partial fix/optimisation for refimgs, only recalc if file is newer than when we build the refimg encoding

pa_job_manager.py

@@ -147,9 +147,10 @@ class Refimg(Base):
     id = Column(Integer, Sequence('refimg_id_seq'), primary_key=True )
     fname = Column(String(256), unique=True, nullable=False)
     encodings = Column(LargeBinary)
+    created_on = Column(Float)
 
     def __repr__(self):
-        return "<id: {}, fname: {}, encodings: {}>".format(self.id, self.fname, self.encodings )
+        return f"<id: {id}, fname: {fname}, created_on: {created_on}, encodings: {encodings}>"
 
 class File_Person_Link(Base):
     __tablename__ = "file_person_link"
@@ -545,10 +546,16 @@ def generateUnknownEncodings(im):
 
 def generateKnownEncodings(person):
     for refimg in person.refimg:
-        img = face_recognition.load_image_file('reference_images/'+refimg.fname)
+        file = 'reference_images/'+refimg.fname
+        stat = os.stat(file)
+        if refimg.created_on and stat.st_ctime < refimg.created_on:
+            print("DEBUG: skipping re-creating encoding for refimg because file has changed since we did this before")
+            continue
+        img = face_recognition.load_image_file(file)
         location = face_recognition.face_locations(img)
         encodings = face_recognition.face_encodings(img, known_face_locations=location)
         refimg.encodings = encodings[0].tobytes()
+        refimg.created_on = time.time()
         session.add(refimg)
     session.commit()