add pre-consume hook

a script hook can be defined in /etc/paperless.conf as
PAPERLESS_PRE_CONSUME_SCRIPT

src/documents/apps.py

@@ -8,11 +8,14 @@ class DocumentsConfig(AppConfig):
     def ready(self):
 
         from .signals import document_consumption_finished
+        from .signals import document_consumption_started
         from .signals.handlers import (
-            set_correspondent, set_tags, run_external_script)
+            set_correspondent, set_tags, run_post_consume_external_script, run_pre_consume_external_script)
 
         document_consumption_finished.connect(set_tags)
         document_consumption_finished.connect(set_correspondent)
-        document_consumption_finished.connect(run_external_script)
+        document_consumption_finished.connect(run_post_consume_external_script)
+
+        document_consumption_started.connect(run_pre_consume_external_script)
 
         AppConfig.ready(self)

src/documents/signals/handlers.py

@@ -57,7 +57,7 @@ def set_tags(sender, document=None, logging_group=None, **kwargs):
     document.tags.add(*relevant_tags)
 
 
-def run_external_script(sender, document, **kwargs):
+def run_post_consume_external_script(sender, document, **kwargs):
 
     if not settings.POST_CONSUME_SCRIPT:
         return
@@ -73,3 +73,13 @@ def run_external_script(sender, document, **kwargs):
         str(document.correspondent),
         str(",".join(document.tags.all().values_list("slug", flat=True)))
     )).wait()
+
+def run_pre_consume_external_script(sender, filename, **kwargs):
+
+    if not settings.PRE_CONSUME_SCRIPT:
+        return
+
+    Popen((
+        settings.PRE_CONSUME_SCRIPT,
+        filename
+)).wait()

src/paperless/settings.py

@@ -230,6 +230,7 @@ SHARED_SECRET = os.getenv("PAPERLESS_SHARED_SECRET", "")
 
 # Trigger a script after every successful document consumption?
 POST_CONSUME_SCRIPT = os.getenv("PAPERLESS_POST_CONSUME_SCRIPT")
+PRE_CONSUME_SCRIPT = os.getenv("PAPERLESS_PRE_CONSUME_SCRIPT")
 
 #
 # TODO: Remove after 0.2