fix: stop holding Pubsub mutex while calling pq.Listener (#12518)

fixes #11950

https://github.com/coder/coder/issues/11950#issuecomment-1987756088 explains the bug

We were also calling into `Unlisten()` and `Close()` while holding the mutex.  I don't believe that `Close()` depends on the notification loop being unblocked, but it's hard to be sure, and the safest thing to do is assume it could block.

So, I added a unit test that fakes out `pq.Listener` and sends a bunch of notifies every time we call into it to hopefully prevent regression where we hold the mutex while calling into these functions.

It also removes the use of a `context.Context` to stop the PubSub -- it must be explicitly `Closed()`.  This simplifies a bunch of the logic, and is how we use the pubsub anyway.

coderd/database/pubsub/pubsub_linux_test.go

@@ -109,60 +109,6 @@ func TestPubsub(t *testing.T) {
 		message := <-messageChannel
 		assert.Equal(t, string(message), data)
 	})
-
-	t.Run("ClosePropagatesContextCancellationToSubscription", func(t *testing.T) {
-		ctx, cancel := context.WithTimeout(context.Background(), testutil.WaitLong)
-		defer cancel()
-		logger := slogtest.Make(t, nil).Leveled(slog.LevelDebug)
-		connectionURL, closePg, err := postgres.Open()
-		require.NoError(t, err)
-		defer closePg()
-		db, err := sql.Open("postgres", connectionURL)
-		require.NoError(t, err)
-		defer db.Close()
-		pubsub, err := pubsub.New(ctx, logger, db, connectionURL)
-		require.NoError(t, err)
-		defer pubsub.Close()
-
-		event := "test"
-		done := make(chan struct{})
-		called := make(chan struct{})
-		unsub, err := pubsub.Subscribe(event, func(subCtx context.Context, _ []byte) {
-			defer close(done)
-			select {
-			case <-subCtx.Done():
-				assert.Fail(t, "context should not be canceled")
-			default:
-			}
-			close(called)
-			select {
-			case <-subCtx.Done():
-			case <-ctx.Done():
-				assert.Fail(t, "timeout waiting for sub context to be canceled")
-			}
-		})
-		require.NoError(t, err)
-		defer unsub()
-
-		go func() {
-			err := pubsub.Publish(event, nil)
-			assert.NoError(t, err)
-		}()
-
-		select {
-		case <-called:
-		case <-ctx.Done():
-			require.Fail(t, "timeout waiting for handler to be called")
-		}
-		err = pubsub.Close()
-		require.NoError(t, err)
-
-		select {
-		case <-done:
-		case <-ctx.Done():
-			require.Fail(t, "timeout waiting for handler to finish")
-		}
-	})
 }
 
 func TestPubsub_ordering(t *testing.T) {