SECURITY: Add a default limit as to when logs should be truncated

Why this change?

This ensures that malicious requests cannot end up causing the logs to
quickly fill up. The default chosen is sufficient for most legitimate
requests to the Discourse application.

When truncation happens, parsing of logs in supported format like
lograge may break down.

config/discourse_defaults.conf

@@ -374,3 +374,6 @@ regex_timeout_seconds = 2
 
 # Allow impersonation function on the cluster to admins
 allow_impersonation = true
+
+# The maximum number of characters allowed in a single log line.
+log_line_max_chars = 160000

config/initializers/102-truncate-logs.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+if Rails.env.production? || ENV["ENABLE_LOGS_TRUNCATION"] == "1"
+  def set_or_extend_truncate_logs_formatter(logger)
+    if logger.formatter
+      logger.formatter.extend(
+        Module.new do
+          def call(*args)
+            truncate_logs_formatter.call(super(*args))
+          end
+
+          def truncate_logs_formatter
+            @formatter ||=
+              TruncateLogsFormatter.new(log_line_max_chars: GlobalSetting.log_line_max_chars)
+          end
+        end,
+      )
+    else
+      logger.formatter =
+        TruncateLogsFormatter.new(log_line_max_chars: GlobalSetting.log_line_max_chars)
+    end
+  end
+
+  Rails.application.config.to_prepare do
+    set_or_extend_truncate_logs_formatter(Rails.logger)
+
+    if Rails.logger.respond_to? :chained
+      Rails.logger.chained.each { |logger| set_or_extend_truncate_logs_formatter(logger) }
+    end
+  end
+end

lib/truncate_logs_formatter.rb

@@ -0,0 +1,29 @@
+# frozen_string_literal: true
+
+# This log formatter limits the number of characters in each log message to prevent malicious requests from filling up the disk
+# in a short amount of time. The number of characters is determined by the `log_line_max_chars` global setting which can be
+# configured via the `DISCOURSE_MAX_LOG_LINES` environment variable or via the `discourse_defaults.conf` file.
+class TruncateLogsFormatter < ::ActiveSupport::Logger::SimpleFormatter
+  include ::ActiveSupport::TaggedLogging::Formatter
+
+  def initialize(log_line_max_chars:)
+    @log_line_max_chars = log_line_max_chars
+  end
+
+  def call(*args)
+    # Lograge formatters are only called with a single argument instead of the usual 4 arguments of `severity`, `datetime`, `progname` and `message`.
+    message =
+      if args.length == 1
+        args[0]
+      else
+        args[3]
+      end
+
+    if message.length > @log_line_max_chars
+      newlines = message.length - message.chomp.length
+      "#{message[0, @log_line_max_chars]}...(truncated)#{"\n" * newlines}"
+    else
+      message
+    end
+  end
+end

spec/lib/truncate_logs_formatter_spec.rb

@@ -0,0 +1,31 @@
+# frozen_string_literal: true
+
+RSpec.describe TruncateLogsFormatter do
+  describe "#call" do
+    describe "when the formatter is initialized with `log_line_max_chars` of 10" do
+      let(:formatter) { TruncateLogsFormatter.new(log_line_max_chars: 10) }
+
+      describe "when the messages is 5 characters long" do
+        it "should not carry out any truncation of the message" do
+          expect(formatter.call(nil, nil, nil, "abcde")).to eq("abcde")
+        end
+      end
+
+      describe "when the message is 10 characters long" do
+        it "should not carry out any truncation of the message" do
+          expect(formatter.call(nil, nil, nil, "aaaaaaaaaa")).to eq("aaaaaaaaaa")
+        end
+      end
+
+      describe "when the message is 11 characters long" do
+        it "should truncate the message with the right postfix" do
+          expect(formatter.call(nil, nil, nil, "aaaaaaaaaaa")).to eq("aaaaaaaaaa...(truncated)")
+        end
+
+        it "should truncate the message with the right postfix while preserving newlines" do
+          expect(formatter.call(nil, nil, nil, "aaaaaaaaaaa\n")).to eq("aaaaaaaaaa...(truncated)\n")
+        end
+      end
+    end
+  end
+end