fix(log): serialize writes to an IO logger so lines from several threads don't mix on JRuby

CHANGELOG.md

@@ -80,6 +80,7 @@
 - `Proxy#rotate` to an upstream proxy failed every HTTPS request with `ERR_TUNNEL_CONNECTION_FAILED`
 - On JRuby `browser.pages`, `windows(:last)` and `workers` came back in random order, so the window a page had just
   opened wasn't the last one. Targets are kept in the order they were created now
+- On JRuby lines written to a `:logger` IO from several threads at once could get mixed up, they're written one by one now
 
 ### Removed
 - `Browser::Process#default_user_agent`, `#browser_version`, `#protocol_version`, `#v8_version` and `#webkit_version`,

lib/ferrum/log.rb

@@ -152,6 +152,7 @@ def initialize(logger, channels: CHANNELS, only: [], skip: [], format: :default,
                end
       @formatter = FORMATS.fetch(format).new(color: @color)
       @commands = Concurrent::Map.new if format == :annotated || only.any? || skip.any?
+      @mutex = Mutex.new
     end
 
     # Whether the channel is written to the logger.
@@ -235,7 +236,8 @@ def write(channel, &block)
       if logger.respond_to?(:debug)
         logger.debug("ferrum:#{channel}", &block)
       else
-        logger.puts(block.call)
+        line = block.call
+        @mutex.synchronize { logger.puts(line) }
       end
     rescue StandardError => e
       warn "Ferrum: couldn't write to the logger, #{e.class}: #{e.message}" unless @write_failed

sig/ferrum/log.rbs

@@ -20,6 +20,7 @@ module Ferrum
     @color: bool
     @formatter: (Default | Json | Annotated)
     @write_failed: bool?
+    @mutex: ::Thread::Mutex
     @commands: ::Concurrent::Map[[untyped, ::Integer], [String, ::Float]]?
 
     def self.parse: (String? value) -> { channels: Array[Symbol], only: Array[String], skip: Array[String] }?

spec/unit/log_spec.rb

@@ -0,0 +1,16 @@
+# frozen_string_literal: true
+
+describe Ferrum::Log do
+  describe "#browser" do
+    it "keeps lines whole when written from several threads at once" do
+      io = StringIO.new
+      log = described_class.new(io)
+      lines = Array.new(8) { |i| i.to_s * 512 }
+
+      lines.map { |line| Thread.new { 200.times { log.browser(line) } } }.each(&:join)
+
+      expect(io.string.lines(chomp: true)).to all(satisfy { |l| lines.include?(l) })
+      expect(io.string.lines.size).to eq(8 * 200)
+    end
+  end
+end