Get pluggy to work with forking workers

src/ocrmypdf/pluginspec.py

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+# © 2020 James R. Barlow: github.com/jbarlow83
+#
+# This file is part of OCRmyPDF.
+#
+# OCRmyPDF is free software: you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation, either version 3 of the License, or
+# (at your option) any later version.
+#
+# OCRmyPDF is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with OCRmyPDF.  If not, see <http://www.gnu.org/licenses/>.
+
+from argparse import Namespace
+
+import pluggy
+from PIL import Image
+
+from ocrmypdf.pdfinfo import PdfInfo
+
+hookspec = pluggy.HookspecMarker('ocrmypdf')
+
+# pylint: disable=unused-argument
+
+
+@hookspec
+def prepare(options: Namespace) -> None:
+    """Called to notify a plugin that a file will be processed.
+
+	The plugin may modify the options. All objects that are in options must
+	be picklable so they can be marshalled to child worker processes.
+
+    Typically, a plugin will call ``registry.register_plugin(__name__)`` to register
+    all of its public functions with the plugin registry. Functions that are
+    not intended for registration should be prefixed with an underscore.
+    Functions that imported from other modules will be ignored by
+    ``.register_plugin()``. For example if you use ``from os import basename``,
+    ``basename`` will not be registered.
+	"""
+
+
+@hookspec
+def validate(pdfinfo: PdfInfo, options: Namespace) -> None:
+    """Called to give a plugin an opportunity to review options and pdfinfo.
+
+    options contains the "work order" to process a particular file. pdfinfo
+    contains information about the input file obtained after loading and
+    parsing.
+
+    The plugin may raise InputFileError or any ExitCodeException to request
+    normal termination. If the plugin raises another exception type, ocrmypdf
+    will abort with an error and hold the plugin responsible.
+    """
+
+
+@hookspec
+def filter_ocr_image(image: Image) -> Image:
+    """Called to filter the image before it is sent to OCR.
+
+    This is the image that OCR sees, not what the user sees when they view the
+    PDF.
+    """