 @@ If the implementation is easy to explain, it may be a good idea.
 Namespaces are one honking great idea -- let's do more of those!
 """
 
+ICON_URL = "https://www.python.org/static/opengraph-icon-200x200.png"
+
 
 class Utils(Cog):
     """A selection of utilities which don't have a clear category."""
@@ -59,6 +61,10 @@ class Utils(Cog):
             await ctx.invoke(self.bot.get_command("help"), "pep")
             return
 
+        # Handle PEP 0 directly because it's not in .rst or .txt so it can't be accessed like other PEPs.
+        if pep_number == 0:
+            return await self.send_pep_zero(ctx)
+
         possible_extensions = ['.txt', '.rst']
         found_pep = False
         for extension in possible_extensions:
@@ -82,7 +88,7 @@ class Utils(Cog):
                     description=f"[Link]({self.base_pep_url}{pep_number:04})",
                 )
 
-                pep_embed.set_thumbnail(url="https://www.python.org/static/opengraph-icon-200x200.png")
+                pep_embed.set_thumbnail(url=ICON_URL)
 
                 # Add the interesting information
                 fields_to_check = ("Status", "Python-Version", "Created", "Type")
@@ -288,6 +294,19 @@ class Utils(Cog):
         for reaction in options:
             await message.add_reaction(reaction)
 
+    async def send_pep_zero(self, ctx: Context) -> None:
+        """Send information about PEP 0."""
+        pep_embed = Embed(
+            title=f"**PEP 0 - Index of Python Enhancement Proposals (PEPs)**",
+            description=f"[Link](https://www.python.org/dev/peps/)"
+        )
+        pep_embed.set_thumbnail(url=ICON_URL)
+        pep_embed.add_field(name="Status", value="Active")
+        pep_embed.add_field(name="Created", value="13-Jul-2000")
+        pep_embed.add_field(name="Type", value="Informational")
+
+        await ctx.send(embed=pep_embed)
+
 
 def setup(bot: Bot) -> None:
     """Load the Utils cog."""
diff --git a/bot/cogs/webhook_remover.py b/bot/cogs/webhook_remover.py
new file mode 100644
index 000000000..49692113d
--- /dev/null
+++ b/bot/cogs/webhook_remover.py
@@ -0,0 +1,72 @@
+import logging
+import re
+
+from discord import Colour, Message
+from discord.ext.commands import Cog
+
+from bot.bot import Bot
+from bot.cogs.moderation.modlog import ModLog
+from bot.constants import Channels, Colours, Event, Icons
+
+WEBHOOK_URL_RE = re.compile(r"((?:https?://)?discordapp\.com/api/webhooks/\d+/)\S+/?", re.I)
+
+ALERT_MESSAGE_TEMPLATE = (
+    "{user}, looks like you posted a Discord webhook URL. Therefore, your "
+    "message has been removed. Your webhook may have been **compromised** so "
+    "please re-create the webhook **immediately**. If you believe this was "
+    "mistake, please let us know."
+)
+
+log = logging.getLogger(__name__)
+
+
+class WebhookRemover(Cog):
+    """Scan messages to detect Discord webhooks links."""
+
+    def __init__(self, bot: Bot):
+        self.bot = bot
+
+    @property
+    def mod_log(self) -> ModLog:
+        """Get current instance of `ModLog`."""
+        return self.bot.get_cog("ModLog")
+
+    async def delete_and_respond(self, msg: Message, redacted_url: str) -> None:
+        """Delete `msg` and send a warning that it contained the Discord webhook `redacted_url`."""
+        # Don't log this, due internal delete, not by user. Will make different entry.
+        self.mod_log.ignore(Event.message_delete, msg.id)
+        await msg.delete()
+        await msg.channel.send(ALERT_MESSAGE_TEMPLATE.format(user=msg.author.mention))
+
+        message = (
+            f"{msg.author} (`{msg.author.id}`) posted a Discord webhook URL "
+            f"to #{msg.channel}. Webhook URL was `{redacted_url}`"
+        )
+        log.debug(message)
+
+        # Send entry to moderation alerts.
+        await self.mod_log.send_log_message(
+            icon_url=Icons.token_removed,
+            colour=Colour(Colours.soft_red),
+            title="Discord webhook URL removed!",
+            text=message,
+            thumbnail=msg.author.avatar_url_as(static_format="png"),
+            channel_id=Channels.mod_alerts
+        )
+
+    @Cog.listener()
+    async def on_message(self, msg: Message) -> None:
+        """Check if a Discord webhook URL is in `message`."""
+        matches = WEBHOOK_URL_RE.search(msg.content)
+        if matches:
+            await self.delete_and_respond(msg, matches[1] + "xxx")
+
+    @Cog.listener()
+    async def on_message_edit(self, before: Message, after: Message) -> None:
+        """Check if a Discord webhook URL is in the edited message `after`."""
+        await self.on_message(after)
+
+
+def setup(bot: Bot) -> None:
+    """Load `WebhookRemover` cog."""
+    bot.add_cog(WebhookRemover(bot))
diff --git a/bot/converters.py b/bot/converters.py
index 1945e1da3..72c46fdf0 100644
--- a/bot/converters.py
+++ b/bot/converters.py
@@ -262,6 +262,34 @@ class ISODateTime(Converter):
         return dt
 
 
+class HushDurationConverter(Converter):
+    """Convert passed duration to `int` minutes or `None`."""
+
+    MINUTES_RE = re.compile(r"(\d+)(?:M|m|$)")
+
+    async def convert(self, ctx: Context, argument: str) -> t.Optional[int]:
+        """
+        Convert `argument` to a duration that's max 15 minutes or None.
+
+        If `"forever"` is passed, None is returned; otherwise an int of the extracted time.
+        Accepted formats are:
+        * <duration>,
+        * <duration>m,
+        * <duration>M,
+        * forever.
+        """
+        if argument == "forever":
+            return None
+        match = self.MINUTES_RE.match(argument)
+        if not match:
+            raise BadArgument(f"{argument} is not a valid minutes duration.")
+
+        duration = int(match.group(1))
+        if duration > 15:
+            raise BadArgument("Duration must be at most 15 minutes.")
+        return duration
+
+
 def proxy_user(user_id: str) -> discord.Object:
     """
     Create a proxy user object from the given id.
@@ -323,7 +351,7 @@ class FetchedUser(UserConverter):
         except discord.HTTPException as e:
             # If the Discord error isn't `Unknown user`, return a proxy instead
             if e.code != 10013:
-                log.warning(f"Failed to fetch user, returning a proxy instead: status {e.status}")
+                log.info(f"Failed to fetch user, returning a proxy instead: status {e.status}")
                 return proxy_user(arg)
 
             log.debug(f"Failed to fetch user {arg}: user does not exist.")
diff --git a/bot/utils/messages.py b/bot/utils/messages.py
index a36edc774..e969ee590 100644
--- a/bot/utils/messages.py
+++ b/bot/utils/messages.py
@@ -92,7 +92,7 @@ async def send_attachments(
             elif link_large:
                 large.append(attachment)
             else:
-                log.warning(f"{failure_msg} because it's too large.")
+                log.info(f"{failure_msg} because it's too large.")
         except HTTPException as e:
             if link_large and e.status == 413:
                 large.append(attachment)
diff --git a/tests/bot/cogs/moderation/__init__.py b/tests/bot/cogs/moderation/__init__.py
new file mode 100644
index 000000000..e69de29bb
--- /dev/null
+++ b/tests/bot/cogs/moderation/__init__.py
diff --git a/tests/bot/cogs/moderation/test_silence.py b/tests/bot/cogs/moderation/test_silence.py
new file mode 100644
index 000000000..3fd149f04
--- /dev/null
+++ b/tests/bot/cogs/moderation/test_silence.py
@@ -0,0 +1,251 @@
+import unittest
+from unittest import mock
+from unittest.mock import MagicMock, Mock
+
+from discord import PermissionOverwrite
+
+from bot.cogs.moderation.silence import Silence, SilenceNotifier
+from bot.constants import Channels, Emojis, Guild, Roles
+from tests.helpers import MockBot, MockContext, MockTextChannel
+
+
+class SilenceNotifierTests(unittest.IsolatedAsyncioTestCase):
+    def setUp(self) -> None:
+        self.alert_channel = MockTextChannel()
+        self.notifier = SilenceNotifier(self.alert_channel)
+        self.notifier.stop = self.notifier_stop_mock = Mock()
+        self.notifier.start = self.notifier_start_mock = Mock()
+
+    def test_add_channel_adds_channel(self):
+        """Channel in FirstHash with current loop is added to internal set."""
+        channel = Mock()
+        with mock.patch.object(self.notifier, "_silenced_channels") as silenced_channels:
+            self.notifier.add_channel(channel)
+        silenced_channels.__setitem__.assert_called_with(channel, self.notifier._current_loop)
+
+    def test_add_channel_starts_loop(self):
+        """Loop is started if `_silenced_channels` was empty."""
+        self.notifier.add_channel(Mock())
+        self.notifier_start_mock.assert_called_once()
+
+    def test_add_channel_skips_start_with_channels(self):
+        """Loop start is not called when `_silenced_channels` is not empty."""
+        with mock.patch.object(self.notifier, "_silenced_channels"):
+            self.notifier.add_channel(Mock())
+        self.notifier_start_mock.assert_not_called()
+
+    def test_remove_channel_removes_channel(self):
+        """Channel in FirstHash is removed from `_silenced_channels`."""
+        channel = Mock()
+        with mock.patch.object(self.notifier, "_silenced_channels") as silenced_channels:
+            self.notifier.remove_channel(channel)
+        silenced_channels.__delitem__.assert_called_with(channel)
+
+    def test_remove_channel_stops_loop(self):
+        """Notifier loop is stopped if `_silenced_channels` is empty after remove."""
+        with mock.patch.object(self.notifier, "_silenced_channels", __bool__=lambda _: False):
+            self.notifier.remove_channel(Mock())
+        self.notifier_stop_mock.assert_called_once()
+
+    def test_remove_channel_skips_stop_with_channels(self):
+        """Notifier loop is not stopped if `_silenced_channels` is not empty after remove."""
+        self.notifier.remove_channel(Mock())
+        self.notifier_stop_mock.assert_not_called()
+
+    async def test_notifier_private_sends_alert(self):
+        """Alert is sent on 15 min intervals."""
+        test_cases = (900, 1800, 2700)
+        for current_loop in test_cases:
+            with self.subTest(current_loop=current_loop):
+                with mock.patch.object(self.notifier, "_current_loop", new=current_loop):
+                    await self.notifier._notifier()
+                self.alert_channel.send.assert_called_once_with(f"<@&{Roles.moderators}> currently silenced channels: ")
+            self.alert_channel.send.reset_mock()
+
+    async def test_notifier_skips_alert(self):
+        """Alert is skipped on first loop or not an increment of 900."""
+        test_cases = (0, 15, 5000)
+        for current_loop in test_cases:
+            with self.subTest(current_loop=current_loop):
+                with mock.patch.object(self.notifier, "_current_loop", new=current_loop):
+                    await self.notifier._notifier()
+                    self.alert_channel.send.assert_not_called()
+
+
+class SilenceTests(unittest.IsolatedAsyncioTestCase):
+    def setUp(self) -> None:
+        self.bot = MockBot()
+        self.cog = Silence(self.bot)
+        self.ctx = MockContext()
+        self.cog._verified_role = None
+        # Set event so command callbacks can continue.
+        self.cog._get_instance_vars_event.set()
+
+    async def test_instance_vars_got_guild(self):
+        """Bot got guild after it became available."""
+        await self.cog._get_instance_vars()
+        self.bot.wait_until_guild_available.assert_called_once()
+        self.bot.get_guild.assert_called_once_with(Guild.id)
+
+    async def test_instance_vars_got_role(self):
+        """Got `Roles.verified` role from guild."""
+        await self.cog._get_instance_vars()
+        guild = self.bot.get_guild()
+        guild.get_role.assert_called_once_with(Roles.verified)
+
+    async def test_instance_vars_got_channels(self):
+        """Got channels from bot."""
+        await self.cog._get_instance_vars()
+        self.bot.get_channel.called_once_with(Channels.mod_alerts)
+        self.bot.get_channel.called_once_with(Channels.mod_log)
+
+    @mock.patch("bot.cogs.moderation.silence.SilenceNotifier")
+    async def test_instance_vars_got_notifier(self, notifier):
+        """Notifier was started with channel."""
+        mod_log = MockTextChannel()
+        self.bot.get_channel.side_effect = (None, mod_log)
+        await self.cog._get_instance_vars()
+        notifier.assert_called_once_with(mod_log)
+        self.bot.get_channel.side_effect = None
+
+    async def test_silence_sent_correct_discord_message(self):
+        """Check if proper message was sent when called with duration in channel with previous state."""
+        test_cases = (
+            (0.0001, f"{Emojis.check_mark} silenced current channel for 0.0001 minute(s).", True,),
+            (None, f"{Emojis.check_mark} silenced current channel indefinitely.", True,),
+            (5, f"{Emojis.cross_mark} current channel is already silenced.", False,),
+        )
+        for duration, result_message, _silence_patch_return in test_cases:
+            with self.subTest(
+                silence_duration=duration,
+                result_message=result_message,
+                starting_unsilenced_state=_silence_patch_return
+            ):
+                with mock.patch.object(self.cog, "_silence", return_value=_silence_patch_return):
+                    await self.cog.silence.callback(self.cog, self.ctx, duration)
+                    self.ctx.send.assert_called_once_with(result_message)
+            self.ctx.reset_mock()
+
+    async def test_unsilence_sent_correct_discord_message(self):
+        """Proper reply after a successful unsilence."""
+        with mock.patch.object(self.cog, "_unsilence", return_value=True):
+            await self.cog.unsilence.callback(self.cog, self.ctx)
+            self.ctx.send.assert_called_once_with(f"{Emojis.check_mark} unsilenced current channel.")
+
+    async def test_silence_private_for_false(self):
+        """Permissions are not set and `False` is returned in an already silenced channel."""
+        perm_overwrite = Mock(send_messages=False)
+        channel = Mock(overwrites_for=Mock(return_value=perm_overwrite))
+
+        self.assertFalse(await self.cog._silence(channel, True, None))
+        channel.set_permissions.assert_not_called()
+
+    async def test_silence_private_silenced_channel(self):
+        """Channel had `send_message` permissions revoked."""
+        channel = MockTextChannel()
+        self.assertTrue(await self.cog._silence(channel, False, None))
+        channel.set_permissions.assert_called_once()
+        self.assertFalse(channel.set_permissions.call_args.kwargs['send_messages'])
+
+    async def test_silence_private_preserves_permissions(self):
+        """Previous permissions were preserved when channel was silenced."""
+        channel = MockTextChannel()
+        # Set up mock channel permission state.
+        mock_permissions = PermissionOverwrite()
+        mock_permissions_dict = dict(mock_permissions)
+        channel.overwrites_for.return_value = mock_permissions
+        await self.cog._silence(channel, False, None)
+        new_permissions = channel.set_permissions.call_args.kwargs
+        # Remove 'send_messages' key because it got changed in the method.
+        del new_permissions['send_messages']
+        del mock_permissions_dict['send_messages']
+        self.assertDictEqual(mock_permissions_dict, new_permissions)
+
+    async def test_silence_private_notifier(self):
+        """Channel should be added to notifier with `persistent` set to `True`, and the other way around."""
+        channel = MockTextChannel()
+        with mock.patch.object(self.cog, "notifier", create=True):
+            with self.subTest(persistent=True):
+                await self.cog._silence(channel, True, None)
+                self.cog.notifier.add_channel.assert_called_once()
+
+        with mock.patch.object(self.cog, "notifier", create=True):
+            with self.subTest(persistent=False):
+                await self.cog._silence(channel, False, None)
+                self.cog.notifier.add_channel.assert_not_called()
+
+    async def test_silence_private_added_muted_channel(self):
+        """Channel was added to `muted_channels` on silence."""
+        channel = MockTextChannel()
+        with mock.patch.object(self.cog, "muted_channels") as muted_channels:
+            await self.cog._silence(channel, False, None)
+        muted_channels.add.assert_called_once_with(channel)
+
+    async def test_unsilence_private_for_false(self):
+        """Permissions are not set and `False` is returned in an unsilenced channel."""
+        channel = Mock()
+        self.assertFalse(await self.cog._unsilence(channel))
+        channel.set_permissions.assert_not_called()
+
+    @mock.patch.object(Silence, "notifier", create=True)
+    async def test_unsilence_private_unsilenced_channel(self, _):
+        """Channel had `send_message` permissions restored"""
+        perm_overwrite = MagicMock(send_messages=False)
+        channel = MockTextChannel(overwrites_for=Mock(return_value=perm_overwrite))
+        self.assertTrue(await self.cog._unsilence(channel))
+        channel.set_permissions.assert_called_once()
+        self.assertIsNone(channel.set_permissions.call_args.kwargs['send_messages'])
+
+    @mock.patch.object(Silence, "notifier", create=True)
+    async def test_unsilence_private_removed_notifier(self, notifier):
+        """Channel was removed from `notifier` on unsilence."""
+        perm_overwrite = MagicMock(send_messages=False)
+        channel = MockTextChannel(overwrites_for=Mock(return_value=perm_overwrite))
+        await self.cog._unsilence(channel)
+        notifier.remove_channel.assert_called_once_with(channel)
+
+    @mock.patch.object(Silence, "notifier", create=True)
+    async def test_unsilence_private_removed_muted_channel(self, _):
+        """Channel was removed from `muted_channels` on unsilence."""
+        perm_overwrite = MagicMock(send_messages=False)
+        channel = MockTextChannel(overwrites_for=Mock(return_value=perm_overwrite))
+        with mock.patch.object(self.cog, "muted_channels") as muted_channels:
+            await self.cog._unsilence(channel)
+        muted_channels.discard.assert_called_once_with(channel)
+
+    @mock.patch.object(Silence, "notifier", create=True)
+    async def test_unsilence_private_preserves_permissions(self, _):
+        """Previous permissions were preserved when channel was unsilenced."""
+        channel = MockTextChannel()
+        # Set up mock channel permission state.
+        mock_permissions = PermissionOverwrite(send_messages=False)
+        mock_permissions_dict = dict(mock_permissions)
+        channel.overwrites_for.return_value = mock_permissions
+        await self.cog._unsilence(channel)
+        new_permissions = channel.set_permissions.call_args.kwargs
+        # Remove 'send_messages' key because it got changed in the method.
+        del new_permissions['send_messages']
+        del mock_permissions_dict['send_messages']
+        self.assertDictEqual(mock_permissions_dict, new_permissions)
+
+    @mock.patch("bot.cogs.moderation.silence.asyncio")
+    @mock.patch.object(Silence, "_mod_alerts_channel", create=True)
+    def test_cog_unload_starts_task(self, alert_channel, asyncio_mock):
+        """Task for sending an alert was created with present `muted_channels`."""
+        with mock.patch.object(self.cog, "muted_channels"):
+            self.cog.cog_unload()
+            alert_channel.send.assert_called_once_with(f"<@&{Roles.moderators}> channels left silenced on cog unload: ")
+            asyncio_mock.create_task.assert_called_once_with(alert_channel.send())
+
+    @mock.patch("bot.cogs.moderation.silence.asyncio")
+    def test_cog_unload_skips_task_start(self, asyncio_mock):
+        """No task created with no channels."""
+        self.cog.cog_unload()
+        asyncio_mock.create_task.assert_not_called()
+
+    @mock.patch("bot.cogs.moderation.silence.with_role_check")
+    @mock.patch("bot.cogs.moderation.silence.MODERATION_ROLES", new=(1, 2, 3))
+    def test_cog_check(self, role_check):
+        """Role check is called with `MODERATION_ROLES`"""
+        self.cog.cog_check(self.ctx)
+        role_check.assert_called_once_with(self.ctx, *(1, 2, 3))
diff --git a/tests/bot/test_converters.py b/tests/bot/test_converters.py
index 1e5ca62ae..ca8cb6825 100644
--- a/tests/bot/test_converters.py
+++ b/tests/bot/test_converters.py
@@ -8,6 +8,7 @@ from discord.ext.commands import BadArgument
 
 from bot.converters import (
     Duration,
+    HushDurationConverter,
     ISODateTime,
     TagContentConverter,
     TagNameConverter,
@@ -271,3 +272,32 @@ class ConverterTests(unittest.TestCase):
                 exception_message = f"`{datetime_string}` is not a valid ISO-8601 datetime string"
                 with self.assertRaises(BadArgument, msg=exception_message):
                     asyncio.run(converter.convert(self.context, datetime_string))
+
+    def test_hush_duration_converter_for_valid(self):
+        """HushDurationConverter returns correct value for minutes duration or `"forever"` strings."""
+        test_values = (
+            ("0", 0),
+            ("15", 15),
+            ("10", 10),
+            ("5m", 5),
+            ("5M", 5),
+            ("forever", None),
+        )
+        converter = HushDurationConverter()
+        for minutes_string, expected_minutes in test_values:
+            with self.subTest(minutes_string=minutes_string, expected_minutes=expected_minutes):
+                converted = asyncio.run(converter.convert(self.context, minutes_string))
+                self.assertEqual(expected_minutes, converted)
+
+    def test_hush_duration_converter_for_invalid(self):
+        """HushDurationConverter raises correct exception for invalid minutes duration strings."""
+        test_values = (
+            ("16", "Duration must be at most 15 minutes."),
+            ("10d", "10d is not a valid minutes duration."),
+            ("-1", "-1 is not a valid minutes duration."),
+        )
+        converter = HushDurationConverter()
+        for invalid_minutes_string, exception_message in test_values:
+            with self.subTest(invalid_minutes_string=invalid_minutes_string, exception_message=exception_message):
+                with self.assertRaisesRegex(BadArgument, exception_message):
+                    asyncio.run(converter.convert(self.context, invalid_minutes_string))