Add new default Tesseract OCR backend

This new backend uses a command call to avoid
Tesseract bug 1670
(https://github.com/tesseract-ocr/tesseract/issues/1670).

Signed-off-by: Roberto Rosario <roberto.rosario.gonzalez@gmail.com>

HISTORY.rst

@@ -176,6 +176,9 @@
 * Remove app top level star imports.
 * Monkeypatch group and user models to make their fields
   translatable.
+* Add new and default Tesseract OCR backend to avoid
+  Tesseract bug 1670 
+  (https://github.com/tesseract-ocr/tesseract/issues/1670)
 
 3.1.11 (2019-04-XX)
 ===================

docs/releases/3.2.rst

@@ -208,6 +208,9 @@ Other changes
 * Remove app top level star imports.
 * Monkeypatch group and user models to make their fields
   translatable.
+* Add new and default Tesseract OCR backend to avoid
+  Tesseract bug 1670
+  (https://github.com/tesseract-ocr/tesseract/issues/1670)
 
 Removals
 --------

mayan/apps/ocr/backends/literals.py

@@ -0,0 +1,4 @@
+from __future__ import absolute_import, unicode_literals
+
+DEFAULT_TESSERACT_BINARY_PATH = '/usr/bin/tesseract'
+DEFAULT_TESSERACT_TIMEOUT = 600  # 600 seconds, 10 minutes

mayan/apps/ocr/backends/tesseract.py

@@ -0,0 +1,119 @@
+from __future__ import absolute_import, unicode_literals
+
+import logging
+import shutil
+
+import sh
+import yaml
+try:
+    from yaml import CSafeLoader as SafeLoader
+except ImportError:
+    from yaml import SafeLoader
+
+from django.utils.encoding import force_text
+from django.utils.translation import ugettext_lazy as _
+
+from mayan.apps.storage.utils import TemporaryFile
+
+from ..classes import OCRBackendBase
+from ..exceptions import OCRError
+from ..settings import setting_ocr_backend_arguments
+
+from .literals import DEFAULT_TESSERACT_BINARY_PATH, DEFAULT_TESSERACT_TIMEOUT
+
+logger = logging.getLogger(__name__)
+
+
+class Tesseract(OCRBackendBase):
+    def __init__(self, *args, **kwargs):
+        super(Tesseract, self).__init__(*args, **kwargs)
+        self.languages = ()
+
+        backend_arguments = yaml.load(
+            Loader=SafeLoader,
+            stream=setting_ocr_backend_arguments.value or '{}',
+        )
+
+        tesseract_binary_path = backend_arguments.get(
+            'tesseract_path', DEFAULT_TESSERACT_BINARY_PATH
+        )
+        self.command_timeout = backend_arguments.get(
+            'timeout', DEFAULT_TESSERACT_TIMEOUT
+        )
+
+        try:
+            self.command_tesseract = sh.Command(path=tesseract_binary_path)
+        except sh.CommandNotError:
+            self.command_tesseract = None
+            raise OCRError(
+                _('Tesseract not found.')
+            )
+        else:
+            # Get version
+            result = self.command_tesseract(v=True)
+            logger.debug('Tesseract version: %s', result.stdout)
+
+            # Get languages
+            result = self.command_tesseract(list_langs=True)
+            # Sample output format
+            # List of available languages (3):
+            # deu
+            # eng
+            # osd
+            # <- empty line
+
+            # Extaction: strip last line, split by newline, discard the first
+            # line
+            self.languages = force_text(result.stdout).strip().split('\n')[1:]
+
+            logger.debug('Available languages: %s', ', '.join(self.languages))
+
+    def execute(self, *args, **kwargs):
+        """
+        Execute the command line binary of tesseract
+        """
+        super(Tesseract, self).execute(*args, **kwargs)
+
+        if self.command_tesseract:
+            image = self.converter.get_page()
+
+            try:
+                temporary_image_file = TemporaryFile()
+                shutil.copyfileobj(image, temporary_image_file)
+                temporary_image_file.seek(0)
+
+                arguments = ['-', '-']
+
+                keyword_arguments = {
+                    '_in': temporary_image_file,
+                    '_timeout': self.command_timeout
+                }
+
+                if self.language:
+                    keyword_arguments['l'] = self.language
+
+                try:
+
+                    result = self.command_tesseract(
+                        *arguments, **keyword_arguments
+                    )
+                    return force_text(result.stdout)
+                except Exception as exception:
+                    error_message = (
+                        'Exception calling Tesseract with language option: {}; {}'
+                    ).format(self.language, exception)
+
+                    if self.language not in self.languages:
+                        error_message = (
+                            '{}\nThe requested OCR language "{}" is not '
+                            'available and needs to be installed.\n'
+                        ).format(
+                            error_message, self.language
+                        )
+
+                    logger.error(error_message)
+                    raise OCRError(error_message)
+                else:
+                    return result
+            finally:
+                temporary_image_file.close()

mayan/apps/ocr/settings.py

@@ -7,7 +7,8 @@ from mayan.apps.smart_settings import Namespace
 namespace = Namespace(label=_('OCR'), name='ocr')
 
 setting_ocr_backend = namespace.add_setting(
-    global_name='OCR_BACKEND', default='mayan.apps.ocr.backends.pyocr.PyOCR',
+    global_name='OCR_BACKEND',
+    default='mayan.apps.ocr.backends.tesseract.Tesseract',
     help_text=_('Full path to the backend to be used to do OCR.')
 )
 setting_ocr_backend_arguments = namespace.add_setting(