#398

backend/app/services/preview/screenshot.rb

@@ -0,0 +1,64 @@
+module Preview
+  class Screenshot
+    class Unavailable < StandardError; end
+    class Busy < Unavailable; end
+    class TimedOut < Unavailable; end
+
+    LIMIT = ENV.fetch('PREVIEW_SCREENSHOT_CONCURRENCY', 2).to_i.clamp(1, 10)
+    TIMEOUT = ENV.fetch('PREVIEW_SCREENSHOT_TIMEOUT', 20).to_i.clamp(5, 60)
+    @mutex = Mutex.new
+    @active = 0
+
+    def self.capture(url)
+      acquire!
+      tempfile = nil
+      pid = nil
+
+      begin
+        tempfile = Tempfile.new(['preview-thumbnail', '.png'])
+        tempfile.close
+        status = nil
+        pid = Process.spawn(
+            'node', Rails.root.join('lib/screenshot.js').to_s,
+            url, tempfile.path,
+            out: File::NULL, err: File::NULL, pgroup: true)
+        Timeout.timeout(TIMEOUT) do
+          _finished_pid, status = Process.wait2(pid)
+        end
+        raise Unavailable, 'スクリーンショットを生成できませんでした．' unless status.success?
+        raise Unavailable, 'スクリーンショットを生成できませんでした．' unless File.size?(tempfile.path)
+
+        File.binread(tempfile.path)
+      rescue Timeout::Error
+        terminate(pid)
+        raise TimedOut, 'スクリーンショット生成が timeout しました．'
+      ensure
+        tempfile&.unlink
+        release!
+      end
+    end
+
+    def self.acquire!
+      @mutex.synchronize do
+        raise Busy, 'スクリーンショット生成が混雑してゐます．' if @active >= LIMIT
+
+        @active += 1
+      end
+    end
+
+    def self.release!
+      @mutex.synchronize { @active -= 1 }
+    end
+
+    def self.terminate(pid)
+      return unless pid
+
+      Process.kill('TERM', -pid)
+      Process.wait(pid)
+    rescue Errno::ESRCH, Errno::ECHILD
+      nil
+    end
+
+    private_class_method :acquire!, :release!, :terminate
+  end
+end