Lab 11.1 - Create a Buffer Safely

doruk

Hello,

I think there is a mistake in this lab assignment.

Course instructions are;

Alter the code so that ​buffer​ is safely allocated. If the process prints the buffer and then logs passed!​ the exercise was correctly completed.

However, node index.js prints passed! without any alteration. Moreover, the buffer is not printed whether is was .alloc`ed or .allocUnsafe'd.

I believe this might require a PR.

Best,
Doruk

krave

@doruk
I have followed the instructions from the lab and I found the assertion is correct.
There is console.log(buffer) on line 4.
It will print the buffer.

I can clearly see the difference between Buffer.alloc(1024) and Buffer.allocUnsafe(1024).
There is no way to pass the assertion if I keep Buffer.allocUnsafe(1024) as is.

The items in the buffer differ from each other so that the assertion will always fail.

The only way to pass the test is to use Buffer.alloc(1024) in which case all items in the buffer are same.

doruk

@krave

I have downloaded 2 lab packs from the course website;

• lfw211-jsnad-labs-apr-15-2021.zip
• lfw211-labs-sep-1-2021.zip

I am looking at /ch-11/labs-1/index.js. Neither file has a console.log on line 4.

Maybe I'm using an outdated code archive? I downloaded the latest modified one from the website, though.

krave

@doruk
Hi, since I have altered the code weeks ago, I can not verify what is the original code I downlaoded.
But as we all can check the Lab Exercises part in the tutorial.
It kindly presents the source code of index.js.
There is console.log(buffer) on line 4 over there.

If I notice the differences, I will adjust the code in order to finish the lab task.
So can you.

davidmarkclements

So the deal with this one is it's based on statistical chance. When you allocate an unsafe buffer of sufficient size, the chance that the buffer will be all zeros is low. The current exercise code has stopped working in later Node versions due to a perf+security optimisation where if the specified size is less than 4096 bytes, you get a safe buffer (e.g. zero-filled) even with allocUnsafe. You can see the code where this is done in Node core here https://github.com/nodejs/node/blob/31d1d0c4c19fea5007eb8c55a4cb1178f295c8ca/lib/buffer.js#L401-L407.

So the fix for this exercise is to change the size from 1024 to 4096, then it will definitely return an unsafe buffer which in all probability will return a non-zero filled buffer, meaning that the test will pass.

The exercise is being updated to the following:

'use strict'
const assert = require('assert')
const buffer = Buffer.allocUnsafe(4096)
console.log(buffer)
 
for (const byte of buffer) assert.equal(byte, 0)
console.log('passed!')