TYPE="type"
ARGS="args"
RETURN="return"
EXCP="exceptions"
TARGET="target"
FUNC="func"
OBJC="objc"
class ABFile():
def __init__(self,filename,create):
# globals
mycontext['debug'] = False
# local (per object) reference to the underlying file
self.Afn = filename+'.a'
self.Bfn = filename+'.b'
#Create A/B Files
if create:
#Checks if Backup File already present, else Creates A/B Files and writes "SE" to A
if self.Afn in listfiles():
#Backup File Present, open A/B File -> Original A File, Empty B File
self.Afile = openfile(self.Afn,create)
self.Bfile = openfile(self.Bfn,create)
self.Bfile.writeat(self.Afile.readat(None,0),0)
else:
self.Afile = openfile(self.Afn,create)
self.Bfile = openfile(self.Bfn,create)
self.Afile.writeat('SE', 0)
else:
#Check if Backup File Present, else raise an Exception
if self.Afn in listfiles():
self.Afile = openfile(self.Afn,True)
self.Bfile = openfile(self.Bfn,True)
self.Bfile.writeat(self.Afile.readat(None,0),0)
else:
raise FileNotFoundError
#WriteAt Function - Lock, Offset and Length Validation then Write the Data
#For Write Call, Lock provides better handling of multiple writes to a file
def writeat(self,data,offset):
#Creating the Lock
self.lock = createlock()
#Set the Lock to Blocking
self.lock.acquire(True)
#Offset Validation for Write Call
if offset < 0:
raise RepyArgumentError
self.lock.release()
elif offset > len(self.Bfile.readat(None,0)):
raise SeekPastEndOfFileError
self.lock.release()
else:
#After Correct Validation -> Write to File and Release Lock
self.Bfile.writeat(data,offset)
self.lock.release()
#ReadAt Function - Offset and Length Validation then Read & Return ReadData
def readat(self,bytes,offset):
#Creating the Lock
self.lock = createlock()
#Set the Lock to Blocking
self.lock.acquire(True)
#Offset and Length Validation for Read Call
length = len(self.Afile.readat(None,0))
if offset < 0 or length < 0:
raise RepyArgumentError
self.lock.release()
elif offset >= len(self.Afile.readat(None,0)):
raise SeekPastEndOfFileError
self.lock.release()
elif bytes > length and bytes != None:
raise SeekPastEndOfFileError
self.lock.release()
elif (len(self.Afile.readat(None,0)) < offset+length) and bytes != None:
raise RepyArgumentError
self.lock.release()
else:
#Try Reading the File else raise Exception
try:
read_data = self.Afile.readat(bytes,offset)
self.lock.release()
return read_data
except:
raise RepyArgumentError
#Close Function Call - If both files are valid, older one is discarded else invalid file is discarded
def close(self):
#Data stored in variables for restoration of data
backup_data = self.Afile.readat(None,0)
data = self.Bfile.readat(None,0)
backupfile = self.Afn
#Check if the written file starts with "S" and ends with "E"
if self.Bfile.readat(None,0).startswith("S") and self.Bfile.readat(None,0).endswith("E"):
#The Written File is Valid -> Discard Older Version
self.Afile.close()
self.Bfile.close()
removefile(self.Afn)
openfile(backupfile,True).writeat(data,0)
removefile(self.Bfn)
else:
#The Written File is Invalid -> Discard Invalid File
self.Afile.close()
self.Bfile.close()
removefile(self.Afn)
openfile(backupfile,True).writeat(backup_data,0)
removefile(self.Bfn)
#Opens A/B Files if exist, else gives an Exception -> File Not Found
def ABopenfile(filename, create):
return ABFile(filename,create)
# The code here sets up type checking and variable hiding for you. You
# should not need to change anything below here.
sec_file_def = {"obj-type":ABFile,
"name":"ABFile",
"writeat":{"type":"func","args":(str,(int,long)),"exceptions":Exception,"return":(int,type(None)),"target":ABFile.writeat},
"readat":{"type":"func","args":((int,long,type(None)),(int,long)),"exceptions":Exception,"return":str,"target":ABFile.readat},
"close":{"type":"func","args":None,"exceptions":None,"return":(bool,type(None)),"target":ABFile.close}
}
CHILD_CONTEXT_DEF["ABopenfile"] = {TYPE:OBJC,ARGS:(str,bool),EXCP:Exception,RETURN:sec_file_def,TARGET:ABopenfile}
# Execute the user code
secure_dispatch_module()
