Respect maximum message length

... to avoid [MESSAGE_TOO_LONG errors](https://core.telegram.org/method/messages.sendMessage#return-errors)

Author: adriankumpf

lib/telegram_logger_backend/formatter.ex

@@ -1,21 +1,34 @@
 defmodule LoggerTelegramBackend.Formatter do
   @moduledoc false
 
+  @max_length 4096
+
   def format_event(message, level, metadata) do
-    """
-    <b>[#{level}]</b> #{format_message(message)}
-    <pre>#{format_metadata(metadata)}</pre>
-    """
+    level_str = "[#{level}]"
+    metadata_str = format_metadata(metadata)
+
+    max_length = @max_length - String.length(level_str) - String.length(metadata_str) - 1
+    message_str = format_message(message, max_length)
+
+    "<b>#{level_str}</b> " <> message_str <> "\n<pre>#{metadata_str}</pre>"
   end
 
-  defp format_message(message) do
+  defp format_message(message, max_length) do
     message
     |> to_string
     |> String.trim()
+    |> limit_length(max_length)
     |> escape_special_chars()
     |> highlight_title()
   end
 
+  defp limit_length(str, max) do
+    case String.split_at(str, max - 3) do
+      {m, ""} -> m
+      {m, _} -> m <> "..."
+    end
+  end
+
   defp escape_special_chars(message) do
     special_chars = [
       {"&", "&amp;"},

test/logger_telegram_backend_test.exs

@@ -4,7 +4,8 @@ defmodule LoggerTelegramBackendTest do
   require Logger
 
   setup_all do
-    Logger.remove_backend(:console)
+    :ok = Logger.configure(truncate: :infinity)
+    :ok = Logger.remove_backend(:console)
   end
 
   test "logs the message to the specified sender" do
@@ -32,14 +33,52 @@ defmodule LoggerTelegramBackendTest do
     }
   end
 
+  test "shortens the message if necessary" do
+    :ok = configure(metadata: [:function, :module])
+
+    message = List.duplicate("E", 9999) |> to_string
+    Logger.info(message)
+    assert_receive {:text, log}
+
+    assert log ==
+             """
+             <b>[info]</b> <b>#{List.duplicate("E", 4000)}...</b>
+             <pre>Function: \"test shortens the message if necessary/1\"
+             Module: LoggerTelegramBackendTest</pre>
+             """
+             |> String.trim_trailing()
+  end
+
+  test "shortens the message based on its graphemes not bytes" do
+    :ok = configure(metadata: [])
+
+    for grapheme <- ["A", "é", "💜"] do
+      message = List.duplicate(grapheme, 9999) |> to_string
+      Logger.info(message)
+
+      assert_receive {:text, log}
+      assert log == "<b>[info]</b> <b>#{List.duplicate(grapheme, 4086)}...</b>\n<pre></pre>"
+    end
+  end
+
+  test "shortens the message and escapes special chars afterwards" do
+    :ok = configure(metadata: [])
+
+    message = List.duplicate("&", 9999) |> to_string
+    Logger.info(message)
+
+    assert_receive {:text, log}
+    assert log == "<b>[info]</b> <b>#{List.duplicate("&amp;", 4086)}...</b>\n<pre></pre>"
+  end
+
   test "escapes special chars" do
     :ok = configure(metadata: [])
 
     Logger.info("<>&")
     Logger.info("<code>FOO</code>")
 
-    assert_receive {:text, "<b>[info]</b> <b>&lt;&gt;&amp;</b>\n<pre></pre>\n"}
-    assert_receive {:text, "<b>[info]</b> <b>&lt;code&gt;FOO&lt;/code&gt;</b>\n<pre></pre>\n"}
+    assert_receive {:text, "<b>[info]</b> <b>&lt;&gt;&amp;</b>\n<pre></pre>"}
+    assert_receive {:text, "<b>[info]</b> <b>&lt;code&gt;FOO&lt;/code&gt;</b>\n<pre></pre>"}
   end
 
   test "logs multiple message smoothly" do