Please help modify the code:

$"""$

This security layer inadequately handles A$/$B storage for files in RepyV$2.$

Note:

This security layer uses encasementlib.r$2$py$,$ restrictions.default, repy.py and Python

Also you need to give it an application to run.

python repy.py restrictions.default encasementlib.r$2$py $[$security$\_$layer$].$r$2$py $[$attack$\_$program$].$r$2$py

$"""$

TYPE$=$"type"

ARGS$=$"args"

RETURN$=$"return"

EXCP$=$"exceptions"

TARGET$=$"target"

FUNC$=$"func"

OBJC$=$"objc"

class ABFile$()$:

def $\_\_$init$\_\_($self$,$filename,create$)$:

# globals

mycontext$[\text{'}$debug$\text{'}]=$ False

# local $($per object$)$ reference to the underlying file

self.Afn $=$ filename$+\text{'}.$a$\text{'}$

self.Bfn $=$ filename$+\text{'}.$b$\text{'}$

self.valid$\_$start$\_$char $=\text{'}$S$\text{'}$

self.valid$\_$end$\_$char $=\text{'}$E$\text{'}$

# make the files and add $\text{'}$SE$\text{'}$ to the readat file...

if create:

self.Afile $=$ openfile$($self$.$Afn,create$)$

self.Bfile $=$ openfile$($self$.$Bfn$,$create$)$

self.Afile.writeat$(\text{'}$SE$\text{'},0)$

else:

# Open existing files without modifying content

self.Afile $=$ openfile$($self$.$Afn,create$)$

self.Bfile $=$ openfile$($self$.$Bfn$,$create$)$

def is$\_$valid$\_$file$($self$,$ data$)$:

$"""$Checks if the data starts with $\text{'}$S$\text{'}$ and ends with $\text{'}$E$\text{'}."""$

return len$($data$)>=2$ and data$[0]==$ self.valid$\_$start$\_$char and data$[-1]==$ self.valid$\_$end$\_$char

def writeat$($self$,$data,offset$)$:

if data.startswith$("")$:

return # Silently reject if the data starts with a space

# Get the current size of B file

bfile$\_$size $=$ len$($self$.$Bfile.readat$($None$,0))$

# Ensure the offset is within valid range and append$/$overwrite correctly

if offset $>=$ bfile$\_$size:

# Append new data if the offset is beyond the current size

self.Bfile.writeat$($data$,$ bfile$\_$size$)$

else:

# Overwrite data at the valid offset

self.Bfile.writeat$($data$,$ offset$)$

def readat$($self$,$bytes,offset$)$:

# Read from the A file using the sandbox's readat...

if $($offset $>=0$ and offset $<$ len$($self$.$Afile.readat$($None$,0)))$:

return self.Afile.readat$($bytes$,$offset$)$

def close$($self$)$:

a$\_$data $=$ self.Afile.readat$($None$,0)$

b$\_$data $=$ self.Bfile.readat$($None$,0)$

self.Afile.close$()$

self.Bfile.close$()$

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$),$"exceptions":Exception,"return":$($int$,$type$($None$)),$"target":ABFile.writeat$\},$

"readat":$\{"$type$"$:"func","args":$(($int$,$type$($None$)),($int$)),$"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$()$

I am getting the following errors:

$1.$ Tests if the ABFile class can successfully open and write valid data. $(0/4\mathrm{.}5)$

$2.$ Tests if the ABFIle class can properly handle initiate $->$ append $->$ close $->$ open $->$ read $($iacor$)$ cases. $(0/4\mathrm{.}5)$

$3.$ Tests if the ABFile class can correctly handle multiple valid writes. $(0/4\mathrm{.}5)$

$4.$ Tests if the ABFile class can correctly handle resource contention during threading. $(0/4\mathrm{.}5)$

$5.$ Tests if the ABFile class can successfully write valid data to an existing file. $(0/4\mathrm{.}5)$

You can test the code using this script

if "testfile.txt$.$a$"$ in listfiles$()$:

removefile$("$testfile$.$txt$.$a$")$

if "testfile.txt$.$b$"$ in listfiles$()$:

removefile$("$testfile$.$txt$.$b$")$

myfile$=$ABopenfile$("$testfile$.$txt$",$True$)$ #Create an AB file

# I should get $\text{'}$SE$\text{'}$ when reading an empty file...

assert$(\text{'}$SE$\text{'}==$ myfile.readat$($None$,0))$

# put some valid data in the file.

myfile.writeat$("$Stest$12345$E$",0)$

# I should still get $\text{'}$SE$\text{'}$ because the file wasn't closed.

assert$(\text{'}$SE$\text{'}==$ myfile.readat$($None$,0))$

#Close the file

myfile.close$()$