Allow running inotify in another process

bin/inotify_watch

@@ -0,0 +1,15 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+$LOAD_PATH.unshift "#{__dir__}/../lib"
+require "listen"
+
+$stdout.sync = true # make sure it works well with the pipe
+files = ARGV.to_a
+listener = Listen.to(*files, { prefer_fork: false }) do |modified, added, removed|
+  modified.each { |file| $stdout.write("M", file, "\0") }
+  added.each { |file| $stdout.write("A", file, "\0") }
+  removed.each { |file| $stdout.write("D", file, "\0") }
+end
+listener.start
+sleep

lib/listen/adapter.rb

@@ -4,12 +4,13 @@
 require 'listen/adapter/bsd'
 require 'listen/adapter/darwin'
 require 'listen/adapter/linux'
+require 'listen/adapter/process_linux'
 require 'listen/adapter/polling'
 require 'listen/adapter/windows'
 
 module Listen
   module Adapter
-    OPTIMIZED_ADAPTERS = [Darwin, Linux, BSD, Windows].freeze
+    OPTIMIZED_ADAPTERS = [Darwin, Linux, BSD, Windows, ProcessLinux].freeze
     POLLING_FALLBACK_MESSAGE = 'Listen will be polling for changes.'\
       'Learn more at https://github.com/guard/listen#listen-adapters.'
 
@@ -18,7 +19,8 @@ def select(options = {})
         Listen.logger.debug 'Adapter: considering polling ...'
         return Polling if options[:force_polling]
         Listen.logger.debug 'Adapter: considering optimized backend...'
-        return _usable_adapter_class if _usable_adapter_class
+        adapter_class = _usable_adapter_class(options)
+        return adapter_class if adapter_class
         Listen.logger.debug 'Adapter: falling back to polling...'
         _warn_polling_fallback(options)
         Polling
@@ -30,8 +32,10 @@ def select(options = {})
 
       private
 
-      def _usable_adapter_class
-        OPTIMIZED_ADAPTERS.find(&:usable?)
+      def _usable_adapter_class(options)
+        usable_adapters = OPTIMIZED_ADAPTERS.select(&:usable?)
+        preferred = usable_adapters.find(&:forks?) if options[:prefer_fork]
+        preferred || usable_adapters.first
       end
 
       def _warn_polling_fallback(options)

lib/listen/adapter/base.rb

@@ -123,6 +123,10 @@ class << self
         def usable?
           const_get('OS_REGEXP') =~ RbConfig::CONFIG['target_os']
         end
+
+        def forks?
+          false
+        end
       end
     end
   end

lib/listen/adapter/process_linux.rb

@@ -0,0 +1,88 @@
+# frozen_string_literal: true
+
+require 'listen'
+
+module Listen
+  module Adapter
+    class ProcessLinux < Base
+      OS_REGEXP = /linux/i
+      BIN_PATH = ::File.expand_path("#{__dir__}/../../../bin/inotify_watch")
+
+      def self.forks?
+        true
+      end
+
+      def _configure(dir, &callback)
+      end
+
+      def _run
+        dirs_to_watch = @callbacks.keys.map(&:to_s)
+        worker = Worker.new(dirs_to_watch, &method(:_process_changes))
+        @worker_thread = Thread.new('worker_thread') { worker.run }
+      end
+
+      def _process_changes(dirs)
+        dirs.each do |dir|
+          dir = Pathname.new(dir.sub(%r{/$}, ''))
+
+          @callbacks.each do |watched_dir, callback|
+            if watched_dir.eql?(dir) || Listen::Directory.ascendant_of?(watched_dir, dir)
+              callback.call(dir)
+            end
+          end
+        end
+      end
+
+      def _process_event(dir, path)
+        Listen.logger.debug { "inotify: processing path: #{path.inspect}" }
+        rel_path = path.relative_path_from(dir).to_s
+        _queue_change(:dir, dir, rel_path, recursive: true)
+      end
+
+      def _stop
+        @worker_thread&.kill
+        super
+      end
+
+      class Worker
+        def initialize(dirs_to_watch, &block)
+          @paths = dirs_to_watch
+          @callback = block
+        end
+
+        def run
+          @pipe = IO.popen([BIN_PATH] + @paths)
+          @running = true
+
+          while @running && IO.select([@pipe], nil, nil, nil)
+            command = @pipe.gets("\0")
+            next unless command
+            # remove status (M/A/D) and terminator null byte
+            dir = command[1..-1].chomp("\0")
+            @callback.call([dir])
+          end
+        rescue Interrupt, IOError, Errno::EBADF
+        ensure
+          stop
+        end
+
+        def stop
+          unless @pipe.nil?
+            Process.kill('KILL', @pipe.pid) if process_running?(@pipe.pid)
+            @pipe.close
+          end
+        rescue IOError, Errno::EBADF
+        ensure
+          @running = false
+        end
+
+        def process_running?(pid)
+          Process.kill(0, pid)
+          true
+        rescue Errno::ESRCH
+          false
+        end
+      end
+    end
+  end
+end

lib/listen/listener/config.rb

@@ -11,6 +11,7 @@ class Config
 
         # Backend selecting options
         force_polling: false,
+        prefer_fork: false,
         polling_fallback_message: nil
       }.freeze
 
@@ -33,7 +34,7 @@ def adapter_instance_options(klass)
       end
 
       def adapter_select_options
-        valid_keys = %w[force_polling polling_fallback_message].map(&:to_sym)
+        valid_keys = %i[force_polling polling_fallback_message prefer_fork]
         Hash[@options.select { |key, _| valid_keys.include?(key) }]
       end
     end

spec/lib/listen/adapter/base_spec.rb

@@ -98,4 +98,10 @@ def _process_event(dir, event)
       end
     end
   end
+
+  describe '.forks?' do
+    it 'is false by default' do
+      expect(described_class.forks?).to be false
+    end
+  end
 end

spec/lib/listen/adapter/process_linux_spec.rb

@@ -0,0 +1,19 @@
+# frozen_string_literal: true
+
+require 'listen/adapter/process_linux'
+
+RSpec.describe Listen::Adapter::ProcessLinux do
+  describe 'class' do
+    subject { described_class }
+
+    if linux?
+      it { should be_usable }
+    else
+      it { should_not be_usable }
+    end
+
+    it '.forks? returns true' do
+      expect(described_class.forks?).to be true
+    end
+  end
+end

spec/lib/listen/adapter_spec.rb

@@ -1,42 +1,58 @@
 # frozen_string_literal: true
 
 RSpec.describe Listen::Adapter do
-  let(:listener) { instance_double(Listen::Listener, options: {}) }
   before do
     allow(Listen::Adapter::BSD).to receive(:usable?) { false }
     allow(Listen::Adapter::Darwin).to receive(:usable?) { false }
     allow(Listen::Adapter::Linux).to receive(:usable?) { false }
+    allow(Listen::Adapter::ProcessLinux).to receive(:usable?) { false }
     allow(Listen::Adapter::Windows).to receive(:usable?) { false }
   end
 
   describe '.select' do
-    it 'returns Polling adapter if forced' do
-      klass = Listen::Adapter.select(force_polling: true)
-      expect(klass).to eq Listen::Adapter::Polling
-    end
+    shared_examples 'chooses usable' do
+      let(:options) { {} }
 
-    it 'returns BSD adapter when usable' do
-      allow(Listen::Adapter::BSD).to receive(:usable?) { true }
-      klass = Listen::Adapter.select
-      expect(klass).to eq Listen::Adapter::BSD
-    end
+      it 'returns Polling adapter if forced' do
+        klass = Listen::Adapter.select(options.merge(force_polling: true))
+        expect(klass).to eq Listen::Adapter::Polling
+      end
 
-    it 'returns Darwin adapter when usable' do
-      allow(Listen::Adapter::Darwin).to receive(:usable?) { true }
-      klass = Listen::Adapter.select
-      expect(klass).to eq Listen::Adapter::Darwin
-    end
+      it 'returns BSD adapter when usable' do
+        allow(Listen::Adapter::BSD).to receive(:usable?) { true }
+        klass = Listen::Adapter.select(options)
+        expect(klass).to eq Listen::Adapter::BSD
+      end
 
-    it 'returns Linux adapter when usable' do
-      allow(Listen::Adapter::Linux).to receive(:usable?) { true }
-      klass = Listen::Adapter.select
-      expect(klass).to eq Listen::Adapter::Linux
+      it 'returns Darwin adapter when usable' do
+        allow(Listen::Adapter::Darwin).to receive(:usable?) { true }
+        klass = Listen::Adapter.select(options)
+        expect(klass).to eq Listen::Adapter::Darwin
+      end
+
+      it 'returns Linux adapter when usable' do
+        allow(Listen::Adapter::Linux).to receive(:usable?) { true }
+        klass = Listen::Adapter.select(options)
+        expect(klass).to eq Listen::Adapter::Linux
+      end
+
+      it 'returns Windows adapter when usable' do
+        allow(Listen::Adapter::Windows).to receive(:usable?) { true }
+        klass = Listen::Adapter.select(options)
+        expect(klass).to eq Listen::Adapter::Windows
+      end
     end
 
-    it 'returns Windows adapter when usable' do
-      allow(Listen::Adapter::Windows).to receive(:usable?) { true }
-      klass = Listen::Adapter.select
-      expect(klass).to eq Listen::Adapter::Windows
+    it_behaves_like 'chooses usable'
+    it_behaves_like 'chooses usable' do
+      let(:options) { { prefer_fork: true } }
+
+      it 'returns ProcessLinux adapter when forks are preferred' do
+        allow(Listen::Adapter::Linux).to receive(:usable?) { true }
+        allow(Listen::Adapter::ProcessLinux).to receive(:usable?) { true }
+        klass = Listen::Adapter.select(options)
+        expect(klass).to eq Listen::Adapter::ProcessLinux
+      end
     end
 
     context 'no usable adapters' do

spec/lib/listen/listener/config_spec.rb

@@ -21,7 +21,11 @@
       end
 
       it 'extract adapter selecting options' do
-        expected = { force_polling: true, polling_fallback_message: nil }
+        expected = {
+          force_polling: true,
+          polling_fallback_message: nil,
+          prefer_fork: false
+        }
         expect(subject.adapter_select_options).to eq(expected)
       end
     end